Dependence of Moisture Content on Microbial Mineralization of Soil Organic Carbon.

In terrestrial ecosystems, soil microbial respiration is one of the largest sources of carbon (C) emissions to the atmosphere. Abiotic factors such as temperature and moisture control microbial processes pertinent to soil C decomposition, but moisture effects are less explored as compared to temperature, especially under field conditions. The aim of this study was to determine how soil moisture content controls C mineralization processes under field conditions. The present study was conducted at the West Tennessee Research and Education Center in Jackson, TN. Soybean (Glycine max L.) plants were subjected to three–irrigation treatments, including: drought (rainout shelters), irrigated and rainfed (non-irrigated) during the experiment. Drought was simulated using rainout shelters from June to October 2018, while the irrigated treatment received irrigation at critical growth stages of soybean starting at R₃ (reproductive stage three: pod development) in addition to the rainfall. Soil respiration was measured weekly using static chamber approach. Soil samples were collected in May, July, September and November 2018 from 0-5, 5-15 and 15-30 cm depths to determine microbial biomass C, dissolved organic C, and hydrolytic enzyme activities in response to moisture changes. Results showed that cumulative CO₂ emission (June-October) from drought plots was 3393 kg ha^-1 which was 32% and 33% lower than rainfed and irrigated plots, respectively. Decreased CO₂ emission due to drought was experienced only after a month of rain-out shelter establishment. At 0-5 cm depth, β-glucosidase activity in drought plots was 27% and 45% lower than rainfed and irrigated plots, respectively. Results from this study strengthen our understanding on how moisture content influences C cycling under field conditions in tandem with other biotic and abiotic changes in natural environment.