Monsoon-phase regulates the decoupling of auto- and heterotrophic respiration by mediating soil nutrient availability and root biomass in tropical grassland

Abstract Soil respiration is considered as one of the most sensitive parameters regulated by the interplay of different soil biophysical parameters, ground cover and climatic conditions. As per the global predictions, monsoon variability in South Asian countries can affect various soil processes. However, studies exploring the interactive role of vegetation and precipitation conditions on soil respiration and its components (viz., auto- and heterotrophic) are lacking. A mini-trenching method was performed to explore the alterations in auto- and heterotrophic components of soil respiration as well as their regulatory variables in grassland vegetation under different summer monsoonal phases. Three vegetation densities (viz. low, medium and high) based on herbaceous cover, and three monsoon phases (viz. onset, active and end) of the tropical summer monsoon were considered. The findings revealed that heterotrophic respiration was the major contributor to the total soil respiration under medium- and high-density vegetation with percent contribution of 62–80% and 53–88% respectively. Contrastingly, autotrophic respiration was the major contributor (54–56%) to soil respiration in low-density vegetation during active- and monsoon-end phases. Amongst the three monsoon phases, variation in soil respiration components was most pronounced during the active-monsoon phase. Structural Equation Modelling results revealed microbial biomass and root biomass as key regulators of auto- and heterotrophic respiration. Remarkably, a partitioning of root biomass parallel to the soil respiration was observed during monsoon-end phase where coarse root biomass and fine root biomass emerged as regulators of auto- and heterotrophic respiration, respectively. Moreover, for monsoon-onset and -end phases, a completely different set of variables involved in regulation of soil respiration components were identified. Our results revealed that the monsoon-phase mediates the root and nutrient dynamics which led to the differential decoupling of the components of soil respiration. Thus, inclusion of (below)-ground cover and monsoon-phase wise variation in soil biophysical parameters in the global biogeochemical models would be more helpful for better prediction of carbon balance in different ecosystems.