An evaluation of a microbial inoculum in promoting organic C decomposition in a paddy soil following straw incorporation

Abstract

Impacts of a commercially available decay-facilitating microbial inoculum on carbon (C) and nitrogen (N) mineralization were evaluated during decomposition of rice straw in a paddy soil. Two incubation experiments were conducted for 105 days with a typical low-yield high-clay soil in central China to monitor effects of straw and the inoculum on CO2 evolution, as well as dissolved organic C (DOC), NH4 +, NO3 −, and pH under conditions of 15 °C 70 %, 25 °C 40 %, 25 °C 70 %, 25 °C 100 %, and 35 °C 70 % of water-holding capacity (WHC) with adequate N, supplied as urea or manure, respectively. Treatments of 25 °C 70 % WHC, 25 °C 100 % WHC, and 35 °C 70 % WHC generally achieved significant higher CO2 evolution while treatment of 25 °C 40 % WHC had least. This was more evident with added manure compared to urea (P < 0.05). The inoculum generally increased the decomposition of C inputs and the largest increases were in the initial 28 day in treatments 25 °C 70 % WHC, 25 °C 100 % WHC, and 35 °C 70 % WHC; only the 25 °C 40 % WHC actually immobilized C. The CO2 release rates were positively correlated with DOC, but with different slopes within treatments. Despite equivalent N application rates, manure treatments had significantly less N (including NO3 −, NH4 +, and total dissolved N) than those with urea. Incubation of 25 °C 40 % WHC decreased soil pH the least, probably due to relative low moisture causing delayed nitrification. The results implied that the inoculum, especially fungi, would adjust to edaphic and N fertilization in regulating organic C mineralization, during which water potential would exhibit a great role in regulating substrate and nutrient availability.