Printer Friendly

Effect of tillage erosion on the distribution of CaC[O.sub.3], phosphorus and the ratio of CaC[O.sub.3]/available phosphorus in the slope landscape.

Introduction

Soils are a limited natural resource that provide fertiliser for growing food, store and filter water, host rich ecosystems and hold our cultural and historical memory in archaeological artefacts (Montanarella 2015). The consideration of soil science can be helpful in solving many of the current issues (environmental, societal, economic, geological and issues related to human health) (Howitt et al. 2009; McBratney et al. 2014; Brevik et al. 2015; Decock et al. 2015). According to Smith et al. (2015), the ability of soils to provide services is principally conferred by two attributes: (1) the range of biogeochemical processes that occur in the soil; and (2) the richness and functionality of soil biodiversity. However, soil erosion has an adverse effect on the functions of soil fertilisation, ecology and services, and is therefore regarded as one of the most serious threats to land resources all over the world (Cerda et al. 2007). Consequently, soil and water conservation is of great importance to the development of modern agriculture (Cerda et al. 2009a).

Lieskovsky and Kenderessy (2014) found that downslope-tilled vineyards were the most eroded among various management practices. No tillage and crop rotation can improve the soil organic matter (SOM) chemical and biochemical properties of Vertosols under a semi-arid environment (Laudicina et al. 2015). Furthermore, coverage with appropriate crops effectively reduces soil erosion under Mediterranean climate conditions (Novara et al. 2011). Also under the Mediterranean climate, conventional tillage and organic farming for a long period of time affect soil organic carbon (SOC), N and exchangeable macroelements ([Ca.sup.2+] and [K.sup.+]) contents, as well as soil texture in particular (Parras-Alcantara and Lozano-Garcia 2014). In the dry areas of northern China, conservation tillage in the form of no post-harvest tillage with stubble retention significantly contributes to increased SOC storage at soil depths of 0-30 cm (Gao et al. 2016). The aforementioned previous studies emphasised the effects of different tillage methods on water erosion, wind erosion and the storage of SOC and soil nutrients (N, P and K). However, there are still knowledge gaps regarding the direct effects of soil redistribution caused by tillage on the distribution of soil nutrients.

Tillage erosion, together with water erosion, wind erosion and freeze-thaw erosion (Yumoto et al. 2006; Cerda et al. 2009b, 2010; Colazo and Buschiazzo 2015), is an important component of soil erosion on sloping croplands, which means gradual soil translocation or displacement downhill caused by tillage (Lindstrom et al. 1990). Tillage erosion modifies the landscape geomorphology by translocating the topsoil from upper to lower slope positions. During this process, net soil loss occurs primarily on the shoulder slope position because of the combination of the effects of gravity and downward drag (Wang et al. 2015). In the meantime, soil accumulates at the lower slope positions due to the transmission effect of tillage erosion at upper slope positions (Zhang et al. 2008). As a result of tillage erosion, shallow soil or even exposed bedrock is observed on convex slopes with a high percentage of rock fragments, CaC[O.sub.3] and low nutrient concentrations. In contrast, a deeper soil layer is found on concave slopes where displaced soil is deposited. Soil degradation caused by tillage erosion is a major reason for the reduction in grain yield in the hillslope landscapes (Su et al. 2010; Karamesouti et al. 2015). Tillage erosion redistributes the soil along the slope, accompanied by many changes in soil properties and migration of soil nutrients. Previous studies have demonstrated that redistribution of soil material determines the distribution of SOC along the hillslope (Zhang et al. 2006, 2015). Li et al. (2013)found that the mixing effect of tillage attenuated the variability of soil nutrients (SOC, P, K) in the vertical direction, whereas the downslope translocation of soil caused by tillage erosion accentuated the variability of soil nutrients (SOC, P, K) in the lateral direction.

Previous studies have demonstrated that available phosphorus (AP) is an important factor that strongly affects plant growth (Hinsinger 2001; Kumar et al. 2015). The CaC[O.sub.3]/AP ratio is a key indicator to measure the degree of immobilisation of AP by CaC[O.sub.3]. The soil stoichiometry of the CaC[O.sub.3]/AP ratio has a considerable effect on plant growth, subsequently affecting plant biomass and density (Zhao et al. 2012). A few studies have contributed to the understanding of the effects of tillage erosion on the horizontal translocation of CaC[O.sub.3] and phosphorus along transects of the toposequence (Braschi et al. 2003; De Alba et al. 2004), yet no studies have investigated the effects of tillage erosion on changes in the CaC[O.sub.3]/AP ratio.

Against this background, the aims of the present study were to: (1) examine the distribution patterns of CaC[O.sub.3], total phosphorus (TP) and AP under different tillage conditions; (2) ascertain the effects of long-term tillage on changes in CaC[O.sub.3] and phosphorus of the soil profile and to reconstruct the historical changes in CaC[O.sub.3] concentrations in soil layers at different landscape positions; and (3) elucidate the mechanism underlying tillage erosion-induced changes in the CaC[O.sub.3]/AP ratio along the transect of slopes.

Materials and methods

Study site

The study area is located in Jianyang County in the eastern part of the Sichuan Basin, south-western China (30[degrees]26'N, 104[degrees]28'E). The climate of the study area is a typically humid subtropical climate, which is characterised by four distinct seasons. The mean annual temperature is 17.4[degrees]C (range -5.4 to 38.7[degrees]C) and there is almost no snow in winter. The mean annual precipitation is 872 mm, >90% of which occurs from May to October. The annual frost-free period is approximately 311 days. The terrain is higher in the north-west than in the south-east. The average elevation is between 400 and 587 m above sea level and approximately 88% of the total area is occupied by hilly landforms that belong to the Sichuan hilly basin. The soils of the study area, derived from purple mudstone in the Jurassic Penglaizhen Formation ([J.sub.2P]), are classified as Regosols according to the Food and Agriculture Organization of the United Nations (FAO) soil taxonomy (FAO 1988). In general, soil layers are quite thin (~20 cm) at the upper slope positions and thick (>40 cm) at the lower slope positions because of intensive soil erosion and accumulation caused by tillage. In particular, bedrocks are exposed on some slope farmlands because of intensive tillage (Zhang et al. 2004). In most cases, the predominant crops are wheat (Triticum aestivum L.), rape (Brassica campestris L.), corn (Zea mays L.) and sweet potatoes (Ipomoea batatas [L.] Lam) on the slope farmlands.

Two similar slopes with a horizontal distance of approximately 500 m were selected as the study sites. The fundamental features of these two slopes are given in Table 1. The two slopes had same horizontal length of 21 m and had similar slope gradients of 17.8% and 19.6%. A similar tillage method, called 'downslope tillage', was used on the two slopes before 1982. During downslope tillage, hoeing to a tillage depth of approximately 20 cm is performed from bottom to hilltop, and the soil moves downhill due to gravity and drag. Downslope tillage was always performed on Slope 1; however, tillage in the opposite direction ('upslope tillage') was performed on Slope 2 after 1982 following implementation of the family contract responsibility system in 1982 and the farmers' awareness of soil loss resulting from downslope tillage. During upslope tillage, tillage by hoeing starts at the summit and proceeds downslope step by step, with the soil being dragged uphill against the gravity by hoeing (Fig. 1). Both downslope and upslope tillage have been conducted once a year since 1982 and are referred to 'normal tillage' conditions. Under normal tillage conditions, the soil is translocated by the effects of both water and tillage erosion. No significant differences (P=0.46) in CaC[O.sub.3] concentrations of bedrocks were detected between Slopes 1 and 2. Similar pH, bulk density and distribution patterns of particle size fractions were also observed in the parent material between the two slopes. The crucial difference in land management between the two slopes has been the opposite tillage direction since 1982. Given the similar landscape elements and properties of parent material for the two slopes, any differences in the distribution patterns of CaC[O.sub.3], TP and AP observed are due to soil redistribution caused by the contrasting tillage directions.

To ascertain the effects of different downslope tillage intensities on the translocation of those soil constituents along the slope, consecutive downslope tillage by hoeing (called 'consecutive tillage') was applied five and 20 times on Slope 1 (short- and long-term tillage respectively). The consecutive tillage was conducted by four farmers over 3 days. During consecutive tillage, no rainfall occurred and therefore the effects of water erosion on the redistribution of soil constituents were eliminated. The redistribution of soil constituents caused by consecutive tillage differed to that seen with normal tillage, because the translocation of soil constituents was affected only by tillage erosion under consecutive tillage conditions and thus the effects of different downslope tillage intensities on soil constituent redistribution were easily examined.

Soil sampling and analysis

Both slopes were divided into five landscape positions: (1) the summit (0 m), (2) the shoulder slope (5 m), (3) the midslope (10m), (4) the backslope (15m) and (5) the toeslope (20m). Three parallel profile lines with a contour distance of 2 m were arranged for each slope. Core samples were taken along the profile lines across the two slopes. The sampling points were set at an interval of 5 m from the hilltop to the bottom along the profile lines. Each core was separated into four to 10 sub-samples (from soil surface to bedrock) with a depth increment of 5 cm in order to analyse the vertical distribution of CaC[O.sub.3], TP and AP. The subsamples from each sampling point with the same depth were mixed and generated a composite sample for each depth. The topsoil layer (0-20 cm) included four subsamples and the subsoil layer (20-35 or 20 40 cm) included three or four subsamples respectively with each 5 cm depth representing a subsample. The topography of each sampling point, including coordinates and elevations, was measured using a survey-grade differential global positioning system (DGPS).

Soil samples were air dried, crushed and passed through a 2-mm mesh sieve to remove coarse gravel material and residual plant roots. Soil particle size fractions were determined by the pipette method following [H.sub.2][O.sub.2] treatment to destroy organic matter and subsequent dispersion of soil suspensions by Na hexametaphosphate (Gee and Or 2002). Soil pH was determined using a digital pH meter. CaC[O.sub.3] was determined using a pressure calcimeter method (Nanjing Institute of Soil Science of the Chinese Academy of Scicnces 1978). Soil TP and AP were determined using the hydrofluoric acid-perchloric acid-molybdenum antimony colourimetric method and sodium bicarbonate extraction colourimetric method respectively (Liu 1996). Soil bulk density was determined using the ratio of the oven-dried mass to the volume of soil cores. These determinations were performed in the Key Laboratory of Mountain Environment Evolution and Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences (Chengdu, China).

Results

Distribution of CaC[O.sub.3] concentrations

Under normal tillage conditions, the summit positions had the highest CaC[O.sub.3] concentrations in the topsoil layer among the five landscape positions; however, under consecutive tillage with both five and 20 tills, the highest CaC[O.sub.3] concentrations in the topsoil layer were detected at the shoulder slope positions. The largest increments of 32.9% and 41.4% in CaC[O.sub.3] concentrations in the topsoil layer were both observed at shoulder slope positions among the five landscape positions for the consecutive tillage with five and 20 tills respectively compared with downslope tillage. For consecutive tillage with 20 tills, CaC[O.sub.3] concentrations of the topsoil layer hardly exhibited any changes at the summit position, compared with consecutive tillage with five tills (Fig. 2). Paired-sample t-tests indicated significant differences (P < 0.01) in CaC[O.sub.3] concentrations in the topsoil layer between downslope tillage and consecutive tillage (five and 20 tills) at each slope position; however, no significant differences (P=0.49) were detected between consecutive tillage with five and 20 tills.

With the exception of only one landscape position of downslope tillage (Table 2), CaC[O.sub.3] concentrations in the soil profile increased exponentially with soil depth under normal tillage conditions, which can be described by the following formula:

[mathematical expression not reproducible] (1)

where [mathematical expression not reproducible] is the depth CaC[O.sub.3] concentration (in g [kg.sup.-1]), D is the depth from the soil surface (in cm), and a and b are coefficients describing the profile shape (in g [kg.sup.-1]). This model significantly fits the data under normal tillage conditions at the P<0.05 or <0.01 levels (Table 2).

Descriptive coefficient a of the profile shape showed different features between downslope and upslope tillage. The mean value of a in Eqn 1 for CaC[O.sub.3] depth concentration was significantly smaller for downslope than upslope tillage, with the former being 0.66-fold that of the latter; however, the mean value of coefficient b was approximately twofold grater for downslope than upslope tillage (Table 2). This indicates that CaC[O.sub.3] depth distribution in the soil profile can be affected by different tillage directions. After consecutive tillage, significant changes occurred in the fitting results of the regression equation compared with downslope tillage. For consecutive tillage with five tills, the depth distribution of CaC[O.sub.3] could be described by Eqn 1 only at the shoulder slope and midslopc positions; however, CaC[O.sub.3] depth distribution did not fit (P>0.05) the exponential equation at any slope position for consecutive tillage with 20 tills, as determined by F-tests (Table 2). The above analysis indicates that consecutive tillage changes the distribution patterns of CaC[O.sub.3] in the vertical direction.

Distribution of TP and AP concentrations

AP concentrations in the topsoil layer under consecutive tillage with five and 20 tills decreased by 7.5% and 21.2% respectively at the summit position and by 22.9% and 24.0% respectively at the toeslope position compared with downslope tillage. Considerable declines in AP concentrations in the topsoil layer were found at the shoulder slope and backslope positions for consecutive tillage with five (34.2% and 32.6% respectively) and 20 tills (37.8% and 37.2% respectively) compared with downslope tillage (Table 3). Significant differences in AP concentrations between the topsoil and subsoil layers were found at each landscape position under normal tillage conditions, but no significant differences were observed in AP concentrations between the topsoil and subsoil layers at the toeslope position for consecutive tillage with 20 tills. For downslope tillage, significant differences in TP concentrations were observed between the topsoil and subsoil layers at the summit, backslope and toeslope positions, whereas highly significant differences in TP between topsoil and subsoil layers were present at each landscape position for upslope tillage. As a whole, significant differences in AP concentrations in the topsoil layer (P<0.01) were detected between consecutive tillage (five and 20 tills) and downslope tillage; however, no significant differences in TP concentrations (P=0.19 and 0.84 respectively) were found (Table 4). These results indicate that the mixing effect of tillage causes a marked decrease in AP concentrations in the topsoil layer and no obvious changes in TP concentrations.

Distribution of CaC[O.sub.3], TP and AP inventories

The distribution of CaC[O.sub.3], TP and AP inventories along the transect of the toposequence is shown in Fig. 3. The highest CaC[O.sub.3] inventories were observed at midslope positions under normal tillage conditions and at the toeslope positions for consecutive tillage with five and 20 tills. For consecutive tillage with five tills, CaC[O.sub.3] inventories decreased at all landscape positions, except toeslope position, compared with downslope tillage, with the most obvious decrease (37.9%) found at the midslope position. For consecutive tillage with 20 tills, CaC[O.sub.3] inventories decreased at the shoulder slope, midslope and backslope positions, with the most obvious decrease (49.4%) also found at the midslope position (Fig. 3a). However, CaC[O.sub.3] inventories increased at the toeslope position for consecutive tillage with five and 20 tills (62.2% and 89.9% respectively) compared with downslope tillage, indicating that both short--and long-term tillage caused considerable CaC[O.sub.3] accumulation at the toeslope position.

The highest TP and AP inventories were found at the backslope and toeslope positions for downslope tillage and at the midslope and toeslope positions for upslope tillage. For consecutive tillage with five tills, TP inventory decreased at the summit, shoulder slope, midslope and backslope positions, whereas AP inventories decreased at all landscape positions compared with downslope tillage. However, for consecutive tillage with 20 tills, TP inventories decreased at the shoulder slope, midslope and backslope positions, and AP inventories decreased at all positions except for the toeslope position. Similar to CaC[O.sub.3] inventory, the TP and AP inventories increased at the toeslope position for consecutive tillage with five and 20 tills (72.1% and 72.0% respectively) compared with downslope tillage (Fig. 3b, c), indicating that consecutive tillage results in the accumulation of TP and AP at the toeslope position.

Changes in the CaC[O.sub.3]/AP ratio

Relationships between CaC[O.sub.3] and AP concentrations under different tillage conditions are shown in Fig. 4. Significant and negative correlations were found between AP and CaC[O.sub.3] concentrations for downslope and upslope tillage ([R.sup.2] = 0.42 (P<0.01, w = 34) and [R.sup.2] = 0.63 (P<0.01, n = 50) respectively; Fig. 4a), whereas no significant correlations were found between AP and CaC[O.sub.3] following consecutive tillage with five and 20 tills ([R.sup.2] = 0.1 (P>0.05, n = 29) and [R.sup.2] = 0.09 (P> 0. 1. n = 25) respectively; Fig. 4b). These results can be explained by the fact that CaC[O.sub.3] can effectively absorb AP under normal tillage conditions, but this absorption is obviously weaker after consecutive tillage because of a lack of sufficient time for chemical reactions to occur.

Changes in the CaC[O.sub.3]/AP ratio of the topsoil layer arc given in Table 5. Of the five landscape positions, the greatest increments in the CaC[O.sub.3]/AP ratio in topsoil layer were detected at the backslope and midslope positions (187.7% and 116.6% respectively), whereas the smallest increments were observed at the toeslope positions for consecutive tillage with five and 20 tills compared with downslope tillage (52.1% and 70.2% respectively). Paired-samples t-tests showed no significant differences (P > 0.05) in the CaC[O.sub.3]/AP ratio of the topsoil layer between downslope and upslope tillage; however, the mean CaC[O.sub.3]/AP ratios increased by 93.5% (range 52.1-187.7%) and 88.4% (range 70.2-116.6%) for consecutive tillage with five and 20 tills respectively compared with downslope tillage. These results suggest that both short--and long-term tillage may contribute to increases in the CaC[O.sub.3]/AP ratio in the topsoil layer.

Discussion

Overall, CaC[O.sub.3] concentrations increased exponentially with soil depth in the soil profile under normal tillage conditions, except for the backslope position under downslope tillage (Table 2). This vertical distribution pattern can be explained by chemical eluviation due to water flow in the soil profile. When rainfall occurs, overland water flow can transport sediments from the upper to lower slope positions; meanwhile, it also infiltrates from the topsoil layer to the deeper soil layer with the eluviation of CaC[O.sub.3] (Schlesinger and Pilmanis 1998; Tan et al. 2008). For consecutive tillage with five tills, CaC[O.sub.3] concentrations still increased exponentially with soil depth at the midslope position (P < 0.05). This is because the midslope position plays the role of a conveyor belt in the tillage erosion process and transports soil from the upper to lower slope positions with slight soil loss and gain (Govers et al. 1996; Quine and Zhang 2002; De Alba et al. 2004). Previous studies also showed that landscape position exert an important effect on soil microbial biomass carbon (C), basal respiration and vegetation recovery (Campos et al. 2014; Pereira et al. 2016). However, Cerda (1998) found that run-off and water erosion rates are barely affected by the relative slope positions under extreme dry conditions during the summer.

For consccutive tillage with 20 tills, CaC[O.sub.3] depth concentrations no longer followed the exponential formula (Eqn 1; P>0.05) at any landscape position (Table 2). This can be attributed to the vertical mixing of CaC[O.sub.3] caused by consecutive tillage. During consecutive tillage, CaC[O.sub.3] was dug out from the parent material or bedrock and was incorporated into the topsoil layer, and the mixed soil was then transported from the upper to lower slope positions where a new plough layer formed. Consequently, the mixing effect of tillage changed the CaC[O.sub.3] depth distribution pattern in the soil profile and weakened the variability of CaC[O.sub.3] concentrations in the vertical direction. This result further confirms previous studies that showed that tillage causes the violent mixture of soil constituents in the soil profile (Li et al. 2008, 2013).

For downslope tillage, the CaC[O.sub.3] concentration in the topsoil layer at the toeslope was significantly lower than that at other slope positions (P < 0.05); however, no significant differences were observed between the toeslope and other slope positions for upslope tillage (Fig. 2). These differences at the toeslope positions can be explained primarily by the opposite directions of tillage. In the case of upslope tillage, soil was conveyed to the upper slope positions from the toeslope position and a hollow was created there because of uphill drag; thus, bedrock rich in CaC[O.sub.3] in the hollow bottom was exposed and some was mixed into the topsoil layer at the toeslope position. However, in the case of downslope tillage, the toeslope was covered with a thick soil layer, the depth of which was apparently greater than that of tillage operations because the accumulation of the soil was caused by a combined effect of gravity and downward drag; thus, it was difficult to till bedrock, but there was a leaching effect at the toeslope. In the meantime, downslope tillage reduced the depth of surface horizons at upper slope positions and thus soil parent materials or bedrock (with high CaC[O.sub.3] concentrations) were tilled and crushed and then incorporated into the topsoil layer (Zhang et al. 2008). The average CaC[O.sub.3] concentrations of the topsoil layer increased significantly for consecutive tillage with five and 20 tills (27.7% and 30.8% respectively) compared with downslope tillage (Fig. 2). These results indicate that the mixing effect of consecutive tillage contributes to the increase in CaC[O.sub.3] concentrations in the topsoil layer, and more CaC[O.sub.3] from the subsoil, parent material or bedrock was incorporated into the topsoil layer with an increase in tillage intensity.

The average AP concentrations of the topsoil layer decreased by 26.1% and 29.0% for consecutive tillage with five and 20 tills respectively compared with downslope tillage; however, no significant differences (P > 0.1) in TP concentrations were found (Tables 3, 4). The different patterns in TP and AP under consecutive tillage conditions can be mostly attributed to the fact that the distribution of TP is relatively uniform in the soil profile compared with AP, and is not easily affected by soil redistribution caused by tillage. Similar results regarding the translocation mechanisms of TP and AP on complex slopes have been reported by Ge et al. (2007), in which AP was highly related to soil redistribution caused by tillage but TP was hardly affected by tillage.

For consecutive tillage with 20 tills, the shoulder slope position had the lowest TP and AP inventories among the five landscape positions. This can be explained by the fact that TP and AP were depleted with the disappearance of the topsoil and the exposure of parent material at the shoulder slope position as a result of long-term tillage (Fig. 3b, c). Similar results were obtained for comparisons of [sup.137]Cs inventories before and after intensive tillage operations in the Sichuan hilly basin of China (Li et al. 2013). In the present study, consecutive tillage with 20 tills decreased the AP inventory of soil profiles by 42.7% at the shoulder slope positions compared with downslope tillage. Among the five landscape positions, the highest TP and AP inventories were present at the toeslope position for consecutive tillage with 20 tills, which can be explained by deposition of soil at the toeslope position as a result of long-term tillage.

AP concentrations (y) decreased with increasing CaC[O.sub.3] concentrations (x) and could be described by the following equations for downslope (Eqn 2) and upslope (Eqn 3) tillage (see also Fig. 4a):

y = 20.49 - 0.25x + 0.0008[x.sup.2] (2)

y = 43.63 - 0.31x (3)

with highly significant correlations ([R.sup.2] = 0.42 and 0.63 respectively; P<0.01 for both). This decrease in AP with increasing CaC[O.sub.3] is due to the fact that CaC[O.sub.3] readily immobilises AP as a result of adsorption and precipitation (von Wandruszka 2006). That is, high levels of CaC[O.sub.3] in the soil greatly decrease the availability of AP; thus, AP concentrations decreased with increasing CaC[O.sub.3] concentrations in the soil profile. However, no significant correlations were detected between CaC[O.sub.3] and AP concentrations after consecutive tillage (Fig. 4b). This can be explained by the fact that the adsorption and precipitation of AP by CaC[O.sub.3] is relatively a long-term process and that CaC[O.sub.3] could not adsorb AP effectively within a relatively short period after consecutive tillage (Tunesi et al. 1999).

No significant differences (P> 0.05) in the CaC[O.sub.3]/AP ratios of the topsoil layer were observed between downslope and upslope tillage (Table 5). This indicates that the CaC[O.sub.3]/AP ratio was hardly affected by the direction of tillage. However, for consecutive tillage, the CaC[O.sub.3]/AP ratios of the topsoil layer were significantly higher (P< 0.05) than those of downslope tillage (Table 5). Changes in the CaC[O.sub.3]/AP ratio can be ascribed to the fact that the subsoil, parent material or bedrock with higher CaC[O.sub.3] and lower AP concentrations was mixed into the topsoil layer because of consecutive tillage, which acted as a mixer for soil with different CaC[O.sub.3] and AP concentrations in the vertical direction. As a result, both short--and long-term tillage may lead to increases in the CaC[O.sub.3]/AP ratio in the topsoil layer. This inference is based on previous studies that showed that conventional tillage decreases soil P stocks in the plough layer (0-20 cm) compared with no-tillage practice (Balota et al. 2014; Gao et al. 2016). However, Singh et al. (2016) found that 5--and 9-year tillage sites had significantly lower CaC[O.sub.3] concentrations in the 0-20 soil depth than the 0-year tillage site in Uttar Pradesh, India. This difference can be attributed to the fact that the effects of water erosion and chemical eluviation due to water flow were not eliminated in the study conducted by Singh et al. (2016). It has been proved that the tillage of sodic soils contributes to the leaching of salts from the surface to deeper layers (Sadiq et al. 2007); thus, the tillage sites had lower CaC[O.sub.3] concentrations in the plough layer than the no-tillage site (Singh et al. 2016). Among the five slope positions, the lowest values of the CaC[O.sub.3]/AP ratio in the topsoil layer were observed at the toeslope positions under normal tillage conditions (Table 5). The linear slope contributed to the development of overland water flow and thus CaC[O.sub.3] was easier to lose by leaching from the toeslope position. However, phosphorus entered the farmland constantly because of the application of phosphorus fertiliser. For consecutive tillage with five and 20 tills, the lowest values of the CaC[O.sub.3]/AP ratio were still observed at the toeslope position. This can be primarily attributed to changes in the CaC[O.sub.3] and AP concentrations, together with the redistribution of soil caused by consecutive tillage. On the one hand, subsoil, parent material or bedrock with high CaC[O.sub.3] concentrations was incorporated into the topsoil layer at the upper landscape positions, and no further phosphorus fertiliser was applied during consecutive tillage. On the other hand, subsoil with lower CaC[O.sub.3] concentrations at the toeslope position than at upper slope positions was mixed into the topsoil layer because of the thick soil layer at the toeslope position. Moreover, parent material or bedrock fragments with high CaC[O.sub.3] concentrations that were incorporated into the topsoil layer at the upper slope positions may not have been fully translocated to the toeslope position after consecutive tillage with five and 20 tills.

Unlike previous expectations, in the present study the decline in AP concentrations in the topsoil layer was not only associated with CaC[O.sub.3], but also linked to the dilution effect by the materials incorporated into the topsoil layer from the subsoil, parent material or bedrock. After tillage, materials with high CaC[O.sub.3] and low AP concentrations were dug out from the subsoil, parent material or bedrock and incorporated into the topsoil layer. On the one hand, high levels of CaC[O.sub.3] decreased AP concentrations in the topsoil layer because of the adsorption and precipitation of AP by CaC[O.sub.3] in the long-term soil-forming processes, as stated earlier. On the other hand, in this action, the mixing effect of tillage decreased AP concentrations in the topsoil layer due to a dilution effect. When rainfall occurs, CaC[O.sub.3] leaches from the topsoil to deeper soil layers because of the infiltration of overland water. The results of the present study revealed that under normal tillage conditions the current status and distribution of CaC[O.sub.3] and AP concentrations within the hillslope resulted from the combined effects of tillage and leaching. CaC[O.sub.3] concentrations of the topsoil layer fluctuated as a result of tillage and rainfall, exhibiting alternate processes between the increase due to the incorporation of bedrock materials to the topsoil and the decline due to leaching. CaC[O.sub.3] was continuously replenished into the topsoil by the incorporation effect of tillage, but its loss as a result of leaching also occurred constantly, which is a dynamic change in the long-term soil-forming process. From this point of view, we would infer that no-tillage or reduced tillage can increase AP concentrations in topsoil layer, thereby diminishing the need for the application of phosphorus fertilisers.

Conclusions

The simulated experiments in the present study reconstructed the processes contributing to changes in CaC[O.sub.3] concentrations in soil layers during long-term tillage practice. CaC[O.sub.3] concentrations increased exponentially with soil depth in the soil profile under normal tillage conditions; however, these vertical distribution patterns changed after consecutive tillage. For consecutivc tillage (five and 20 tills), CaC[O.sub.3] concentrations in the topsoil layer also increased compared with downslope tillage, but AP concentrations decreased markedly. AP concentrations decreased with increasing CaC[O.sub.3] concentrations due to the adsorption and precipitation of AP by CaC[O.sub.3] under normal tillage conditions. However, under conditions of consecutive tillage, the lack of correlation between CaC[O.sub.3] and AP concentrations implied that the soil mixing and translocation due to tillage altered the patterns of CaC[O.sub.3] and AP that arc normally seen with long-term soil-forming processes. For consecutive tillage (five and 20 tills), the mean CaC[O.sub.3]/AP ratios of the topsoil layer were much greater than those seen with downslope tillage, whereas no significant differences in mean CaC[O.sub.3]/AP ratios in the topsoil layer were seen between upslope and downslope tillage practices. The present study shows that tillage is a process of CaC[O.sub.3] replenishment and AP dilution in the surface layer of soil derived from carbonate-rich bedrock.

http://dx.doi.org/10.1071/SR16077

Received 21 March 2016, accepted 5 January 2017, published online 14 February 2017

Acknowledgements

This study was performed with financial support from the National Natural Science Foundation of China (Grant nos 41571267 and 41271242) and the Key Scientific and Technological Program of China National Tobacco Corporation Sichuan Branch (Grant no. SCYC201504).

References

Balota EL, Yada IF, Amaral H, Nakatani AS, Dick RP, Coyne MS (2014) Long-term land use influences soil microbial biomass P and S, phosphatase and arylsulfatase activities, and S mineralization in a

Brazilian oxisol. Land Degradation & Development 25, 397-406. doi: 10.1002/ldr.2242

Braschi I, Ciavatta C, Giovannini C, Gessa C (2003) Combined effects of water and organic matter on phosphorus availability in calcareous soils. Nutrient Cycling in Agroecosystems 67, 67-74. doi: 10.1023/A: 102514 3809825

Brevik EC, Cerda A, Mataix-Solera J, Pereg L, Quinton JN, Six J, Van Oost K (2015) The interdisciplinary nature of SOIL. Soil 1, 117-129. doi:10.5194/soil-1-117-2015

Campos AC, Etchevers JB, Oleschko KL, Hidalgo CM (2014) Soil microbial biomass and nitrogen mineralization rates along an altitudinal gradient on the Cofre de Perote Volcano (Mexico): the importance of landscape position and land use. Land Degradation & Development 25, 581-593. doi: 10.1002/ldr.2185

Cerda A (1998) The influence of geomorphological position and vegetation cover on the erosional and hydrological processes on a Mediterranean hillslope. Hydrological Processes 12, 661-671. doi: 10.1002/(SICI) 1099-1085(19980330)12:4<661:: AID-H YP607>3.0.CO;2-7

Cerda A, Imeson AC, Poesen J (2007) Soil water erosion in rural areas. Catena 71, 191-266.

Cerda A, Flanagan DC, le Bissonnais Y, Boardman J (2009a) Soil erosion and agriculture. Soil & Tillage Research 106, 107-108. doi:10.1016/ j.still.2009.10.006

Cerda A, Gimenez-Morera A, Bodi MB (20096) Soil and water losses from new citrus orchards growing on sloped soils in the western Mediterranean basin. Earth Surface Processes and Landforms 34, 1822-1830. doi: 10.1002/esp. 1889

Cerda A, Lavee H, Romero-Diaz A, Hooke J, Montanarella L (2010) Soil erosion on Mediterranean type-ecosystems. Land Degradation & Development 21, 71-74. doi:10.1002/ldr.968

Colazo JC, Buschiazzo D (2015) The impact of agriculture on soil texture due to wind erosion. Land Degradation & Development 26, 62-70. doi: 10.1002/ldr.2297

De Alba S, Lindstrom M, Schumacher TE, Malo DD (2004) Soil landscape evolution due to soil redistribution by tillage: a new conceptual model of soil catena evolution in agricultural landscapes.

Catena 58, 77-100. doi:10.1016/j.catena.2003.12.004 Decock C, Lee J, Necpalova M, Pereira E1P, Tendall DM, Six J (2015) Mitigating N20 emissions from soil: from patching leaks to transformative action. Soil 1, 687-694. doi: 10.5194/soil-1-687-2015

FAO (1988) Soil map of the world (revised legend). World Soil Resources Report 60, FAO, Rome.

Gao Y, Dang X, Yu Y, Li Y, Liu Y, Wang J (2016) Effects of tillage methods on soil carbon and wind erosion. Land Degradation & Development 27, 583-591. doi:10.1002/ldr.2404

Ge FL, Zhang JH, Su ZA, Nie XJ (2007) Response of changes in soil nutrients to soil erosion on a purple soil of cultivated sloping land. Acta Ecologica Sinica 27, 459-463. doi: 10.1016/SI872-2032(07)60018-3

Gee GW, Or D (2002) Particle size analysis. In 'Methods of soil analysis. Part 4. Physical methods'. Book Series no. 5. (Eds JH Dane, GC Topp) pp. 255-293. (Soils Science Society of America: Madison, WI)

Govers G, Quine TA, Desmet PJJ, Walling DE (1996) The relative contribution of soil tillage and overland flow erosion to soil redistribution on agricultural land. Earth Surface Processes and Landforms 21, 929-946. doi: 10.1002/(SICI)1096-9837(199610)21:10 <929::A1D-ESP631>3.0.CO;2-C

Hinsinger P (2001) Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review. Plant and Soil 237, 173-195. doi: 10.1023/A: 1013351617532

Howitt RE, Catala-Luque R, De Gryze S, Wicks S, Six J (2009) Realistic payments could encourage farmers to adopt practices that sequester carbon. California Agriculture 63, 91-95. doi:10.3733/ca.v063n02p91

Karamesouti M, Detsis V, Kounalaki A, Vasiliou P, Salvati L, Kosmas C (2015) Land-use and land degradation processes affecting soil resources: evidence from a traditional Mediterranean cropland (Greece). Catena 132, 45-55. doi: 10.1016/j.catena.2015.04.010

Kumar B, Dhaliwal SS, Singh ST, Lamba JS, Ram H (2015) Herbage production, nutritional composition and quality of teosinte under Fe fertilization. International Journal of Agriculture and Biologv 614-615. 1583-1586. doi: 10.17957/IJAB/15.0089

Laudicina VA, Novara A, Barbera V, Egli M, Badalucco L (2015) Longterm tillage and cropping system effects on chemical and biochemical characteristics of soil organic matter in a Mediterranean semiarid environment. Land Degradation & Development 26, 45-53. doi:10.1002/ ldr.2293

Li S, Lobb DA, Lindstrom MJ, Papiernik SK, Farenhorst A (2008) Modeling tillage-induced redistribution of soil mass and its constituents within different landscapes. Soil Science Society of America Journal 72. 167-179. doi: 10.2136/sssaj2006.0418

Li FC, Zhang JH, Su ZA, Fan HZ (2013) Simulation and [sup.137]Cs tracer show tillage erosion translocating soil organic carbon, phosphorus, and potassium. Journal of Plant Nutrition and Soil Science 176, 647-654.

Lieskovsky J, Kenderessy P (2014) Modelling the effect of vegetation cover and different tillage practices on soil erosion in a case study in vrable (Slovakia) using WATEM/SEDEM. Land Degradation & Development 25, 288-296. doi:10.1002/ldr.2162

Lindstrom MJ. Nelson WW, Schumacher TE, Lemme GD (1990) Soil movement by tillage as affected by slope. Soil & Tillage Research 17, 255-264. doi:10.1016/0167-1987(90)90040-K

Liu GS (1996) 'Soil physical and chemical analysis and description of soil profiles.' (Chinese Standard Press: Beijing)

McBratney A, Field DJ, Koch A (2014) The dimensions of soil security. Geoderma 213, 203 213. doi:10.1016/j.geoderma.2013.08.013

Montanarella L (2015) Agricultural policy: govern our soils. Nature 528, 32-33. doi: 10.1038/528032a

Nanjing Institute of Soil Science of the Chinese Academy of Sciences (1978) 'Analysis of soil physics and chemistry.' (Shanghai Press of Science and Technology: Shanghai)

Novara A, Gristina L, Saladino SS, Santoro A, Cerda A (2011) Soil erosion assessment on tillage and alternative soil managements in a Sicilian vineyard. Soil & Tillage Research 117, 140-147. doi: 10.1016/j.still. 2011.09.007

Parras-Alcantara L, Lozano-Garcia B (2014) Conventional tillage versus organic farming in relation to soil organic carbon stock in olive groves in Mediterranean rangelands (southern Spain). Solid Earth 5, 299-311. doi:I0.5194/se-5-299-2014

Pereira P, Cerda A, Lopez AJ, Zavala LM, Mataix-Solera J, Arcenegui V, Misiune I, Keesstra S, Novara A (2016) Short-term vegetation recovery after a grassland fire in Lithuania: the effects of fire severity, slope position and aspect. Land Degradation & Development 21, 458-466. doi: 10.1002/ldr.2498

Quine TA, Zhang Y (2002) An investigation of spatial variation in soil erosion, soil properties, and crop production within an agricultural field in Devon, United Kingdom. Journal of Soil and Water Conservation 57, 55-65.

Sadiq M, Hassan G, Mehdi SM, Hussain N, Jamil M (2007) Amelioration of saline-sodic soils with tillage implements and sulfuric acid application. Pedosphere 17, 182-190. doi: 10.1016/S1002-0160(07)60024-1

Schlesinger WH, Pilmanis AM (1998) Plant-soil interact ions in deserts. Biogeochemistry Al, 169-187. doi:10.1023/A:1005939924434

Singh K, Mishra AK., Singh B, Singh RP, Patra DD (2016) Tillage effects on crop yield and physicochemical properties of sodic soils. Land Degradation & Development 27, 223-230. doi: 10.1002/ldr.2266

Smith P, Cotrufo MF, Rumpel C, Paustian K, K ink man PJ, Elliott JA, McDowell R, Griffiths Rl, Asakawa S, Bustamante M, House .11, Sobocka J. Harper R, Pan G, West PC, Gerber JS, Clark JM, Adhya T, Scholes RJ, Scholes MC (2015) Biogeochemical cycles and biodiversity as key drivers of ecosystem services provided by soils. Soil 1, 665-685. doi: 10.5194/soil-l-665-2015

Su ZA, Zhang JH, Nie XJ (2010) Effect of soil erosion on soil properties and crop yields on slopes in the Sichuan Basin, China. Pedosphere 20, 736-746. doi: 10.1016/S1002-0160(10)60064-1

Tan LP, He XD, Wang HT, Zhang N, Gao YB (2008) Analysis of soil water content in relation to accumulation of pedogenic calcium carbonate of Artemisia ordosica community in Tengger Desert. Journal of Desert Research 28, 701-705. [In Chinese]

Tunesi S, Poggi V, Gessa C (1999) Phosphate adsorption and precipitation in calcareous soils: the role of calcium ions in solution and carbonate minerals. Nutrient Cycling in Agroecosystems 53, 219-227. doi: 10.1023/ A: 1009709005147

von Wandruszka R (2006) Phosphorus retention in calcareous soils and the effect of organic matter on its mobility. Geochemical Transactions 7, 6-14. doi:10.1186/1467-4866-7-6

Wang Y, Zhang JH, Zhang ZH (2015) Influences of intensive tillage on water-stable aggregate distribution on a steep hillslope. Soil & Tillage Research 151, 82-92. doi: 10.1016/j.still.2015.03.003

Yumoto M, Ogata T, Matsuoka N, Matsumoto E (2006) Riverbank freeze-thaw erosion along a small mountain stream, Nikko Volcanic area, central Japan. Permafrost and Periglacial Processes 17, 325-339. doi: 10.1002/ppp.569

Zhang JH, Frielinghaus M, Tian G, Lobb DA (2004) Ridge and contour tillage effects on soil erosion from steep hillslopes in the Sichuan Basin, China. Journal of Soil and Water Conservation 59, 277-284.

Zhang JH, Quine TA, Ni SJ, Ge FL (2006) Stocks and dynamics of SOC in relation to soil redistribution by water and tillage erosion. Global Change Biology 12, 1834-1841. doi: 10.1111/j.1365-2486.2006.01206.x

Zhang JH, Nie XJ, Su ZA (2008) Soil profile properties in relation to soil redistribution by intense tillage on a steep hillslope. Soil Science Society of America Journal!!, 1767-1773. doi:10.2136/sssaj2007.0228

Zhang JH, Wang Y, Li FC (2015) Soil organic carbon and nitrogen losses due to soil erosion and cropping in a sloping terrace landscape. Soil Research 53, 87-96. doi: 10.1071/SR14151

Zhao XL, He XD, Xue PP, Zhang N, Wu W, Li R, Ci HC, Xu JJ, Gao YB, Zhao HL (2012) Effects of soil stoichiometry of the CaC[O.sub.3]/available phosphorus ratio on plant density in Artemisia ordosica communities. Chinese Science Bulletin 57, 80-87.

L. Z. Jia (A,B), J. H. Zhang (A,E), Y. Wang (C), Z. H. Zhang (A,B), and B. Li (D)

(A) Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Conservancy, 9 Section 4, South Renmin Road, Chengdu 610041, China.

(B) University of Chinese Academy of Sciences, Yuquan Road, Shijingshan District, Beijing 100049, China.

(C) College of Water Conservancy and Hydropower Engineering, Sichuan Agricultural University, No. 46, Xinkang Road, Yucheng District, Ya'an 625014, China.

(D) China National Tobacco Corporation Sichuan Branch, No. 936, Century City Road, Chengdu 610000, China.

(E) Corresponding author. Email: zjh@imde.ac.cn

Caption: Fig. 1. Soil movement direction for (a) downslope tillage and consecutive tillage (with five and 20 tills) and (b) upslope tillage.

Caption: Fig. 2. Distributions of CaC[O.sub.3] concentrations (mean values) in the topsoil layer (0-20 cm) along the transects of the toposequence following downslope tillage, consecutive tillage (with five and 20 tills) and upslope tillage.

Caption: Fig. 3. (a) CaC[O.sub.3], (b) total phosphorus (TP) and (c) available phosphorus (AP) inventories (absolute values) at different landscape positions following downslope tillage, consecutive tillage (with five and 20 tills) and upslope tillage.

Caption: Fig. 4. Relationships between CaC[O.sub.3] and available phosphorus (AP) under different tillage conditions: (a) normal tillage and (b) consecutive tillage (with five and 20 tills).
Table 1. Landscape elements and properties
of parent material for the two slopes

Where appropriate, data are given as the mean [+ or -] s.e.m.
Within columns, values with different letters differ
significantly (P<0.05. Fisher's least significant
difference)

          Slope    Slope      Tillage method
          length   gradient   since 1982
          (m)      (%)

Slope 1   21       17.8       Downslope tillage
Slope 2   21       19.6       Upslope tillage

          Consecutive      Bulk density               pH
          tillage         (g cm.sup.-3])

Slope 1   5 tills,      1.76 [+ or -] 0.05a   8.36 [+ or -] 0.06a
          20 tills

Slope 2   No            1.75 [+ or -] 0.07a   8.54 [+ or -] 0.02a

                                   Soil particle size
                                      fraction (%)
               CaC[O.sub.3]
             (g [kg.sup.-1])              Sand
                                       (2-0.02 mm)

Slope 1   125.77 [+ or -] 23.43a   46.4 [+ or -] 12.7a

Slope 2   145.80 [+ or -] 52.73a   41.8 [+ or -] 13.7a

              Soil particle size fraction (%)

                 Silt                  Clay
            (0.02 0.002 mm)        (<0.002 mm)

Slope 1   44.5 [+ or -] 2.8a    9.1 [+ or -] 2.1a

Slope 2   46.9 [+ or -] 13.3a   11.3 [+ or -] 2.5a

Table 2. Coefficients of the regression equations
of CaC[O.sub.3] depth distribution in the soil profile

                      Parameter    Summit    Shoulder slope

Normal tillage
  Downslope tillage       a         74.52            73.65
                          b         0.023           0.0196
                          r        0.81 *          0.93 **
                          n             5                7
  Upslope tillage         a        119.07           118.27
                          b        0.0052           0.0024
                          r       0.81 **          0.71 **
                          n             7               12
Consecutive tillage
  5 tills                 a        112.84           103.60
                          b        0.0031           0.0112
                          r          0.46           0.96 *
                          n             4                5
  20 tills                a        109.79           112.24
                          b         0.007           0.0102
                          r          0.68             0.81
                          n             5                4

                                  Landscape position

                      Parameter   Midslope    Backslope

Normal tillage
  Downslope tillage       a           76.3       81.45
                          b         0.0177      0.0027
                          r        0.85 **        0.22
                          n              9           8
  Upslope tillage         a         116.37      110.17
                          b          0.004      0.0062
                          r        0.81 **     0.93 **
                          n             10           8
Consecutive tillage
  5 tills                 a         101.18       100.7
                          b         0.0123      0.0084
                          r         0.91 *        0.93
                          n              5           4
  20 tills                a          94.97       97.46
                          b         0.0147      0.0139
                          r           0.83        0.80
                          n              5           5

                      Parameter   Toeslope     Mean

Normal tillage
  Downslope tillage       a         78.53      76.89
                          b        0.0055     0.0137
                          r       0.95 **     0.75 *
                          n             8          7
  Upslope tillage         a        115.67     115.91
                          b        0.0038     0.0043
                          r       0.95 **    0.84 **
                          n             8          9
Consecutive tillage
  5 tills                 a        102.79     104.22
                          b        0.0021     0.0074
                          r          0.63       0.78
                          n             9          5
  20 tills                a        113.49     105.59
                          b        0.0009     0.0093
                          r          0.21       0.67
                          n             9          6

** P < 0.01, * P < 0.05. a and b, coefficients describing
the profile shape (in g [kg.sup.-1]) based on the regression
equation [mathematical expression not reproducible] where D
is soil depth; r, correlation coefficient; n. number of
depth subsamples

Table 3. Distribution of total (TP) and available phosphorus (AP)
concentrations in the topsoil (0-20 cm) and subsoil (20-40 cm)
layers Data are the mean [+ or -] s.e.m. TP and AP concentrations
between the topsoil and subsoil layers were compared using
t-tests. ** P<0.01, * P<0.05

                                    Normal tillage

                                   Downslope tillage

Landscape   Soil layer   TP (g [kg.sup.-1])   AP (mg [kg.sup.-1])
position

Summit      Topsoil      0.67 [+ or -] 0.06   4.77 [+ or -] 1.05
            Subsoil              --                   --
              t-value            --                   --
Shoulder    Topsoil      0.71 [+ or -] 0.12   5.64 [+ or -] 2.40
  slope     Subsoil      0.55 [+ or -] 0.02   1.39 [+ or -] 0.51
              t-value           2.29                2.95 *
Midslope    Topsoil      0.79 [+ or -] 0.10   5.31 [+ or -] 1.63
            Subsoil      0.52 [+ or -] 0.02   0.97 [+ or -] 0.25
              t-value           5.33                5.27 *
Backslope   Topsoil      0.83 [+ or -] 0.07   6.44 [+ or -] 1.92
            Subsoil      0.55 [+ or -] 0.01   1.39 [+ or -] 0.76
              t-value         7.32 **               4.88 **
Toeslope    Topsoil      0.82 [+ or -] 0.03   6.19 [+ or -] 1.73
            Subsoil      0.58 [+ or -] 0.09   2.02 [+ or -] 0.67
              t-value         5.32 **               4.48 **

                                     Normal tillage

                                     Upslope tillage

Landscape   Soil layer   TP (g [kg.sup.-1])   AP (mg [kg.sup.-1])
position

Summit      Topsoil      0.87 [+ or -] 0.07   8.40 [+ or -] 1.12
            Subsoil      0.56 [+ or -] 0.03   1.35 [+ or -] 0.60
              t-value         7.22 **               9.76 **
Shoulder    Topsoil      0.83 [+ or -] 0.11   6.21 [+ or -] 2.13
  slope     Subsoil      0.57 [+ or -] 0.01   1.19 [+ or -] 0.33
              t-value         4.62 **               4.66 *
Midslope    Topsoil      0.93 [+ or -] 1.49   6.44 [+ or -] 2.61
            Subsoil      0.59 [+ or -] 0.03   1.25 [+ or -] 0.20
              t-value         4.51 **               3.97 *
Backslope   Topsoil      0.95 [+ or -] 0.15   6.63 [+ or -] 2.86
            Subsoil      0.60 [+ or -] 0.02   1.49 [+ or -] 0.45
              t-value         4.54 **               3.54 *
Toeslope    Topsoil      1.10 [+ or -] 0.05   8.47 [+ or -] 1.39
            Subsoil      0.70 [+ or -] 0.04   2.74 [+ or -] 0.60
              t-value         12.43 **              7.60 **

                                  Consecutive tillage

                                        5 tills

Landscape   Soil layer   TP (g [kg.sup.-1])   AP (mg [kg.sup.-1])
position

Summit      Topsoil      0.67 [+ or -] 0.04   4.41 [+ or -] 1.24
            Subsoil              --                   --
              t-value            --                   --
Shoulder    Topsoil      0.66 [+ or -] 0.11   3.71 [+ or -] 1.61
  slope     Subsoil              --                   --
              t-value            --                   --
Midslope    Topsoil      0.70 [+ or -] 0.10   3.84 [+ or -] 1.10
            Subsoil              --                   --
              t-value            --                   --
Backslope   Topsoil      0.80 [+ or -] 0.14   4.00 [+ or -] 1.82
            Subsoil              --                   --
              t-value            --                   --
Toeslope    Topsoil      0.84 [+ or -] 0.07   4.77 [+ or -] 0.39
            Subsoil      0.67 [+ or -] 0.08   2.05 [+ or -] 0.78
              t-value          3.21 *               6.17 **

                                  Consecutive tillage

                                        20 tills

Landscape   Soil layer   TP (g [kg.sup.-1])   AP (mg [kg.sup.-1])
position

Summit      Topsoil      0.68 [+ or -] 0.04   3.76 [+ or -] 1.07
            Subsoil              --                   --
              t-value            --                   --
Shoulder    Topsoil      0.71 [+ or -] 0.07   3.81 [+ or -] 1.01
  slope     Subsoil              --                   --
              t-value            --                   --
Midslope    Topsoil      0.77 [+ or -] 0.17   3.72 [+ or -] 1.76
            Subsoil              --                   --
              t-value            --                   --
Backslope   Topsoil      0.77 [+ or -] 0.08   4.04 [+ or -] 1.52
            Subsoil              --                   --
              t-value            --                   --
Toeslope    Topsoil      0.87 [+ or -] 0.01   4.70 [+ or -] 0.15
            Subsoil      0.73 [+ or -] 0.09   4.77 [+ or -] 0.65
              t-value          3.13 *                -0.21

Table 4. Comparison of differences in total and available phosphorus
concentrations in the topsoil layer between consccutivc tillage and
downslope tillage (paired-samples f-test)

                    Total             Available
                  phosphorus          phosphorus
Consecutive
tillage        t-value   P-value   t-value   P-value

5 tills         1.56      0.19      4.43      0.01
20 tills        0.22      0.84      7.32      0.002

Table 5. Ratio of CaC[O.sub.3] to available phosphorus (AP)
in the topsoil layer (0-20 cm) along the transect

Data are the mean [+ or -] s.e.m. Average ([+ or -] s.e.m.)
refers to the mean values obtained for the five landscape
positions. CaC[O.sub.3]/AP ratio of all landscape positions
for a given tillage treatment. Within columns, values with
different letters differ significantly (P<0.05, Duncan's
multiple range test)

Tillage operation             Summit             Shoulder slope

Downslope tillage      19.03 [+ or -] 5.68    20.18 [+ or -] 14.89
Upslope tillage        15.15 [+ or -] 3.09    21.45 [+ or -] 10.02
Consecutive tillage
  5 tills              29.68 [+ or -] 3.64    38.37 [+ or -] 20.52
  20 tills             34.71 [+ or -] 15.45   36.14 [+ or -] 15.28

Tillage operation            Midslope               Backslope

Downslope tillage      18.39 [+ or -] 9.23    14.73 [+ or -] 5.54
Upslope tillage        22.73 [+ or -] 14.57   20.91 [+ or -] 11.03
Consecutive tillage
  5 tills              33.39 [+ or -] 14.24   42.37 [+ or -] 40.40
  20 tills             39.82 [+ or -] 30.92   28.57 [+ or -] 7.65

Tillage operation            Toeslope                Average

Downslope tillage      14.59 [+ or -] 5.86    17.38 [+ or -] 8.24a
Upslope tillage        14.50 [+ or -] 2.62    18.95 [+ or -] 8.23a
Consecutive tillage
  5 tills              22.19 [+ or -] 1.68    33.20 [+ or -] 16.10b
  20 tills             24.84 [+ or -] 0.31    32.85 [+ or -] 13.92b
COPYRIGHT 2017 CSIRO Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2017 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Jia, L.Z.; Zhang, J.H.; Wang, Y.; Zhang, Z.H.; Li, B.
Publication:Soil Research
Article Type:Report
Date:Oct 1, 2017
Words:8778
Previous Article:A survey of total and dissolved organic carbon in alkaline soils of southern Australia.
Next Article:Quantifying above and belowground biomass carbon inputs for sugar-cane production in Brazil.
Topics:

Terms of use | Privacy policy | Copyright © 2019 Farlex, Inc. | Feedback | For webmasters