Influence of quality and quantity of crop residues on organic carbon dynamics and microbial activity in soil

An incubation study investigated the decomposition patterns, microbial activities, soil aggregates distribution, and soil organic carbon (SOC) contents, using four crop residues; wheat (Triticum aestivum L.), rice (Oryza sativa L.), cotton (Gossypium hirsutum L.), and maize (Zea mays L.), at low and high rates (10 and 20 g kg–1). An average increase of about 35% in cumulative C-CO2 emissions and 26% in soil microbial biomass carbon (MBC) was observed at high residue rates as compared to low rates. A positive and strong correlation (0.96) was found between C-CO2 emissions and MBC, and the effect of residue quality on decomposition was found to be less at high rates. The relative reduction in C-CO2 emissions by adding rice residues may be due to their high silica content. Differences in extracellular enzyme activities (EEA) with different residues indicated the effect of residue quality on microbial activities. Cotton residues increased β-glucosidase and chitinase activities by 41 and 38%, respectively, at high rates than at low rates. High maize residues also markedly increased chitinase activity by 41%, implying higher N cycling and fungal prevalence. Acid phosphatase activity was found to increase by about 25 to 32% with high rice and wheat residues, respectively. Higher EEA at high residues input likely reflected the microbial nutritional limitations. Crop residues raised SOC content from 0.7 under control to 1.0% at low residues input but not at high input. Increased macro-aggregates fraction at high residues input may be attributed to higher microbial activities. In summary, high residues input can minimize residue biochemical quality’s effect on decomposition, with no further increase in SOC content. While a positive effect on the soil MBC, but not on the SOC content, was observed at high residue input, suggesting moderation in the additions of organic amendments is key for SOC buildup. Crop residues, added at 10 g kg–1, may help maintain a positive SOC balance in the arid agroecosystems by moderating higher microbial activities and soil respiration.