Effects of irrigation and tillage on soil organic carbon and nutrients in mining-induced subsided cropland.
Mining subsidence is a global problem. In China and the United States alone, subsided land now amounts to more than 700 000 and 44030 [km.sup.2] respectively (USGS 2000; Wang et al. 2016;). Mining subsidence may result in serious environmental problems, such as soil degradation and crop yield reduction (Bell et al. 2000; Shepley et al. 2008; Thompson et al. 2011; Darmody et al. 2014; Jing et al. 2018). In recent years, land reclamation has been widely practiced in coal mine districts to reduce soil degradation (Barnhisel and Hower 1997; Akala and Lai 2000). However, there are still considerable proportions of subsided lands not subjected to reclamation in economically undeveloped coal-producing countries, such as China and India (Bian et al. 2010). Thus, determining a method to sufficiently manage soils in unrestored subsidence lands is essential.
Soil organic carbon (SOC), total nitrogen (N) and total phosphorus (P) are three critical indicators of soil quality and previous studies reported severe depletion and increased variations in all three after land subsidence (Tripathi et al. 2009; Shi et al. 2017; Wang et al. 2015). For example, Tripathi et al. (2009) reported that total N and P concentrations respectively declined by 17% and 34% in subsidence-formed slopes compared with undisturbed land in the Singareni coalfields, India. In previous studies, researchers often presumed that soil erosion was responsible for the dynamics of these soil properties in subsidence-formed slopes. Although it has been well documented that mining subsidence leads to formation of ground cracks and sloped terrain, which trigger soil erosion (Shepley et al. 2008; James and Mossa 2013), the mechanism of subsidence-driven soil changes has not been identified and direct evidence on the impact of soil erosion is lacking.
In fact, the mechanism of subsidence-driven soil changes is complex due to field management practices, such as irrigation and tillage, performed on subsidence-formed slopes in coal mine districts. It is well documented that both irrigation and tillage may cause soil erosion on natural slopes (Koluvek et al. 1993; Fernandez-Gomez et al. 2004; Van Oost et al. 2006; Li et al. 2008) and resultant soil degradation (Heckrath et al. 2005; Zhang et al. 2012). For example, Zhang et al. (2012) reported that tillage not only increases the spatial variability in SOC but also accelerates its depletion when combined with water erosion within the same slope. For subsidence-formed slopes in mine districts, however, the effects of irrigation and tillage on soil quality are poorly understood. Therefore, the effects of irrigation and tillage on soil erosion and SOC, and nutrients (e.g. total N and P) in mining subsidence landscapes must be determined to provide targeted guidance for management of subsided land.
The objectives of this study were to: (1) investigate the SOC, total N and total P stocks, and soil erosion in cropland of the Jiaozuo coal mine district in China, 15-18 years after subsidence; (2) examine relationships between the distribution of these soil properties and soil erosion along the slope under irrigation and tillage; and (3) explore the effects of irrigation and tillage on the dynamics of SOC, total N and total P in subsided cropland.
Materials and methods
The study area lies in the Jiaozuo coal mining district (35[degrees]16'N, 113[degrees] 17'E), Henan Province, China (Fig. 1). The district is one of the oldest coal production bases in China, and has been subjected to underground mining for 120 years. Landforms in this district include low mountains, hills and plains (alluvial-proluvial plains), with elevations in the range of 88-299 m a.s.l. Coal and crops are produced mainly on the plains of this district. Large areas of flat cropland have been damaged by mining subsidence and turned into sloped cropland. The district is characterised by a temperate continental monsoon climate (Cheng et al. 2014); average annual temperature is 15[degrees]C, average annual precipitation is 600 mm and annual mean evaporation is 1850 mm. Approximately 70% of annual precipitation occurs during June-September. Soils in this district are classified as a calcareous cinnamon soil (CaC[O.sub.3], 180 g [kg.sup.-1]) in the Chinese soil taxonomy (Gong et al. 1999) or as Calcisols in the FAO soil taxonomy (FAO 1998). Crop rotation mainly involves wheat (Triticum aestivum L.) and maize (lea mays L.).
In this district, mining subsidence has resulted in serious damage to original irrigation ditches. Most damaged ditches were actively repaired by farmers after land subsidence. As a result, two water management practices are observed in subsided sites of this district: irrigation and non-irrigation (i.e. rainfed). Moreover, two tillage methods are conducted in subsided sites: tillage along a longitudinal slope-direction ([T.sub.L]), in which the subsidence gradient is greater; and tillage along a transverse slope-direction ([T.sub.T]), in which the subsidence gradient is smaller. Practicing [T.sub.L] is relatively easy on subsided cropland because there is less lateral inclination of tractors compared to [T.sub.T] Therefore, [T.sub.L] is more widely adopted by local farmers. Overall, there are four types of subsided cropland with different soil management practices in this study area: rainfed under [T.sub.L] (R[T.sub.L]-cropland), rainfed under [T.sub.T] (R[T.sub.T]-cropland), irrigated under [T.sub.L] (I[T.sub.L]-cropland) and irrigated under [T.sub.T] (I[T.sub.T]-cropland).
Soil sampling and laboratory analysis
In this study, R[T.sub.L]-, R[T.sub.T]-, I[T.sub.L]--and I[T.sub.T]-cropland were selected from three subsided sites (A, B and C; Fig. 1) spaced ~4 km apart. Both R[T.sub.T]--and I[T.sub.L]-cropland were located in site A and had areas of 2500 and 3000 [m.sup.2] respectively. The R[T.sub.L]-(2880 [m.sup.2]) and I[T.sub.T]-cropland (1500 [m.sup.2]) were located in sites B and C respectively. All selected subsided cropland was formed over a period of ~ 15-18 years after surface subsidence, and had an average slope gradient of <4[degrees] and horizontal lengths of 50-100 m (Table 1). There were no visible ground cracks on the subsided croplands because of small thickness of the extracted coal seams and thick unconsolidated layers above the coal seam in these sites (Dai et al. 2011). Flat cropland (1800 [m.sup.2]) in an unmined site near the three subsided sites was chosen as the control cropland.
Before 2008, subsided and control cropland were tilled twice per year with a mouldboard plough and rake. Soil was tilled to a depth of 0.2 m after wheat (first operation) and maize (second operation) harvests of each year, with two passes per tillage operation (one each for the plough and the rake). After 2008, subsided and control cropland were tilled once per year with a rotary tiller. Soil was tilled to a depth of 0.2 m after maize harvest, with only one pass. For irrigated subsided (I[T.sub.L] and I[T.sub.T]) and control cropland, irrigation was by flood irrigation with frequency and wetting depth generally 5-6 times per year and 0.5-0.8 m respectively.
All cropland soils had a silty loam texture (10-37, 55-76 and 8-14 g 100 [g.sup.-1] sand, silt and clay respectively) with pH mean of 8.13 and range 8.10-8.21 (Table 1). Compound fertiliser (165, 165 and 105 kg [ha.sup.-1] y of N, [P.sub.2][O.sub.5] and [K.sub.2]O respectively) and urea (345 kg N [ha.sup.-1] [y.sup.-1]) was uniformly applied to cropland. Management of crop (wheat-maize) residues in cropland consisted of directly burning straw in fields before 2009 and returning straw to fields thereafter. Measured soil water contents in irrigated subsided cropland were 7.5 [+ or -] 0.9 g 100 [g.sup.-1] during November-April and 9.5 [+ or -] 1.1 g 100 [g.sup.-1] during May-October, and were higher than for rainfed subsided cropland with corresponding contents of 5.8 [+ or -] 1.1 and 6.7 [+ or -] 1.1 g 100 [g.sup.-1].
The landform of each subsided cropland was measured using a Real-time Kinematic Global Positioning System (Zhonghaida V30, Guangzhou, China). Within each subsided cropland, five plots of fields were selected as replications to conduct soil sampling (Fig. 2). For each field, bulked soil samples from its top, upper, lower and foot positions were collected and divided at depth intervals of 0-0.2 m (tillage layer) and 0.2-0.3 m. There were 20 samples for each subsided cropland, with five replicated samples at each slope position. Replicate soil cores (n = 12) were also collected from control cropland. Sampling took place in October 2014, after maize had been cut.
Samples from the 0-0.2 m depth were analysed to determine the SOC, total N, total P, soil particle size fractions and pH. Samples from 0-0.2 m and 0.2-0.3 m depths were analysed for [sup.137]Cs. Prior to experimental analysis, soil samples were air-dried, crushed and sieved through a 2-mm mesh screen to remove coarse materials (e.g. roots, coarse plant debris and gravel). The SOC was determined via wet oxidation with potassium dichromate (Liu 1996), total N by classical Kjeldahl digestion (McKenzie 1994) and total P by sodium hydroxide digestion (Dick and Tabatabai 1977). Soil particle size fractions were determined with the pipette method following hydrogen peroxide treatment to destroy organic matter, and suspensions with Na-hexametaphosphate (Liu 1996). The soil pH of suspensions with a soil: water ratio of 1:2.5 was determined with an automatic acid-base titrator (Metrohm 702, Switzerland).
Air-dried soil samples for [sup.137]Cs determination were packed into 320-[cm.sup.3] PVC beakers, and [sup.137]Cs activity measured using a hyperpure lithium-drifted germanium detector coupled with a Nuclear Data 6700 multichannel g-ray spectrophotometer (Ortec, Oak Ridge, TN, USA) (Zhang et al. 2006). Contents of [sup.137]Cs were determined at 662 keV, and the range of count time for each sample was 40 000-60000 s. Contents of [sup.137]Cs were originally expressed on a unit mass basis (Bq [kg.sup.-1]), then converted and expressed on an area basis (Bq [m.sup.-2]).
The measured contents of SOC, total N and total P were initially expressed on a unit mass basis (g [kg.sup.-1]). However, due to highly variable gravel concentrations (range 3-8 g 100 [g.sup.-1]) in soils, direct comparisons of SOC, total N and total P concentrations (g [kg.sup.-1]) among different subsided cropland would not accurately reflect the real difference in these stocks. Thus, we reported these stocks (0-0.2 m) on an area basis. The stock per unit area (kg [m.sup.-2]) for SOC, total N and total P was obtained by multiplying the original mass concentration (g [kg.sup.-1]) by the total net weight of oven-dried soil. The total net weight of oven-dried soil was obtained using the total weight of bulked core sample minus the weight of gravel. This data conversion was also adopted for [sup.137]Cs (0-0.2 and 0.2-0.3 m).
Statistical analyses were conducted using SPSS 19.0 software (SPSS, Chicago, IL, USA). The two-independent sample nonparametric tests were used to compare differences in SOC, total N and total P between subsided and control cropland. The K-independent samples nonparametric tests were used to compare differences in these parameters among slope positions of subsided cropland. Differences between groups at P < 0.05 were considered significant.
Soil erosion as affected by irrigation and tillage
Comparison of [sup.137]Cs data with the reference inventory (1197 [+ or -] 71 Bq [m.sup.-2] measured in control cropland) confirmed that the top, upper and lower positions of the four subsided croplands were erosional, whereas foot positions were relatively depositional (Fig. 3a). The most eroded soils were in the upper and lower positions of each subsided cropland. The most severe erosion was in the R[T.sub.L]-cropland but there was only slight erosion in the I[T.sub.T]-cropland (Fig. 3a). An obvious increase in clay particles (<0.002 mm) was observed from the lower and upper positions to the foot positions in the I[T.sub.L]-, R[T.sub.L]-and R[T.sub.T]-cropland (Fig. 3b), suggesting selective transport of fine soil particles by water erosion.
The [sup.137]Cs data also confirmed more severe soil erosion in rainfed compared with irrigated cropland (926 [+ or -] 66 and 1086 [+ or -] 65 Bq [m.sup.-2] for erosional sites respectively) and in cropland under [T.sub.L] compared with [T.sub.T] (977 [+ or -] 15 and 1035 [+ or -] 12 Bq m 2 for erosional sites respectively). Both [T.sub.L] and being rainfed induced more severe soil erosion in subsided cropland, compared with [T.sub.T] and irrigation.
SOC and nutrient stocks as affected by irrigation and tillage
Compared with control, the I[T.sub.L]-, R[T.sub.L]--and R[T.sub.T]-cropland had much lower SOC concentrations (P < 0.05), but I[T.sub.T]-cropland had greater SOC (P > 0.05) (Fig. 4a). The concentrations of total N and total P significantly decreased in the R[T.sub.L]-cropland compared with control (P < 0.05) (Fig. 4b and c). Overall, the values of SOC, total N and total P decreased in R[T.sub.L]--but increased in I[T.sub.T]-cropland compared with control.
There were lower means of SOC, total N and total P concentrations in rainfed compared with irrigated cropland (P < 0.05), and in cropland under [T.sub.L] compared with under [T.sub.T] (P < 0.05). For both irrigated cropland and those under [T.sub.T], mean values of SOC, total N and total P were similar to those of control cropland (P > 0.05). These findings suggest that both [T.sub.L] and being rainfed after surface subsidence induced obvious depletions in SOC, N and P stocks in subsided cropland, whereas [T.sub.T] and irrigation had no marked impact on the three.
Effects of irrigation and tillage on SOC and nutrient distribution along the slope
The distribution of SOC and nutrients on the four types of cropland differed (Fig. 5). There were lower values of SOC, total N and total P in lower positions in the I[T.sub.L]--and upper positions in the R[T.sub.T]-cropland (P < 0.05). In the R[T.sub.L]-cropland, lower values of SOC and total N were observed in upper positions (P < 0.05). However, no significant within-field variations of the three parameters were observed in I[T.sub.T]-cropland (P > 0.05). These findings suggest that integrated irrigation-[T.sub.T] management in subsided cropland obviously reduced within-field variations in the SOC, total N and total P concentrations. It is noteworthy that the patterns of the three parameters were similar to the [sup.137]Cs distribution in subsided cropland.
There were different changes in SOC and nutrient concentrations in subsided cropland due to the different soil management practices. After cropland subsidence, both [T.sub.L] and being rainfed induced obvious depletions in SOC, N and P stocks, which can be attributed to stronger soil erosion under [T.sub.L] and rainfed conditions (Fig. 3a). Under both [T.sub.T] and irrigation, there were no significant depletions in these stocks in subsided cropland, likely due to weaker soil erosion (Fig. 3a). The distribution patterns of [sup.137]Cs and soil clay on slopes are useful tools for evaluating soil erosion by water (Kjaergaard et al. 2004; Zhang et al. 2006; Nguetnkam and Dultz 2011; Nie et al. 2013; Laura et al. 2016). Our observation of the distribution of [sup.137]Cs and clay particles suggests that water erosion was a major process of soil redistribution on the studied subsided cropland. Erosion by tillage is also considered an important soil redistribution process in sloped farmland (Van Oost et al. 2006). Soil loss by tillage erosion occurs mainly at the top slope positions (Zhang et al. 2012, 2015; Li et al. 2013; Nie et al. 2016). In our study, eroded soils were observed at the top positions of subsided croplands, suggesting tillage erosion. Similar distribution patterns of SOC, total N and total P to [sup.137]Cs indicate that the distribution of SOC, total N and total P was associated with soil redistribution by erosion in the subsided cropland.
In subsided cropland, irrigation and tillage affected variations in SOC, total N and total P concentrations with slope position. Under irrigation, subsided cropland exhibited humid soil conditions for most of the year (see Material and Methods). Humid soil conditions can increase cohesion, thereby weakening the within-field SOC and nutrient redistribution induced by erosion (Truman et al. 1990; Wuddivira et al. 2009). Compared with [T.sub.L], [T.sub.T] induced smaller within-field variations of SOC, total N and total P concentrations in irrigated cropland due to small subsidence gradients. The impact of subsidence gradient on variations of soil nutrient concentrations within subsided land was reported by Meng et al. (2009). As a result, integrated irrigation-[T.sub.T] management in subsided cropland obviously reduced within-field variations in the SOC, total N and total P concentrations. In summary, our study suggests that irrigation and tillage affected soil erosion and thereby the dynamics of SOC, total N and total P on subsided cropland, which enhances our understanding of subsidence-driven soil changes.
The presence of severe erosion, depleted stocks and notable within-field variations of SOC and nutrients in the R[T.sub.L]-cropland suggests that [T.sub.L] combined with being rainfed was the most detrimental to soil conservation in subsided cropland. In contrast, [T.sub.T] combined with irrigation promoted soil conservation in subsided cropland. These findings provide a new solution for agricultural land consolidation of mining areas, i.e. improving soil management may replace traditional engineering reclamation of subsided cropland, such as filling with soils imported from other places and filling with coal gangue or fly ash. At present, the traditional process of reclaiming subsided lands in mining areas is not popular due to high cost and substantial use of filling material. Accordingly, our findings may provide a theoretical and technical reference for agricultural land rehabilitation in mining subsidence areas.
The results showed that irrigation and tillage had pronounced effects on the dynamics of SOC, N and P in subsided cropland at 15-18 years after surface subsidence in the Jiaozuo mining district, China. Tillage along a longitudinal slope-direction and being rainfed in subsided croplands induced severe soil erosion and resultant depletions in SOC, N and P stocks. In contrast, both tillage along a transverse slope-direction and irrigation caused slight soil erosion and showed no marked impact on SOC, N and P stocks. The distribution of SOC, total N and total P was associated with soil redistribution by erosion in subsided cropland. Combining small subsidence gradients with irrigation is a good solution for soil conservation and is a possible alternative to traditional engineering reclamation of subsided cropland.
Conflicts of interest
The authors declared no conflict of interest.
This work was supported by the Plan for Scientific Innovation Team of Henan Provincial Universities (18IRTSTHN008) and the Key Scientific Research Projects of Institutions of Higher Education, Henan Province (19A170003). We express our gratitude to Dr Xu Keke and his students for assistance in topographic surveying.
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X. J. Nie (ID) (A,B), H. B. Zhang (A), and S. Y. Li (A)
(A) School of Surveying and Land Information Engineering, Henan Polytechnic University, Jiaozuo City 454000, China.
(B) Corresponding author. Email: firstname.lastname@example.org
Caption: Fig. 1. Location of selected subsidence sites and the unmined site in the study area.
Caption: Fig. 2. Sampling points (*) on the four types of subsided cropland.
Caption: Fig. 3. Distribution of [sup.137]Cs and clay contents in subsided cropland: (a) [sup.137]Cs and (b) soil clay percentage. Asterisks indicate significance differences (P < 0.05) among slope positions. The error bars are standard deviations.
Caption: Fig. 4. Differences in SOC, total N and total P concentrations between subsided and control cropland. Asterisk indicates significance difference (P < 0.05) between subsided and control cropland. The error bars are standard deviations.
Caption: Fig. 5. Distribution of SOC, total N and total P concentrations in the four types of subsided cropland. Asterisk indicates significance difference (P < 0.05) among slope positions. The error bars are standard deviations.
Table 1. Selected landscape elements and soil properties of control and different subsided cropland R, rainfed; I, irrigated; TL, tilled along a longitudinal slope-direction; [T.sub.T], tilled along a transverse slope-direction Cropland Age since Slope Horizontal subsidence gradient length (years) ([omicron]) (m) R[T.sub.L] 15 3.7 96 R[T.sub.L] 17 2.2 50 I[T.sub.L] 17 3.5 100 I[T.sub.L] IB 2.1 50 Control Cropland Soil texture Soil pH (% sand: silt: clay) R[T.sub.L] silty loam (37:55: 8) 8.21 R[T.sub.L] silty loam (16:74: 10) 8.12 I[T.sub.L] silty loam (13:76: 11) 8.12 I[T.sub.L] silty loam (17:72: 11) 8.10 Control silty loam (10:76: 14) 8.11
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|Author:||Nie, X.J.; Zhang, H.B.; Li, S.Y.|
|Date:||Aug 1, 2019|
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