SoilUse and Management Soil Use and Management, March 2011, 27, 28–35 doi: 10.1111/j.1475-2743.2010.00308.x

Soil organic carbon and nutrient content in aggregate-size fractions of a subtropical rice soil under variable tillage

X. Jiang1 ,Y.Hu 1 ,J.H.Bedell1 ,D.Xie1 &A.L.Wright2 1College of Resources and Environment, Southwest University, 2 Tiansheng Road, Beibei, Chongqing 400715, China, and 2Everglades Research & Education Center, University of Florida, Belle Glade, FL 33430, USA

Abstract The effects of tillage on soil organic carbon (SOC) and nutrient content of soil aggregates can vary spatially and temporally, and for different soil types and cropping systems. We assessed SOC and nutrient levels within water-stable aggregates in ridges with no tillage (RNT) and also under conventional tillage (CT) for a subtropical rice soil in order to determine relationships between tillage, cation concentrations and soil organic matter. Surface soil (0–15 cm) was fractionated into aggregate sizes (>4.76 mm, 4.76–2.00 mm, 2.00–1.00 mm, 1.00–0.25 mm, 0.25–0.053 mm, <0.053 mm) under two tillage regimes. Tillage significantly reduced the proportion of macroaggregate fractions (>2.00 mm) and thus aggregate stability was reduced by 35% compared with RNT, indicating that tillage practices led to soil structural change for this subtropical soil. The patterns in SOC, total N, exchangeable Ca2+,Mg2+ and total exchangeable bases (TEB) were similar between tillage regimes, but concentrations were significantly higher under RNT than CT. This suggests that RNT in subtropical rice soils may be a better way to enhance soil productivity and improve soil C sequestration potential than CT. The highest SOC was in the 1.00–0.25 mm fraction (35.7 and 30.4 mg ⁄ kg for RNT and CT, respectively), while the lowest SOC was in microaggregate (<0.025 mm) and silt + clay (<0.053 mm) fractions (19.5 and 15.7 mg ⁄ kg for RNT and CT, respectively). Tillage did not influence the patterns in SOC across aggregates but did change the aggregate-size distribution, indicating that tillage affected soil fertility primarily by changing soil structure.

Keywords: Tillage management, aggregates, organic carbon, subtropical soil

These properties are important to fertility and productivity of Introduction soils (Bronick & Lal, 2005). Tillage disrupts incipient soil Soil management is crucial to agricultural productivity and aggregates (Kirchhof et al., 2000; Jiang & Xie, 2009), thereby environmental protection. Loss of soil organic matter (SOM) exposing previously protected organic matter to microbial because of inappropriate land management practices can attack (Beare et al., 1994). Adoption of minimal tillage or no decrease availability of soil nutrients, destroy soil structure tillage (NT) regimes often results in increases in SOM and soil and increase CO2 emissions (West & Marland, 2002; Bronick fertility. There is a strong link between organic matter and & Lal, 2005). Soil management, including the use of physical aggregation in soils (Bronick & Lal, 2005). As SOM is minimum tillage (MT), can increase SOM sequestration distributed heterogeneously, ‘hot-spots’ of aggregation which consequently enhances nutrient availability to crops develop where labile organic matter is most abundant. There (Wright et al., 2007). Therefore, understanding the effects of are also chemical drivers of aggregation, such as Ca2+ and soil management practices, especially tillage, can lead to Mg2+, which form cationic bridges with clay particles and soil better management. organic carbon (SOC) and to thus help to protect organic Soil management aims to generate favourable soil structure matter from decomposition. with high aggregate stability to thus protect organic matter. The effects of SOC and cations on soil structure are well documented (Armstrong & Tanton, 1992; Zhang & Norton, Correspondence: Dr. X. Jiang. E-mail: [email protected] 2002). Many studies have confirmed the relationship of SOC Received April 2010; accepted after revision September 2010 and aggregate-size fractions and one example by Wright &

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Hons (2005) found that SOC was higher under NT than Hydrgric Anthrosol (FAO ⁄ UNESCO, 1988) that developed under CT in all aggregate-size fractions down to <53 lm. In from purple mudstone. Soil physical and chemical properties contrast, Onweremadu et al. (2007) report that SOC in water- were pH (H2O) = 7.1, SOC = 21.7 g ⁄ kg, total stable aggregates varied according to tillage methods. In N = 1.74 g ⁄ kg, total P = 0.75 g ⁄ kg, total K = 22.7 g ⁄ kg, another study Castro et al. (2002) state that SOC was greater available N = 120.1 mg ⁄ kg, available P = 7.5 mg ⁄ kg and in <2 mm aggregates for NT than CT. Greater SOC was available K = 71.1 mg ⁄ kg. The field was planted with rape found within >250 lm aggregates under MT, whereas the (Brassica napus L.) in winter and rice (Oryza sativa L.) in reverse was the case under CT (Onweremadu et al., 2007). summer with residues returned to soil. Factors such as climate, soil type, texture, and dominant mineralogy also influence aggregation and relationships with Tillage regimes SOM (Mbagwu & Piccolo, 1998; Potter et al., 1998; Stevenson et al., 2005), and may be responsible in part for The experiment was laid out in a randomized complete block variable responses of SOC to tillage. design with four replications. There were two tillage These studies demonstrate the inconsistencies in the treatments, RNT and CT. The size of the plots was 4 · 5m2. relationships between SOC and aggregate size. Research All fields were planted with rape (B. napus L.) in winter and using different soils under variable climates is needed. rice (O. sativa L.) in summer with residues returned to soil. Furthermore, most studies have examined only SOM pools For CT, rice was transplanted in flooded fields with a layer of (Tisdall & Oades, 1980; Christensen & Sorensen, 1985; water until maturation. For CT, rice was transplanted in Mbagwu & Piccolo, 1990; Christensen, 1992; Cambardella & flooded fields with a layer of water until maturation. After Elliot, 1993) with little attention given to exchangeable the rice harvest, fields were drained, dried, ploughed to a cations that can influence SOM and aggregate stability depth of 20–30 cm, and then rape was grown. After the rape (Adesodun et al., 2007). The distribution of cations can harvest, the soil was puddled and harrowed twice and rice influence both aggregation and microbial degradation of seedlings transplanted again. Rice was planted at the end of SOM, so quantifying their location within aggregate sizes will April and harvested in mid-August, while rape was planted in improve our understanding of mechanisms for sequestering early October and harvested in mid-April. The RNT SOM. Also, as different aggregate fractions are selectively cropping system treatment was similar to CT except for the removed during erosion (Lal, 1976), characterization of ploughing and harrowing with ridges kept intact. A no-tillage eroded versus retained aggregates will improve our treatment was imposed on soil that was ridge tilled in 1990, understanding of nutrient dynamics. with the ridges kept intact since then (Jiang & Xie, 2009). This study for a subtropical purple rice soil (Hydrgric From rice transplantation to the vegetative stage, the water Anthrosols, FAO ⁄ UNESCO, 1988), addresses the lack of surface was kept even with ridge tops, whereas during other understanding of the factors influencing the distribution of times of the year water depth in furrows was 15–20 cm below SOC and cations among different aggregate sizes under the top of the ridges. For all treatments, the annual conventional and conservation tillage. Subtropical purple rice application of fertilizers included 270 kg ⁄ ha urea, 500 kg ⁄ ha soils are responsible for rice production that feed 200 million calcium superphosphate and 150 kg ⁄ ha KCl. people in southwest China, and very little information is available regarding mechanisms to increase C sequestration Aggregate-size distribution and stability and nutrient storage. We compare a novel tillage method that combines ridges with no tillage (RNT) which has shown great Surface soil samples (0–20 cm) were collected in April, 2007. promise in lowland rice-based cropping compared with Fractionation of soil aggregates was achieved using a wet- conventional tillage (CT) (Jiang & Xie, 2009). The objective sieving procedure (Elliott & Cambardella, 1991; Cambardella of this study was to examine the relationships between soil & Elliott, 1994). Approximately 100 g soil was placed on a aggregation and the distribution of SOC, N, P and base set of five nested sieves (4.76, 2.00, 1.00, 0.25, and 0.053 mm). cations under two tillage regimes in a subtropical rice soil. Soils on the 4.76 cm sieve were capillary-wetted to field capacity to prevent slaking following immersion. Wetted soil was immersed 50 times in water with a vertical displacement Material and methods of 3 cm during a 2-min period and the aggregates retained on each sieve were collected. The silt + clay fraction Experimental location (<0.053 mm) was collected after being centrifuged. Samples The Sichuan Basin is in southwestern China (latitude 28– of different aggregate sizes were air-dried and ground before 32N, longitude 103–108E) and covers 165 000 km2. Annual chemical analysis. The method of Van Bavel (1950) as mean temperature ranges between 14 and 19 C and mean modified by Kemper & Rosenau (1986) was used to rainfall between 1000 and 1400 mm. The field experiment has determine the mean-weight diameter (MWD) of water-stable been in operation at Southwest University since 1990 on a aggregates.

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40 Chemical analysis (a) RNT Soil organic carbon was determined by acid dichromate wet CT oxidation as described by Nelson & Sommers (1996). Total N was determined by the micro-Kjeldahl method (Bremner, 30 1996) and available P by Bray I following Olsen & Sommers (1982). Exchangeable bases were determined by ammonium acetate replacement as described by Thomas (1982). The Ca and Mg concentrations were measured by atomic absorption 20 Organic C (g/kg) spectrophotometer with K and Na by flame photometry. Data (measured or calculated) were subjected to ANOVA and mean values were separated using Duncan’s New

Multiple Range Test at P < 0.05. All statistical analyses 10 were performed using the SPSS statistical package. >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053 Aggregate size (mm) Results (b) 3.6 Effects of tillage on aggregate-size distribution and stability RNT CT The proportion of silt + clay sized aggregates (<0.053 mm) comprised the greatest fraction of whole soil for CT while 2.4 the aggregates size (<0.25 mm) and silt + clay fraction constituted the greatest fraction for RNT (Table 1). The RNT regime in contrast to CT increased the proportion of macroaggregates (>1.00 mm) by 67% (Table 1). The observed Total N (g/kg) trends indicate that CT disrupted soil macroaggregates into 1.2 microaggregates or individual particles. Similar results are also reported by You et al. (2006) and Abid & Lal (2008). The MWD was significantly affected by tillage treatments, showing a higher value under NT than CT. This result indicates that CT 0 reduced the aggregate stability by 35% compared with NT >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053 (Table 1). The degree of macroaggregation in this soil was Aggregate size (mm) much lower than in most other agricultural systems, due Figure 1 Soil organic carbon and total N distribution in aggregates primarily to the puddling of soil which tends to destroy under different tillage regimes (RNT, combination of ridge with no aggregates (Wright & Hons, 2005; Abid & Lal, 2008). tillage in a rice rotation ecosystem; CT, conventional tillage in a rice rotation ecosystem). Effects of tillage on C, N, and P concentrations the microaggregates and silt + clay fractions (19.5 and Figure 1a shows SOC distribution in aggregates under 15.7 mg ⁄ kg for RNT and CT, respectively). No significant different tillage regimes with a range from 15.7 to differences were found for other aggregate fractions under 35.7 mg ⁄ kg. SOC distribution among aggregate sizes was not either tillage regime. While similar distribution patterns were influenced by tillage regime. The highest SOC was observed observed under RNT and CT, SOC was significantly higher in the 1.00–0.25 mm size fraction (35.7 and 30.4 mg ⁄ kg under RNT than CT by an average of 23.4%. One of the for RNT and CT, respectively), while the lowest SOC was in reasons for the significant difference may be that the RNT

Table 1 Aggregate-size distribution (%) and stability (MWD) of soils under RNT and CT

Aggregate size (mm)

Tillage >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053 MWD (mm)

RNT 4.2a 26.1a 1.6a 6.6a 33.1a 26.6b 1.211a CT 2.4a 16.2b 0.5a 8.5a 27.7a 42.6a 0.787b aValues (MWD) with different letters in column indicates significant differences (P < 5%). bRNT, no-till treatment was imposed on the soil that was ridge tilled before the experiment, and the ridges were kept intact from 1990. CT, conventional tillage; MWD, mean-weight diameter.

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system has greater crop residue production resulting in higher 1.4 residual returns to the soil each year. The slower RNT decomposition of this residue under RNT than CT is due to CT less physical destruction during tillage and is also likely to be 1.2 responsible for the higher SOC levels under RNT. The resulting higher crop and residue production is a response to higher SOM levels, and higher nutrient-supplying capacity of RNT soils compared with CT. From 2001 to 2006, the 1 average crop yield was 7168 kg ⁄ ha for RNT and 5426 kg ⁄ ha for CT (Jiang & Xie, 2009). Total P (g/kg) The total N distribution of soils from different tillage treatments follows the same trend as SOC (Figure 1b). 0.8 Similar relationships between SOC and total N distribution are commonly observed (Wright & Hons, 2005). Total N ranged from 1.3 to 3.0 mg ⁄ kg, with an average of 2.3 and 0.6 2.13 mg ⁄ kg for RNT and CT, respectively. The highest total >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053 N for both RNT and CT was in the 1.00–0.25 mm fraction Aggregate size (mm) while the lowest N for both tillage methods was in the 0.25– Figure 2 Total P distribution in aggregates under different tillage 0.053 mm fraction. Similar to total C, total N was higher regimes (RNT, combination of ridge with no tillage in a rice rotation under RNT than CT. There is a positive linear correlation ecosystem; CT, conventional tillage in a rice rotation ecosystem). between SOC and TN (r2 = 0.84, n = 12). The C:N ratio is lower under RNT than CT for most aggregate-size fractions, indicating better soil conditions under MT (Table 2). The tillage regimes in which Ca2+ was mainly concentrated in the higher ratio under CT indicates that this system has a lesser silt + clay fraction (<0.053 mm) (14.5 and 14.4 cmol ⁄ kg for ability to release N to available forms. RNT and CT, respectively), with lowest concentrations in Figure 2 shows the total P in aggregates which range from microaggregates of 0.25–0.053 mm (12.9 and 11.7 cmol ⁄ kg 0.80 to 1.13 g ⁄ kg under different tillage regimes. The trend of for RNT and CT, respectively). Exchangeable Ca2+ in each total P in the macroaggregates (>1.00 mm) is similar for aggregate fraction is significantly higher under RNT than CT both tillage regimes. No tillage effect is evident for the except for the silt + clay fraction. <0.053 mm fraction. Similar to SOC and total N, total P Trends in exchangeable Mg2+ concentrations do not differ was about 23% higher for RNT than CT in 0.053–1.00 mm between tillage regimes as a function of aggregate size. aggregates of with no significant differences for other Exchangeable Mg2+ ranged from 3.9 to 4.0 cmol ⁄ kg. The aggregate sizes. The soil C:P ratio was higher in the 1.00– highest exchangeable Mg2+ was in the 2.00–1.00 mm fraction 0.25 mm fraction for both tillage regimes, and in contrast to for both RNT and CT, while the lowest concentration was in C:N, was generally higher under RNT than CT (Table 2). 0.25–0.053 mm microaggregates. Furthermore, there were no significant differences in exchangeable Mg2+ between tillage regimes (Figure 3b). Effects of tillage on distribution of exchangeable cations Figure 4 shows the distribution patterns of exchangeable within aggregates K+ and Na+ under RNT and CT. Exchangeable K+ ranged The concentration of exchangeable Ca2+ ranged from 11.7 to from 0.12 to 0.29 cmol ⁄ kg. The highest and lowest 14.5 cmol ⁄ kg (Figure 3a). Similar patterns occurred for both exchangeable K+ for both tillage regimes were in the

Table 2 The soil C ⁄ N ⁄ P ratio of aggregates under RNT and CT

Aggregate size (mm)

>4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053

Tillage C ⁄ NC⁄ PC⁄ NC⁄ PC⁄ NC⁄ PC⁄ NC⁄ PC⁄ NC⁄ PC⁄ NC⁄ P

RNT 10.7 27.7 11.0 26.6 10.2 26.4 11.8 31.6 11.0 21.3 9.6 19.8 CT 11.4 23.3 11.3 22.3 12.0 24.2 13.0 33.1 11.8 19.5 9.3 19.0

Each value in this table were means of four replicates. RNT, no-till treatment was imposed on the soil that was ridge tilled before the experiment, and the ridges were kept intact from 1990. No significant differences due to tillage was found. CT, conventional tillage.

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15 0.4 (a) RNT (a) CT RNT 14 0.3 CT (cmol/kg)

(cmol/kg) 2+ + 13 0.2

0.1 12 Exchange K Exchange Ca

11 0 >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053 >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053 Aggregate size (mm) Aggregate size (mm)

(b) 4.1 (b) 0.8 RNT RNT CT CT 0.7

4 (cmol/kg) + (cmol/kg) 0.6 2+

3.9 0.5 Exchange Na Exchange Mg

0.4 >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053

3.8 Aggregate size (mm) >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053 Figure 4 Exchangeable K+ and Na+ in aggregates under different Aggregate size (mm) tillage regimes (RNT, combination of ridge with no tillage in a rice Figure 3 Exchangeable Ca2+ and Mg2+ in aggregates under rotation ecosystem; CT, conventional tillage in a rice rotation different tillage regimes (RNT, combination of ridge with no tillage ecosystem). in a rice rotation ecosystem; CT, conventional tillage in a rice rotation ecosystem). purple rice soil under tillage treatment for 18 yr. The trends <0.053 mm and 0.25–0.053 mm fractions, respectively. in SOC, total N, exchangeable Ca2+,Mg2+and TEB Exchangeable Na+ ranged from 0.48 to 0.78 cmol ⁄ kg, with distribution in aggregates are similar between the different highest concentrations in the silt + clay fraction tillage regimes (Figures 1, 3, 4 and 5), although total (<0.053 mm) for both tillage regimes. The K+ and Na+ concentrations differ. There are differences in total P in concentrations in meso (2.00–0.25 mm) and microaggregates macro-aggregates between the two tillage systems (Figure 2). (0.25–0.053 mm) were higher under RNT than CT. TEB in These findings are in contrast to other studies indicating aggregates are shown in Figure 5. There are similar that the impacts of tillage on SOM and nutrient content distribution patterns of TEB for RNT and CT. The TEB vary greatly between aggregates. For example, for the within all fractions are significantly higher under RNT than Highlands of Madagascar, Razafimbelo et al. (2008) report CT, except for the silt + clay fraction (<0.053 mm). that SOC is mainly associated with microaggregates (<0.02 mm) under NT with no significant differences apparent for CT among macro-, meso- (0.02–0.2 mm), and Discussion microaggregates (<0.02 mm). For Fluvisols of southeastern This paper reports results on SOC and nutrient distribution Nigeria, Onweremadu et al. (2007) report that SOC in within aggregates under RNT and CT in a subtropical aggregates vary according to tillage method. They found

ª 2010 The Authors. Journal compilation ª 2010 British Society of Soil Science, Soil Use and Management, 27, 28–35 Distribution of nutrients in soil aggregates 33

20 aggregate fractions is higher under NT than CT except for RNT CT the silt + clay fraction. For exchangeable cations, concentrations of divalent cations (Ca2+and Mg2+) are 19 higher than the monovalent cations (K+ and Na+)in macroaggregates, perhaps because divalent cations are more tightly held at the exchange complexes of macroaggregates. 18 For example, the average Ca2+concentration (13.9 and 13.3 cmol ⁄ kg for RNT and CT, respectively) within 17 aggregates represent 73.9% of the TEB (18.8 and 18.0 cmol ⁄ kg for RNT and CT, respectively); Adesodun et al. (2007) report similar results. Moreover, the concentrations of 16 SOC, total N and exchangeable Ca2+ are higher under RNT Total exchange bases (cmol/kg) than CT, which implies that soil tillage management may play a crucial role in sustaining agricultural productivity in 15 >4.76 4.76–2.00 2.00–1.00 1.00–0.25 0.25–0.053 <0.053 this system. Aggregate size (mm) Although the distribution patterns of SOC, total N, exchangeable Ca2+,Mg2+ and TEB in aggregates were not Figure 5 Total exchangeable bases in aggregates under different affected by tillage, the size distribution patterns are different tillage regimes (RNT, combination of ridge with no tillage in a rice between the two tillage regimes (Table 1). The proportion of rotation ecosystem; CT, conventional tillage in a rice rotation macroaggregates (>1.00 mm) is higher under RNT than CT, ecosystem). probably because tillage disrupted soil macroaggregates and reduced macroaggregates by 67% compared with RNT (Table 1). The finding that tillage methods do not change the that the highest SOC was distributed in 4.75–2.0 mm distribution patterns of SOC and nutrients in aggregate-size aggregates under MT, whereas 1.0–0.5 mm had the highest fractions, but do change the size distribution of aggregates, SOC under CT. However, for the present study, the indicates that tillage influences soil fertility through changes distribution trends in SOC, total N, exchangeable Ca2+, in soil structure. Mg2+ and TEB in each size fraction are similar for both tillage regimes. The results indicate that tillage treatments Conclusions have little impact on the distribution patterns of these soil quality parameters. Wright & Hons (2005) also report that Soil organic carbon, total N, exchangeable Ca2+,Mg2+ SOC concentrations are similar among aggregate-size and TEB in aggregates are similar under RNT and CT. fractions between NT and CT at 5–15 cm for a south- This indicates that tillage affects soil fertility through central Texas soil. Madari et al. (2005) found that for a changes in soil structure in this subtropical purple rice soil Ferralsol in Brazil, the difference in SOC distribution ecosystem. CT significantly reduces macroaggregates to between aggregate-size fractions is greater under cultivation smaller ones, thus aggregate stability was reduced by 35% than forest, regardless of the tillage system. Impacts of compared with RNT, further indicating that tillage tillage on SOC in different size fractions vary greatly practices led to soil structural damage. The concentrations because of many factors, including climate, soil type, of SOC, total N, and other nutrients are also significantly texture, pH and dominant mineralogy (Mbagwu & Piccolo, higher under RNT than CT, implying that RNT may be an 1998). In the present study, climate, soil type, texture, pH ideal enhancer of soil productivity in this subtropical rice and dominant mineralogy are identical, as are the crop soil ecosystem through improving soil structure which rotations. The factors affecting SOC and other nutrient leads to the protection of SOM and nutrients, and the distributions are limited to tillage management and soil maintenance of higher nutrient content. This can structure. Therefore, the results indicate that the distribution subsequently contribute to greater crop yields under a RNT patterns are mostly governed by soil structure. Other regime. evidence substantiates this conclusion such as similar patterns in soil pore-size distribution under RNT and CT Acknowledgements (Shi et al., 2008). This study shows that SOC, total N, exchangeable Ca2+ We are grateful to the Natural Science Foundation of China and Mg2+ in aggregates are higher for RNT than CT. The (No. 40501033) and National Key Technology R D Program trend in exchangeable Ca2+,Mg2+,K+ and thus TEB in CT (2007BAD87B10). We wish to acknowledge useful and NT is similar, while the concentration of TEB in all suggestions by the reviewers and the editor.

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