Soil respiration in a tropical montane grassland ecosystem is largely heterotroph-driven and increases under simulated warming

Soil respiration, a major source of atmospheric carbon (C), can feed into climate warming, which in turn can amplify soil CO2 efflux by affecting respiration by plant roots, arbuscular mycorrhizal fungi (AMF) and other heterotrophic organisms. Although tropical ecosystems contribute >60% of the global soil CO2 efflux, there is currently a dearth of data on tropical soil respiration responses to increasing temperature. Here we report a simulated warming and soil respiration partitioning experiment in tropical montane grasslands in the Western Ghats in southern India. The study aimed to (a) evaluate soil respiration responses to warming, (b) assess the relative contributions of autotrophic and heterotrophic components to soil respiration, and (c) assess the roles of soil temperature and soil moisture in influencing soil respiration in this system. Soil respiration was tightly coupled with instantaneous soil moisture availability in both the warmed and control plots, with CO2 efflux levels peaking during the wet season. Soil warming by ˜1.4 °C nearly doubled soil respiration from 0.62 g CO2 m−2 hr−1 under ambient conditions to 1.16 g CO2 m−2 hr−1 under warmed conditions. Warming effects on soil CO2 efflux were dependent on water availability, with greater relative increases in soil respiration observed under conditions of low (with a minimum of 2.6%), compared to high (with a maximum of 64.3%), soil moisture. Heterotrophs contributed to the majority of soil CO2 efflux, with respiration remaining unchanged when roots and/or AMF hyphae were excluded as the partitioning treatments were statistically indistinguishable. Overall, our results indicate that future warming is likely to substantially increase the largely heterotroph-driven soil C fluxes in this tropical montane grassland ecosystem.