Carbon and nitrogen mineralization dynamics in different soils of the tropics amended with legume residues and contrasting soil moisture contents

Seasonal drought in tropical agroecosystems may affect C and N mineralization of organic residues. To understand this effect, C and N mineralization dynamics in three tropical soils (Af, An1, and An2) amended with haricot bean (HB; Phaseolus vulgaris L.) and pigeon pea (PP; Cajanus cajan L.) residues (each at 5 mg g−1 dry soil) at two contrasting soil moisture contents (pF2.5 and pF3.9) were investigated under laboratory incubation for 100–135 days. The legume residues markedly enhanced the net cumulative CO2–C flux and its rate throughout the incubation period. The cumulative CO2–C fluxes and their rates were lower at pF3.9 than at pF2.5 with control soils and also relatively lower with HB-treated than PP-treated soil samples. After 100 days of incubation, 32–42% of the amended C of residues was recovered as CO2–C. In one of the three soils (An1), the results revealed that the decomposition of the recalcitrant fraction was more inhibited by drought stress than easily degradable fraction, suggesting further studies of moisture stress and litter quality interactions. Significantly (p < 0.05) greater NH4+–N and NO3−–N were produced with PP-treated (C/N ratio, 20.4) than HB-treated (C/N ratio, 40.6) soil samples. Greater net N mineralization or lower immobilization was displayed at pF2.5 than at pF3.9 with all soil samples. Strikingly, N was immobilized equivocally in both NH4+–N and NO3−–N forms, challenging the paradigm that ammonium is the preferred N source for microorganisms. The results strongly exhibited altered C/N stoichiometry due to drought stress substantially affecting the active microbial functional groups, fungi being dominant over bacteria. Interestingly, the results showed that legume residues can be potential fertilizer sources for nutrient-depleted tropical soils. In addition, application of plant residue can help to counter the N loss caused by leaching. It can also synchronize crop N uptake and N release from soil by utilizing microbes as an ephemeral nutrient pool during the early crop growth period.