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Effects of crop rotation on properties of a Vietnam clay soil under rice-based cropping systems in small-scale farmers' fields.

Introduction

Rice is one of the major crops in the world and the most important crop in the Mekong Delta. Continuous irrigated rice cultivation with three rice crops grown each year in the same field is the dominant cropping system in this area. With the introduction of short-duration varieties and adoption of intensive monocultivation, rice has become the main agricultural commodity and has the highest production of agricultural crops grown in the area. However, yields of rice cultivated on alluvial clay soil in monocultures with three harvests per year are declining in the Mekong Delta, even though farmers yearly add more fertiliser (Khoa 2002; Linh et al. 2014). One of the major constraints on rice yield is soil compaction, which leads to reduced root growth (Linh et al. 2015). Continued cultivation of rice leads to declining soil fertility (Dwivedi et al. 2001; Guong et al. 2010), as well as low soil productivity (Mandai et al. 2014). Rice fields in the Mekong delta arc also subjected, in each rice season, to conventional puddling with repeated ploughing and field levelling to reduce water loss through percolation and help weed control (Farooq et al. 2011). However, long-term rice monoculture with puddling to the same shallow depth may result in the establishment of a compacted zone just below the ploughed layer.

Previous studies in the Mekong Delta have identified soil compaction as an obstacle to sustainable production (Khoa 2002). It was observed that in areas of intensive rice cultivation, soil compaction started at the 20-40 cm depth, with the thickness of the compacted layer varying from 35 to 50 cm. This compaction was physically apparent in terms of high soil density, low soil porosity (<50%) and low hydraulic conductivity. Chemical soil degradation also occurred, as evidenced by nutrient depiction and acidification (Khoa 2002).

Many agricultural practices are known to affect soil properties, including crop type (Scott et al. 1994), cultivation methods (Gantzer and Blake 1978) and application of organic residues (Anderson et al. 1990; Ekwue 1990). Effects of cropping systems on soil physical properties are often related to changes in soil organic matter (SOM; Ghidey and Alberts 1997; Haynes 2000). Studying the effects of cropping systems and management practices on soil properties provides essential information for assessing their sustainability and environmental effects (Ishaq and Lai 2002).

Interviews with farmers have indicated that rice yield in the Mekong Delta was lower in rice monocultures with three rice crops per year (RRR) than in rotation systems with rice being rotated with one (RUR) or two (RUU) upland crops per year (Linh et al. 2014). In addition, the cost : benefit ratio was lower for rice monocultures compared with systems where rice was rotated with upland crops (Nguyen 2010; Linh et al. 2014). Field experiments in the Mekong Delta have shown that rice monocultures result in physical and chemical deterioration of soils, thus rendering them less productive, whereas rotating rice with upland crops results in substantial soil improvement (Linh et al. 2015). The aim of the present study was to compare the properties of alluvial heavy clay soils under different cropping systems in small-scale farmers' fields in the Mekong Delta. This study was performed in collaboration with several farmers who recently shifted from rice monocultures to the newly introduced cropping systems in which at least one upland crop is grown per year. The four cropping systems evaluated in the present study were a conventional intensive rice monoculture with three rice crops per year (RRR) and alternative cropping systems consisting of upland crops cultivated in the summer-autumn season in rotation with two rice crops in the winter-spring and autumn-winter seasons (RUR), upland crops cultivated in the summer-autumn and autumn-winter seasons in rotation with rice in the winter-spring season (RUU) and upland crop monocultures with three upland crops per year (UUU). Because RRR is the traditional farming practice in the area, it is considered as the reference against which the results obtained from the three alternative cropping systems are compared.

Materials and methods

Study area

The study was performed in the Cai Lay district, Tien Giang province (10[degrees]21'-10[degrees]22'N, l06[degrees]04'-106[degrees]07'E) in the Vietnamese Mekong Delta. The study area is a plain with flat topography (<1% slope) and an elevation of 2m above sea level. The mean annual precipitation is 1500 mm [year.sup.-1] and mean monthly temperature is 26-31[degrees]C. The soil in the district is described as poorly drained, brown and fine textured (>650g [kg.sup.-1] clay), and the soil texture is classified as clay (Soil Survey Staff 1998). The soil has been classified as Gleyic Fluvisol (FAO 2014).

Upland crops, including cucumber (Cucumis sativus), tomato (Solanum lycopersicum), maize (Zea mays), chili pepper (Capsicum annuum), okra (Abelmoschus esculentus), onion (Allium fistulosum), mung bean (Vigna radiara) and sesame (Sesamum indicum), are normally cultivated on raised beds approximately 20cm high above the field surface, with the soil dug by hoe to a depth of 20-30cm to create the raised beds and furrows. Prior to sowing of upland crops, farmers clear any remaining crop residue and post-season weeds. Rice is planted with a conventional flooding system on flat fields after ploughing, puddling and levelling the top 10cm under wet conditions with a small tractor before sowing.

Soil sampling

A farm household sampling was undertaken in four villages (Long Khanh, Cam Son, Binh Phu and Long l ien). All villages included four types of crop cultivation, namely RRR. UUU, RUU and RUR (Table 1).

In all, 40 fields were randomly chosen for soil sampling, with 10 fields sampled for each cropping system. All fields had the same cropping history in that they had been cultivated as RRR for more than 30 years before the present study. At the time of soil sampling, RUR, RUU and UUU fields had been under that cropping system for 5 years.

Undisturbed soil samples were taken with l00-[cm.sup.3] rings following the core method at depths of 0-10, 10-20 and 20-30 cm (Dirksen 1999). A push press ring was placed on a sharpened steel sample ring and the latter was gently driven into the soil by hand. The samples were then dug out, the soil was carefully removed without breaking it away from the inside of the ring and the cores were trimmed. Core samples were used for the determination of soil water retention and bulk density (BD). Disturbed soil samples at the same three depths were taken to measure soil texture, soil particle density (PD), aggregate stability and chemical properties; each disturbed sample consisted of 10 cores taken at different locations within one field. All soil samples were taken in the dry season after the rice harvest (RRR, RUR and RUU) or harvest of upland crops (UUU) when the soil moisture was near field capacity. In rice fields (RRR, RUR, RUU), both disturbed and undisturbed samples were taken on the flat, but for upland crop fields the soil samples were taken in the bed where the plants were cultivated. The disturbed soil samples were brought to the laboratory, air dried and ground to pass through 0.5- and 2-mm sieves before being analysed. Soil penetration resistance (SPR) was measured with an electronic penetrometer (Eijkelkamp Agrisearch Equipment) directly in the field up to a depth of 30cm at 1-cm intervals, and soil moisture content was determined in disturbed samples.

Soil physical analysis

Soil texture was determined by the Robinson pipette method (Gee and Bander 1986). The sand (0.05-2 mm), silt (0.002-0.05 mm) and clay (<0.002mm) fractiFons of the soil sample were determined and the USDA/Soil Taxonomy texture triangle (Soil Survey Staff 1998) was used to classify soil texture. Dry BD was calculated as dried soil weight of undisturbed soil samples (oven dried at 105[degrees]C) per bulk volume using the core method (Grossman and Reinsch 2002). PD was determined using the pycnometer method (Blake and Hartge 1986). Total soil porosity (SP) was then calculated from the BD and PD (Vomocil 1965). Soil water content was determined using a sand-box apparatus (Eijkelkamp Agrisearch Equipment) at water tensions between 1 and -10 kPa, whereas pressure chambers (Soilmoisture Equipment) were used for tensions between -33 and -1500kPa, following the procedure described in Cornelis et al. (2005). A matric potential of -10kPa was taken as corresponding to field capacity, according to field observations reported in Khoa (2002). Wilting point was considered at a matric potential of 1500kPa (Reynolds et al. 2007). Then, plant-available water capacity (PAWC) was calculated as follows:

PAWC = [[theta].sub.fc] - [[theta].sub.pwp]

where [[theta].sub.fc] and [[theta].sub.pwp] are water content ([m.sup.3] [m.sup.-3]) at field capacity and at permanent wilting point respectively.

Macroporosity (MacP) was defined as the volume of pores with pore diameter >0.3 mm and was calculated as follows:

MacP = [[theta].sub.s] MatP

where [[theta].sub.s] ([m.sup.3] [m.sup.-3]) is the saturated volumetric water content of the soil and MatP is matric porosity and MatP is equal to [[theta].sub.m], the volumetric water content at matric potential of -1 kPa (Reynolds et al. 2007).

The wet and dry sieving method of de Leenheer and de Boodt (1959) was used on air-dried soil with aggregates <8 mm to determine the aggregate stability index (SI). Sieve fractions of 8.0-4.76, 4.76-2.83, 2.83-2.0, 2.0-1.0, 1.0-0.5 and 0.5-0.3 mm were collected for dry and wet sieving. The SI was calculated as the inverse of the difference in mean weighted diameter after dry and wet sieving.

Soil chemical analysis

Soil organic carbon (SOC) concentrations were determined by wet oxidation (Walkley and Black 1934). This method assumes that 77% of the organic carbon is oxidised and that SOM contains 58% C. Acid hydrolysis by 6 M HCl was used to quantify a relatively labile C pool ([C.sub.hydrolysable]; Silveira et al. 2008), calculated by subtracting the 6M HCI hydrolysis-resistant C content from the total SOC content. SOC and [C.sub.hydrolysable] stocks were calculated per hectare for each soil depth (0-10, 10-20 and 20-30cm) as follows:

SOC or [C.sub.hydrolysable] stocks (Mg [ha.sup.-1]) = SOC or [C.sub.hydrolysable] (%) concentration x BD (Mg [m.sup.-3]) x d x 10000

where d is the thickness of the soil layer (in m). The pH and electrical conductivity (EC5) of air-dried soil samples were measured in a 1:5 soil : deionised water suspension using pH and EC meters respectively (Rayment and lligginson 1992).

Cation exchange capacity (CEC) was determined according to the method of Gillman (1979). Briefly, Ba (from Ba[Cl.sub.2]) was used to displace adsorbed cations, after which Ba was precipitated as BaS[O.sub.4] with MgS[O.sub.4]. The concentration of [Mg.sup.2+] left in solution was analysed by titration and used to calculate CEC.

Statistical analyses

Analysis of variance (ANOVA) was conducted on all the soil properties following a randomised complete design (Gomez and Gomez 1984) in order to compare differences among cropping systems and among depths using SPSS 20.0 software (IBM Corporation 2011). The significance of the effects of the cropping system or soil depth were tested by using Duncan's multiple range tests at 5% probability. The s.d. is provided to indicate the variation within a set of values. In addition, linear correlation analyses were conducted to identify the relationship between soil BD, soil SP and SOC.

Results

The soil texture at all depths in all cropping systems was classified as clay, with mean sand content ranging from 15 to 25 g [kg.sup.-1], silt content ranging from 291 to 318g [kg.sup.-1] and clay content ranging from 656 to 692 g [kg.sup.-1] (Table 2). There were no significant differences in sand, silt and clay content among the cropping systems at three different depths (0-10, 10-20 and 20-30 cm), nor among depths within each cropping system.

Neither cropping system nor soil depth significantly affected pH and CEC when comparing RRR to RUR, RUU or UUU (Table 3). The CEC values ranged from 23.0 to 24.8 [cmol.sup.+] [kg.sup.-1]. EC5 at the 0- 10 cm depth was significantly smaller for UUU than for RUR. However, there were no significant differences in EC5 at the 10-20 and 20-30 cm depths among cropping systems, or among depths within each cropping system. Overall, EC5 values ranged from 491 to 643 dS [m.sup.-1]

Cropping systems significantly affected SOC content and SOC stock, with the highest values found at the 0-10 cm depth and values decreasing with depth in all cropping systems, except UUU (Figs 1, 2). The SOC content in the top 10cm in RRR was significantly greater than in the top horizon of RUR, RUU and UUU (33 vs 26, 24 and 18g [kg.sup.-1] respectively; P<0.05). In contrast, at a depth of 20-30cm, the highest SOC contents ranged from I 7 to 19g [kg.sup.-1] for UUU, RUU and RUR, and RRR had the lowest SOC (10g [kg.sup.-1]). Numeric trends in SOC stocks followed those of SOC concentrations (Fig. 2). Considering the total 0-30 cm profile, the mean value of total SOC stocks per hectare was greatest in RUR (72.3t [ha.sup.-1]), followed by RUU (68.31t [ha.sup.-1]), RRR (66.42t [ha.sup.-1]) and UUU (59.31t [ha.sup.-1]) and was significantly greater for RUR, RUU and RRR than for UUU. I however, differences in SOC stocks were not significant among RRR, RUR and RUU.

Replacement of long-term RRR with RUR, RUU or UUU significantly increased [C.sub.hydrolysable] at depths of 10-20 and 20-30cm (Fig. 1b). At depths of 10-20 and 20-30cm [C.sub.hydrolysable] under RUR, RUU and UUU were two- and threefold greater respectively than with RRR. A similar trend existed for total [C.sub.hydrolysable] stocks per hectare at depths of 10-20 and 20-30cm (Fig. 2).

Despite differences in SOC content among cropping systems or among depths, no significant effect of the cropping system on PD was observed at each depth (i.e. 0-10, 10-20 and 20-30 cm; Table 3). PD was not significantly different among the three layers. Conversely, BD values were significantly different between RRR and RUR, RUU and UUU at all depths (Fig. 3a). Indeed, BD at the 0 10 cm depth was significantly lower for RRR than for the rice-upland crop rotation systems (RUR and RUU) and upland crop monoculture (UUU). However, at depths of 10-20 and 20-30 cm, BD was significantly greater for RRR than for all other cropping systems.

Within each cropping system, BD increased numerically with soil depth, except for UUU. The significant differences in SP among depths or among cropping systems (Fig. 3b) are a direct result of differences in BD. An increase in SPR (Table 3; Fig. 4) was also associated with the reduction in SP and increase in soil BD with increasing depth. SPR increased significantly with depth for all cropping systems, whereas it had a wider range of values for RRR than for the other cropping systems (Fig. 4; Table 3). Below 20 cm, RRR had significantly greater SPR than the other cropping systems.

For the topsoil, PAWC did not differ significantly among cropping systems (Table 3). At a depth of 10-20cm, UUU had significantly lower PAWC than RUR and RUU. At a depth of 20-30 cm, RRR had significantly lower PAWC (20-32% lower) than all other cropping systems.

MacP was significantly affected by both cropping system and depth, and decreased from depths of 0-20 to 20-30cm for all cropping systems (Table 3). RRR and both RUR and RUU had similar MacP values at a depth of 0-10 cm, which were significantly lower than the MacP of the UUU system. The RRR system had a significantly lower MacP for depths of 10-20 and 20-30cm (26-37% and 33-40% respectively) than the RUR, RUU and UUU systems.

The use of upland crops in the rotations resulted in significant improvement in SI compared with rice monoculture at a depth of 20-30 cm (Table 3). Comparison of overall SI in the top 30 cm reveals that UUU had the highest SI, followed by RUU, RUR and RRR (Table 4).

Discussion

Five years after replacing long-term rice monoculture of three rice crops per year (RRR) with rice upland crop rotations (RUR and RUU) or continuous cultivation of upland crops (UUU), effective changes were observed in total soil carbon stocks among cropping systems (Table 5) and in SOC concentrations and stocks among cropping systems for each soil depth (Figs 1, 2). At a depth of 20-30cm, RUR, RUU or UUU had significantly higher SOC (P<0.05) than RRR; the opposite trend was seen for the 0 10 cm depth, with the highest SOC value found under RRR. This could be explained by the deeper tillage (0-30 cm depth) and hence the greater soil mixing during the upland crop season compared with rice monoculture (0-10 cm depth only). Breaking up the hardpan and mixing the soil likely resulted in greater crop residue input at a depth of 20-30cm under the RUR, RUU and UUU systems than under RRR. Further, rice is most frequently grown under Hood irrigated conditions, where the upper part of the soil profile is completely saturated (Norman et al. 2003). This reduces the mineralisation rate, thus slowing the decomposition of fibrous residues, as reported by Olk et al. (2009b), which causes the accumulation of SOM, especially at the 0-10 cm depth. This may reduce nitrogen mineralisation due to phenol accumulation in flooded rice soil (Olk et al. 1996; Cassman et al. 1997).

Throughout the entire 0-30 cm depth, there were no significant differences in SOC stock when shifting from RRR to the RUR and RUU systems. However, of major concern is that changing long-term RRR to UUU decreased SOC stock: UUU had significantly less SOC content (17.9 g [kg.sup.-1]) than did RRR. RUR and RUU (21.2-22.6 g [kg.sup.-1]; Table 5) over the top 30 cm.

According to the farmers, nearly all aboveground biomass from the upland crops was collected after harvest as livestock feed, resulting in very limited addition of crop residues to the soil. In contrast, addition of organic residue in the form of crop residue was greater after the rice crop season, with approximately 50% of crop residue (rice stubble) remaining on the soil surface for incorporation before the subsequent crop season. Furthermore, changes in the soil environment associated with year-round aerobic conditions under UUU may have contributed to accelerated organic residue decomposition, causing rapid breakdown of accumulated SOM and thus low SOC accumulation. Guo and Gifford (2002) reported that changes in land use are necessarily followed by changes in soil carbon storage. SOC stock under the rotation system of two rice with one upland crop seasons per year did not differ significantly from that of rice with two upland crop seasons per year. In the present study, this rotation was only applied for 5 years, so this limited duration may be the reason for not finding differences in SOC content between RUR and RUU.

The [C.sub.hydrolysable] values did not show a similar trend with SOC for all cropping systems. Despite a much larger SOC content in RRR at the 0-10 cm depth, [C.sub.hydrolysable] did not significantly differ in RRR compared with the other cropping systems (Fig. 1). Averaged across the 0-30cm depth, the ratio of [C.sub.hydrolysable] to SOC was more than 70-80% greater in RUR and RUU than in RRR (Fig. 2). Remarkably, the proportion of [C.sub.hydrolysable] to SOC for UUU (9.5%), always being under aerobic conditions, was more than twofold greater that under continuous rice monoculture (4.5%). This heightened proportion of hydrolysable C could reflect the aerobic soil conditions prevailing during the upland crop seasons, hence enhanced microbial activity, which stimulates the decomposition of organic residues in rotation systems with upland crops (Dung et al. 2010; Xuan et al. 2012). The results of the present study are generally consistent with those of Xu et al. (2007), who reported that the relative rate of SOM decomposition generally increases with the introduction of periods of aerobic conditions (Pulleman et al. 2000; Norman et al. 2003).

Increasing the organic matter stock as well as organic matter decomposability (i.e. [C.sub.hydrolysable]) through rotation of rice with upland crops could have contributed to the increase in soil physical quality (i.e. BD, SP, SPR, PA WC, MacP and SI). After 5 years of rice-upland crop rotations, soil BD at the 20-30 cm depth was 9% and 12% lower under RUR (1.22 g [cm.sup.-3]) and RUU (1.17 g [cm.sup.-3]) respectively than under RRR (1.33 g [cm.sup.-3]; Fig. 3a). Relative to the other cropping system, UUU had the greatest BD in the topsoil (0-10cm), whereas BD at depths of 10-20 and 20-30 cm was greatest in RRR (P<0.05), coinciding with a hardpan. Along with BD, SPR was lower at the 20-30 cm depth in rice with upland crop rotations (Table 3; Fig. 4). These systems returned more organic residues to the subsoil (below 10 cm depth), whereas in RRR this return occurs primarily in the topsoil. This difference can be explained by seasonal ploughing and puddling, and repeated machinery trafficking (Mahboubi et al. 1993) under wet conditions that characterise RRR. With degradation of soil structure under such challenging conditions as with RRR, the soil BD increased and compaction occurred at depths of 20-30 cm. The soil of the study area had high clay content (>60%) and may therefore be sensitive to compaction under wet conditions. It is clear in Fig. 3a that the BD in RUR, RUU and UUU was not only above the optimum range for field crop production, but also below the upper limit for soil compaction in fine-textured soil (0.9-1.2 Mg [m.sup.-3]; Reynolds et al. 2003; Drewry and Paton 2005). According to Joncs et al. (2003), the threshold BD for soil compaction for these soils with clay content of approximately 650 g [kg.sup.-1] is approximately 1.2 Mg [m.sup.-3] ([BD.sub.threshold] = 1.75 Mg [m.sup.-3]-0.9Cl, where Cl is clay content in g [g.sup.-1]). This threshold value was clearly exceeded in RRR at a depth of 20-30 cm, whereas RUR. RUU and UUU showed BD values close to the threshold. Soil compaction at 20-30 cm depth under the RRR system can impede mechanical rice root penetration in the soil and hence inhibit deep rooting (Linh et al. 2015). According to local farmers, under the RRR system rice plants can lodge in the ripening phase, which makes harvest difficult and results in yield loss. This was less the case in the other cropping systems (i.e. RUR, RUU and UUU), which received a lower number of machinery passes compared with RRR. Furthermore, rotations with upland crops provide a deeper and higher degree of soil mixing. Thus, BD showed little variation with depth and was significantly lower at the 20-30 cm depth than in RRR. Other researchers found that deep tillage reduces SPR and thus promotes deep rooting (Kundu et al. 1996; Khan et al. 1998).

Correlation analysis showed that SOC increased with increasing SP and decreasing soil BD. We observed a rather high positive correlation between SOC and SP (r = 0.79), and consequently a negative correlation between SOC and BD (r = -0.80, P<0.01; Figs 5, 6). Negative correlations between organic carbon and BD were also observed by Cotching et al. (2002), Reynolds et al. (2007) and Diana et al. (2008). However, the correlation between SOM content and aggregate SI was low (r = 0.31). The low correlation may be due to independent effects on aggregate stability by specific organic components rather than by the bulk SOM as such (Lal and Shukla 2004).

Increased compaction probably reduced PAWC, SI and MacP in RRR. RUR, RUU and UUU had greater SI values than did RRR by 11-63% (Table 4). An increase in aeration in the upland crop season(s) likely caused the increased SI. With three puddling operations per year under wet conditions and prolonged flooded conditions during the rice season of RRR, this probably promotes the dispersion of soil aggregates (Hillel 2004) and thus lowers the aggregate SI. RRR also produced some surprising differences in MacP at the 20-30 cm depth. Although the other cropping systems yielded a large MacP (0.0417-0.0522 [m.sup.3][m.sup.-3]), the RRR value was approximately 60-70% smaller (0.0281 [m.sup.3] [m.sup.-3]; Table 3). The higher MacP at depth under the RUR and RUU systems may be attributable to the increase in SOC, which, in turn, led to increased SI, which generally results in additional total pore space. Moreover, deeper soil preparation and a coarse root system of upland crops in UUU, RUR and RUU would favour the creation of macropores, thus increasing SP. This scenario is consistent with the documented gain of SOC causing a rapid and significant increase in MacP (Carter 1990). The low PAWC under RRR relative to RUR and RUU appears to be due primarily to a decrease in the number of pores, which is consistent with the low matrix porosity of RRR relative to the other cropping systems.

Surprisingly, UUU produced SI and MacP values that were much greater than those of RRR, RUR and RUU, although the SOC stock of this system was lower than that of RRR, RUR and RUU (P<0.05; Table 5). This can probably be explained by the higher number of earthworms and increased soil fauna activity paired with accelerating organic residue decomposition in the aerobic conditions under the UUU system. The soil physical properties of the UUU system did generally not deteriorate 5 years after replacing continuous rice monoculture with continuous upland crop monoculture and the soil quality indicator values of the UUU system still fell within ideal ranges as presented by Reynolds et al. (2007), although UUU caused a decrease in SOC content. This decline can reduce soil productivity in the future if the farmers do not apply organic fertiliser or keep crop residue on the fields after harvesting.

Several soil properties in the present study, including pH, CEC, particle size distribution (sand, silt and clay) and soil particle density, were not affected by changing the cropping system from RRR to RUR, RUU and UUU (Tables 3, 5). However, EC trended downward in UUU at the 0-10 cm depth and also over the 0-30 cm depth, probably reflecting enhanced eluviation of nutrients from the soil beds.

Yet, with the introduction of rice-upland crop rotation systems, several soil properties that are often considered as indices of soil quality improved within 5 years. Limited soil compaction reflected by BD and SPR and greater SOC quality expressed in terms of [C.sub.hydrolysable] may have explained the higher rice grain yield and income observed by Linh et al. (2014) in rice--upland crop rotation systems in a farm household survey with 109 farmers on the same fields where the present study was conducted. The significance of soil physical properties to rice yields and net profitability in the Mekong Delta was also highlighted by Linh et al. (2015). They concluded that soil physical properties contributed to enhanced rice yield with rice upland crop rotation systems. In these systems, the improved soil physical properties increased rooting depth and rice yield compared with the rice monoculture system.

Conversely, other factors can also contribute to rice yield problems with RRR, such as inhibited N supply. Cassman et al. (1997) discussed the complete reversal of a long-term yield decline in similar RRR fields in the Philippines by better synchronising N fertiliser application with plant N demand, which compensated for decreasing availability of soil N. The yield reversal occurred despite no effort to improve the physical properties of those soils. Presuming that soil N supply is therefore a key contributor to the long-term yield decline, we speculate that rotation with an upland crop and the resulting improvement of soil physical properties would facilitate a long-term yield reversal by enabling the root system to access deeper soil masses and their N supply. Increased soil aeration during upland crop rotation can in cases also improves soil N mineralisation and rice crop uptake of soil N (Olk et al. 2009a). Accordingly, many continuous rice farmers in Asia drain their fields sometimes during the growing season with the belief that aeration improves soil rooting depth or increases crop N uptake (Kaneta et al. 1989).

Conclusions

After 5 years of alternative cropping systems consisting of rotations of rice with upland crops on clay soil, several soil physical and chemical properties were changed in small-scale farmers' fields. The soil quality of the RRR system was the poorest in relation to the major soil function in the area (i.e. sustainable rice production). Soil quality improved with rotation of rice with upland crops, showing higher SP, [C.sub.hydrolysable], PAWC, MacP and SI, and lower BD and SPR, especially at the 20-30 cm depth. Overall, those rotation systems resulted in a reduction in soil compaction and improvement in soil structure, which may result from better organic carbon quality in terms of higher [C.sub.hydrolysable] content compared with the long-term rice monoculture. However, although UUU showed no compaction, it produced 10%, 15% and 18% less SOC stocks than RRR, RUR and RUU respectively, suggesting the potential of the UUU system for degrading SOC. Therefore, the UUU system is susceptible to loss of SOC if the farmers do not apply organic fertiliser. Such SOC decline can affect soil productivity and the sustainability of this system in the long term. From the farm field study, it could be concluded that cropping systems with rice-upland crop rotations are to be preferred over rice monocultures because of their higher SOC quality and lower degree of soil compaction. Upland crop monocultures should not be promoted because they tend to reduce SOC content. The results of the present study may lead to increased awareness and development of more sustainable agricultural practices that, in turn, may lead to less degraded soil for future productivity.

http://dx.doi.org/10.1071/SR16002

Acknowledgements

The authors sincerely thank the farmers at the Long Khanh, Cam Son, Binh Phu and Long Tien villages (Cai Lay district, Tien Giang province) for kindly allowing the collection of soil samples on their fields. The authors are grateful to the staff of the Department of Soil Science, Can Tho University and Department of Agriculture of Cai Lay for their help and support throughout the study.

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Tran Ba Linh (A,B,D), Vo Thi Cuong (A), Vo Thi Thu Tran (A), Le Van Khoa (A), Daniel Olk (C), and Wim M. Cornelis (B)

(A) Department of Soil Science, Can Tho University, 3/2 Street, Ninh Kieu District, Can Tho city, Vietnam.

(B) Department of Soil Management, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium.

(C) USDA-ARS, National Laboratory for Agriculture and the Environment, 2110 University Blvd Ames, IA 50011, USA.

(D) Corresponding author. Email: tblinh@ctu.edu.vn

Caption: Fig. 1. Effects of the cropping system on (a) soil organic carbon and (b) HCl-hydrolysable carbon ([C.sub.hydrolysable]) content at three soil depth intervals. RRR. monoculture with three rice crops per year; RUR, rice upland crop rice rotation; RUU, rice upland crop-upland crop rotation; UUU, monocultures with three upland crops per year. Data show the mean [+ or -] s.d. Different lowercase letters denote statistically significant differences (P< 0.05) within each cropping system using Duncan's multiple range test (DMRT); different uppercase letters denote significant differences (P<0.05) among the cropping systems for a given depth.

Caption: Fig. 2. Effects of the cropping system on soil organic carbon stocks and HCl-hydrolysable soil C stocks at depths of 0 10, 10-20 and 20-30cm. RRR. monoculture with three rice crops per year; RUR, rice upland crop-rice rotation; RUU, rice-upland crop-upland crop rotation: UUU, monocultures with three upland crops per year. Data show the mean [+ or -] s.d. Different lowercase letters denote statistically significant differences (P<0.05) within each cropping system using DMRT; different uppercase letters denote significant differences (P< 0.05) among the cropping systems for a given depth.

Caption: Fig. 3. Effects of the cropping system on (a) soil bulk density and (b) soil porosity at three depth intervals. RRR, monoculture with three rice crops per year; RUR, rice upland crop-rice rotation; RUU, rice upland crop upland crop rotation; UUU, monocultures with three upland crops per year. Data show the mean [+ or -] s.d. Different lowercase letters denote statistically significant differences (P<0.05) within each cropping system using DMRT; different uppercase letters denote significant differences (P<0.05) among the cropping systems for a given depth.

Caption: Fig. 4. Mean ([+ or -] s.d.) soil penetration resistance and corresponding soil water content for the 0-30cm depth (n = 10) in the different cropping systems. RRR, monoculture with three rice crops per year; RUR, rice upland crop-rice rotation; RUU, rice upland crop-upland crop rotation; UUU, monocultures with three upland crops per year.

Caption: Fig. 5. Regression equation and coefficient of determination ([R.sup.2]) describing the relationship between soil porosity and soil organic carbon. ** P<0.01.

Caption: Fig. 6. Regression equation and coefficient of determination ([R.sup.2]) describing the relationship between soil bulk density and soil organic carbon. ** P<0.01.
Table 1. Cropping systems in the study area

RRR, monoculture with three rice crops per year; RUR, rice-upland
crop-rice rotation; RUU, rice upland crop upland crop rotation; UUU,
monocultures with three upland crops per year

Cropping                   Cropping season
system     Late wet      Dry           Wet

RRR        Rice          Rice          Rice
RUR        Rice          Upland crop   Rice
RUU        Rice          Upland crop   Upland crop
UUU        Upland crop   Upland crop   Upland crop

Table 2. Particic size distribution for four cropping systems at three
soil depths

Data are the mean [+ or -] s.d. Using DMTR, there were no
significant differences in any parameter (P>0.05) among cropping
systems at any depth or among depths within any cropping system. RRR,
monoculture with three rice crops per year; RUR, rice upland crop
rice rotation; RUU, rice upland crop-upland crop rotation; UUU,
monocultures with three upland crops per year

Cropping   Soil    Sand 50-2000 [micro]m   Silt 2-50 [micro]m
system     depth   (g [kg.sup.-1])         (g [kg.sup.-1])
           (cm)

RRR        0-10    22.8 [+ or -] 11.2      310.1 [+ or -] 38.1
           10-20   19.2 [+ or -] 7.2       308.8 [+ or -] 31.6
           20-30   13.6 [+ or -] 4.4       299.7 [+ or -] 35.6

RUR        0-10    15.0 [+ or -] 7.0       301.8 [+ or -] 38.0
           10-20   21.2 [+ or -] 13.2      303.9 [+ or -] 33.6
           20-30   16.4 [+ or -] 8.4       309.0 [+ or -] 20.3

RUU        0-10    25.3 [+ or -] 11.3      318.2 [+ or -] 48.3
           10-20   23.2 [+ or -] 7.4       313.3 [+ or -] 37.3
           20-30   21.0 [+ or -] 6.4       307.4 [+ or -] 34.0

UUU        0-10    19.8 [+ or -] 8.2       311.5 [+ or -] 44.0
           10-20   18.8 [+ or -] 7.3       308.0 [+ or -] 39.0
           20-30   15.9 [+ or -] 6.3       291.4 [+ or -] 28.5

Cropping   Soil
system     depth   Clay <2 [micro]m
           (cm)    (g [kg.sup.-1])

RRR        0-10    668.1 [+ or -] 39.9
           10-20   672.0 [+ or -] 34.3
           20-30   686.7 [+ or -] 37.0

RUR        0-10    683.2 [+ or -] 33.1
           10-20   674.9 [+ or -] 34.4
           20-30   674.6 [+ or -] 24.1

RUU        0-10    656.5 [+ or -] 47.8
           10-20   663.5 [+ or -] 37.3
           20-30   671.5 [+ or -] 34.7

UUU        0-10    668.8 [+ or -] 47.0
           10-20   673.2 [+ or -] 42.2
           20-30   692.8 [+ or -] 28.1

Table 3. Effects of crop rotations on selected soil chemical and
physical quality indicators and parameters

Data are the mean [+ or -]  s.d. Within columns, values with
different letters differ significantly (P< 0.05, DMRT). Different
uppercase letters indicate significant differences among cropping
systems at each depth; different lowercase letters indicate
significant differences among depths within a cropping system. RRR,
monoculture with three rice crops per year; RUR, rice-upland crop
rice rotation; RUU, rice upland crop upland crop rotation; UUU,
monocultures with three upland crops per year; EC5, electric
conductivity at a 1 : 5 soil : water ratio; CEC, cation exchange
capacity; PD, particle density; PAWC, plant available water capacity;
MacP. macroporosity; SI, stability index; PR. penetration resistance

Cropping   Depth            PH                   EC5
system      (cm)                           (dS [m.sup.-1])

RRR         0-10    5.41 [+ or -] 0.32   0.62 [+ or -] 0.18AB
           10-20    5.38 [+ or -] 0.21   0.62 [+ or -] 0.15
           20-30    5.52 [+ or -] 0.20   0.63 [+ or -] 0.14

RUR         0-10    5.46 [+ or -] 0.28   0.64 [+ or -] 0.12A
           10-20    5.50 [+ or -] 0.25   0.62 [+ or -] 0.16
           20-30    5.53 [+ or -] 0.23   0.61 [+ or -] 0.13

RUU         0-10    5.49 [+ or -] 0.27   0.52 [+ or -] 0.13AB
           10-20    5.58 [+ or -] 0.24   0.53 [+ or -] 0.20
           20-30    5.59 [+ or -] 0.25   0.58 [+ or -] 0.14

UUU         0-10    5.56 [+ or -] 0.24   0.49 [+ or -] 0.14B
           10-20    5.51 [+ or -] 0.25   0.51 [+ or -] 0.19
           20-30    5.65 [+ or -] 0.19   0.51 [+ or -] 0.16

Cropping   Depth           CEC                  PD
system      (cm)     ([cmol.sup.+]        (Mg [m.sup.3]
                      [kg.sup.-1])

RRR         0-10    24.4 [+ or -] 3.0   2.47 [+ or -] 0.07
           10-20    24.4 [+ or -] 3.1   2.50 [+ or -] 0.05
           20-30    24.5 [+ or -] 3.0   2.54 [+ or -] 0.02

RUR         0-10    24.0 [+ or -] 1.0   2.49 [+ or -] 0.06
           10-20    23.5 [+ or -] 1.5   2.51 [+ or -] 0.05
           20-30    23.0 [+ or -] 1.2   2.53 [+ or -] 0.04

RUU         0-10    24.8 [+ or -] 3.7   2.51 [+ or -] 0.04
           10-20    24.3 [+ or -] 4.2   2.50 [+ or -] 0.02
           20-30    24.2 [+ or -] 3.2   2.51 [+ or -] 0.03

UUU         0-10    23.9 [+ or -] 3.0   2.50 [+ or -] 0.04
           10-20    23.8 [+ or -] 2.8   2.49 [+ or -] 0.03
           20-30    24.2 [+ or -] 2.3   2.51 [+ or -] 0.03

Cropping   Depth             PAWC
system      (cm)     ([m.sup.3][m.sup.3])

RRR         0-10    0.250 [+ or -] 0.033a
           10-20    0.241 [+ or -] 0.024ABa
           20-30    0.178 [+ or -] 0.022Cb

RUR         0-10    0.254 [+ or -] 0.042a
           10-20    0.261 [+ or -] 0.020Aa
           20-30    0.222 [+ or -] 0.032Bb

RUU         0-10    0.253 [+ or -] 0.035
           10-20    0.256 [+ or -] 0.023A
           20-30    0.261 [+ or -] 0.041A

UUU         0-10    0.231 [+ or -] 0.031
           10-20    0.226 [+ or -] 0.025B
           20-30    0.227 [+ or -] 0.036B

Cropping   Depth              MacP                       SI
system      (cm)      ([m.sup.3][m.sup.3])

RRR         0-10    0.0437 [+ or -] 0.0076Ba    l.52 [+ or -] 0.27Ba
           10-20    0.0389 [+ or -] 0.0088Ba    1.28 [+ or -] 0.20Bb
           20-30    0.0281 [+ or -] 0.0055Cb    0.85 [+ or -] 0.12Dc

RUR         0-10    0.0490 [+ or -] 0.0141Bab   1.54 [+ or -] 0.31Ba
           10-20    0.0524 [+ or -] 0.0073Aa    1.35 [+ or -] 0.28Bab
           20-30    0.0417 [+ or -] 0.0082Bb    1.19 [+ or -] 0.18Cb

RUU         0-10    0.0537 [+ or -] 0.0127ABa   1.82 [+ or -] 0.30Ba
           10-20    0.0543 [+ or -] 0.0l09Aa    1.36 [+ or -] 0.25Bb
           20-30    0.0435 [+ or -] 0.0083ABb   1.44 [+ or -] 0.29Bb

UUU         0-10    0.0622 [+ or -] 0.0104Aa    2.18 [+ or -] 0.51Aa
           10-20    0.0612 [+ or -] 0.0132Aa    2.05 [+ or -] 0.35Aab
           20-30    0.0502 [+ or -] 0.0079Ab    l.73 [+ or -] 0.30Ab

Cropping   Depth              PR
system      (cm)            (MPa)

RRR         0-10    0.436 [+ or -] 0.083c
           10-20    1.126 [+ or -] 0.103b
           20-30    1.781 [+ or -] 0.122Aa

RUR         0-10    0.445 [+ or -] 0.095c
           10-20    0.925 [+ or -] 0.090b
           20-30    1.385 [+ or -] 0.142Ba

RUU         0-10    0.397 [+ or -] 0.091c
           10-20    0.932 [+ or -] 0.114b
           20-30    l.435 [+ or -] O.135Ba

UUU         0-10    0.405 [+ or -] 0.115c
           10-20    0.841 [+ or -] 0.127b
           20-30    1.221 [+ or -] 0.106Ba

Table 4. Effects of crop rotations on selected soil physical quality
indicators and parameters for the overall 0-30 cm depth

Within columns, values with different letters differ significantly
(P<0.05, DMRT). RRR, monoculture with three rice crops per year; RUR.
rice upland crop rice rotation; RUU, rice-upland crop-upland crop
rotation; UUU, monocultures with three upland crops per year; BD,
bulk density; PD, particle density; SP, soil porosity; PAWC,
plant-available water capacity; MacP, macroporosity; SI, stability
index

Cropping       Sand            Silt            Clay
system     (g[kg.sup.1])   (g[kg.sup.1])   (g[kg.sup.1])

RRR            18.2            306.2           675.6
RUR            17.5            304.8           677.5
RUU            23.1            312.9           663.8
UUU            18.1            303.6           678.2

Cropping        BD              PD           SP
system     (Mg[m.sup.3])   (Mg[m.sup.3])    (%)

RRR            1.13A           2.50        54.9B
RUR            1.09B           2.51        56.7A
RUU            1.07B           2.51        57.1A
UUU           1.11AB           2.50        55.7AB

Cropping          PAWC                   MacP            SI
system     ([m.sup.3][m.sup.])   ([m.sup.3][m.sup.3])

RRR              0.225B                 0.036C          1.22C
RUR              0.246A                 0.047B          1 36C
RUU              0.258A                 0.050B          I.54B
UUU              0.227B                 0.057A          1.99A

Table 5. Effects of crop rotations on selected soil chemical quality
indicators and parameters for the overall 0-30 cm depth

Within columns, values with different letters differ significantly
(P<0.05, DMR T). RRR, monoculture with three rice crops per year;
RUR, rice-upland crop rice rotation; RUU, rice upland crop upland
crop rotation; UUU, monocultures with three upland crops per year;
HC, electric conductivity; CEC, cation exchange capacity; SOC, soil
organic carbon; [C.sub.hydrolysable], HCl-hydrolysable soil carbon

Cropping    pH         EC5                  CEC
system            (dS[m.sup.1])   ([cmol.sup.+][kg.sup.1])

RRR        5.44       0.623                 24.4
RUR        5.50       0.624                 23.5
RUU        5.56       0.541                 24.5
UUU        5.57       0.503                 23.9

Cropping        SOC         SOC stocks     [C.sub.hydrolysable]
system     (g[kg.sup.1])   (t[ha.sup.1])      (g[kg.sup.1])

RRR            21.2A          66.42A              0.97B
RUR            22.6A          72.26A              1.69A
RUU            21.5A          68.30A              1.79A
UUU            17.9B          59.31B              1.72A

Cropping   [C.sub.hydrolysable]
system            Stocks
              (t[ha.sup.1])

RRR               3.0IB
RUR               5.44A
RUU               5.71A
UUU               5.72A
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Author:Linh, Tran Ba; Cuong, Vo Thi; Tran, Vo Thi Thu; Van Khoa, Le; Olk, Daniel; Cornelis, Wim M.
Publication:Soil Research
Article Type:Report
Date:Mar 1, 2017
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