Organic amendment impact on SOC dynamics in dry tropics: A possible role of relative availability of inorganic-N pools

Abstract

Organic amendment in agro-ecosystems positively affects the soil organic C (SOC) dynamics, both alone and in combination with mineral fertilizers. Therefore, it is being widely advocated in recent times for the restoration of SOC. Limited understanding exists on how these agro-management practices affect SOC dynamics in the dry tropical agro-ecosystems. We, therefore, aim to investigate the impact of sole organic and combined (organic+inorganic) amendment on SOC dynamics, taking the precursor native forest as a reference. SOC, soil CO2 efflux, soil physico-chemical, microbial and aggregate characteristics were observed in these systems. Our results demonstrate that organic and combined amended systems behaved distinctly as compared to reference system. However, combined amended system was found more C protective as compared to organic amended system. Further, combined amended system showed higher SOC, soil bulk density, NH4+-N, NH4+-N to NO3−-N ratio, microbial biomass C/N ratio (MBC/MBN), β-glucosidase activity, mineral fractions, and aggregate-associated total C as compared to organic system. However, soil CO2 efflux, pH, moisture content, NO3−-N, MBC, MBN, percent labile C in meso- and micro-aggregate, and labile C stock in micro-aggregate showed an opposite trend. Soil moisture and macro aggregate water stability were found important drivers of SOC dynamics in dry tropical ecosystems. SOC dynamics across the studied agroecosystems was found associated with the shift in MBC, MBC/MBN and β-glucosidase activity. The consequent shift in the relative availability of soil inorganic-N pools (i.e. soil NH4+-N to NO3−-N ratio) was identified crucial in this regard. Its relationship with soil microbial biomass, in addition to bulk density, aggregate size distribution and labile C concentration and stock in meso- and micro-aggregates depicts its possible importance in SOC dynamics. Results suggest that higher soil NH4+-N to NO3−-N ratio in combined amended system may be responsible for its relatively higher C protective nature. It might be attributed to its relationship with labile C in meso- and micro-aggregate fractions; however, micro-aggregate appears to be relatively more important in this regard.