Soil carbon efflux and sequestration as a function of relative availability of inorganic N pools in dry tropical agroecosystem

• Soil temperature and SMC determine SOC dynamics affecting soil NH4+-N/NO3−-N ratio (ANR).
• Organic amendment increases SOC increasing ANR by shift in microbial community.
• Organic amendment increases SOC and SCE close to native forest ecosystem.
• Organic amendment may deny positive global climate change-carbon cycle feedback.

The complex interaction between nitrogen (N) availability and soil organic carbon (SOC) dynamics in the nutrient-limited dry tropical agroecosystem requires investigation in the changing climatic conditions. Organic amendment shows a direct linkage between nutrient availability and SOC dynamics, and improves SOC by mediating soil physicochemical and biological properties. Therefore, plots under various ages of organic cultivation (viz., organic plots under one (O1), three (O3), five (O5) and ten (O10) years) along with nearby native forest ecosystem (REF) were taken in this study to explore the linkage between the nutrient availability and SOC dynamics. Soil moisture content (SMC), soil temperature, SOC, soil carbon efflux (SCE), microbial biomass carbon (MBC), soil ammonium-N (NH4+-N) and nitrate-N (NO3−-N) were investigated. SMC showed positive (power and linear) while the soil temperature showed polynomial (quadratic) relationship with SCE. Interestingly, the soil temperature and SMC also showed contrasting relationships with the soil ammonium-N:nitrate-N ratio (ANR) in the summer season. Further, the ANR was also observed to indirectly govern SOC by affecting MBC and SCE, particularly in summer. It suggests that the long term organic amendment indirectly sequesters SOC by moderating SCE, through increase in the ANR, possibly by affecting the microbial community associated with nitrification. Therefore, consideration of the ANR in SOC management assessment in the dry tropical agroecosystem, owing to its significant coverage and C-sink potential, may help to mitigate the rapidly increasing, CO2-induced global climate change. It also indicates that the organic amendments may potentially nullify the predicted positive climate change-carbon cycle feedback. Thus, inclusion of the ANR in climate models may better predict the future course of climate.