Effects of crop residue incorporation on soil organic carbon pools and nutrient dynamics in semi-arid Alfisols

Introduction

Approximately 140 billion tonnes of biomass are generated from agriculture globally each year, yet in many semi-arid regions more than half of this material is burned in the open a practice that destroys the very organic matter these carbon-depleted soils need most [1,2] . Semi-arid Alfisols, which cover over 300 million hectares across tropical and subtropical Asia, are characterized by low organic carbon content (typically 3-8 g kg⁻¹), poor water-holding capacity, and susceptibility to structural degradation under continuous cultivation [3,4] . Crop residue incorporation offers a direct pathway to reverse SOC decline by adding particulate organic matter that soil microorganisms gradually transform into stable humic fractions [5] . Rice straw, with its high C:N ratio (60-80:1), decomposes slowly and contributes primarily to structural carbon pools, while legume residues with lower C:N ratios (15-25:1) decompose rapidly and stimulate microbial activity and nitrogen mineralization [6,7] . Combining cereal and legume residues may therefore generate synergistic effects: the legume fraction accelerates decomposition and nitrogen supply, while the cereal fraction provides a sustained carbon substrate for long-term stabilization [8] . Most residue management research in Southeast Asia has focused on single-residue systems over 1-2 seasons, leaving uncertainty about the cumulative effects of combined residue incorporation on different SOC pools over multiple years [9,10] . This research aimed to: (a) quantify the effects of three residue treatments on total SOC and its labile, particulate, and mineral-associated fractions after four seasons, (b) measure changes in microbial biomass carbon and enzyme activity, and (c) assess the impact on available N, P, and K [11] .

Reagents and Chemicals

Potassium dichromate, concentrated sulfuric acid, ferrous ammonium sulfate, and diphenylamine indicator were used for SOC determination by the Walkley-Black wet oxidation method. Chloroform, potassium sulfate (0.5 M), and sodium hydroxide were used for microbial biomass carbon extraction by the fumigation-extraction method. Sodium hexametaphosphate served as the dispersing agent for particlesize fractionation. All reagents were analytical grade, purchased from Merck (Darmstadt, Germany) [5,12] .

Soil and Water Quality Analysis

Baseline soil characterization (0-20 cm) confirmed a sandy clay loam texture (56% sand, 22% silt, 22% clay) with pH 6.4, EC 0.23 dS m⁻¹, initial SOC 7.61 g kg⁻¹, and CEC 11.8 cmol₂ kg⁻¹. Irrigation water analysis showed pH 7.1, EC 0.41 dS m⁻¹, and sodium adsorption ratio (SAR) of 2.3, all within acceptable limits for sustained irrigation per FAO guidelines. Soil temperature at 10 cm depth ranged from 24.3°C to 33.7°C across the experimental period [3,13] .

Material and Methods Material

The research was conducted at Kasetsart University's Kamphaeng Saen campus (14°01′N, 99°58′E, 27 m asl) in Nakhon Pathom province, Thailand, from June 2021 to May 2023. Institutional approval was granted (KU-KPS/2021-034, approved 10 May 2021). Three treatments were arranged in a randomized complete block design with four replications: T1 (no residue control), T2 (rice straw at 5 t ha⁻¹ per season), and T3 (rice straw 5 t ha⁻¹ + mung bean residue 2.5 t ha⁻¹ per season). Residues were incorporated to 15 cm depth by rotary tillage within 7 days of harvest [6,9] .

Methods

Soil samples were collected at 0-20 cm depth at the end of each season and at termination (May 2023). Total SOC was determined by Walkley-Black oxidation. Carbon fractionation followed a combined physical-chemical protocol: labile C by 333 mM KMnO₄ oxidation, particulate organic C (POC, >53 μm) by wet sieving, and mineral-associated organic C (MAOC, <53 μm) by difference. Microbial biomass carbon (MBC) was measured by chloroform fumigation-extraction. Available N (alkaline KMnO₄), P (Olsen), and K (ammonium acetate) were analyzed by standard methods. Data were analyzed by one-way ANOVA with LSD at p<0.05 in SPSS 28 [5,11,12] . Available N increased by 48.7%, P by 71.8%, and K by 47.8% under combined residue incorporation compared to the control (Table 2). The legume component contributed disproportionately to N and P availability through its low C:N ratio and rapid mineralization.

Results

Discussion

The 38.6% increase in total SOC after just four seasons of combined residue incorporation is substantially higher than the 15-25% range reported in most single-residue trials of similar duration [8,10] . The synergistic effect of cereal plus legume residues is evident in the disproportionate increase in labile C (73.9%) and MBC (62.7%), which indicates that the legume fraction's rapid decomposition primes microbial communities to process the more recalcitrant rice straw more efficiently [6,14] . The 67.9% increase in POC compared to only 24.6% for MAOC confirms that residue-derived carbon first enters the particulate fraction and transitions gradually to mineral-associated forms through microbial processing and organo-mineral interactions [5,15] . The 71.8% increase in available P under the combined treatment is noteworthy because Alfisols typically fix phosphorus strongly. Organic acid release during residue decomposition may have competed with phosphate for sorption sites, effectively liberating previously fixed P [16] . From a practical standpoint, these results suggest that returning just 7.5 t ha⁻¹ of mixed residues per season can meaningfully rebuild SOC and nutrient status in degraded Alfisols within two years a timeline relevant to the planning horizons of smallholder rice-legume farmers across Southeast Asia [9,11] .

Conclusion

References

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