Effect of Temperature On SOM Decomposition Modulated by Soil Moisture Content.

Increasing atmospheric CO2 along with other greenhouse gases has been implicated in global climate change. Increased temperature could increase soil organic matter (SOM) decomposition resulting in a positive feedback for climate change. There are constraints in predicting effect of temperature on SOM decomposition due to uncertainties in vulnerability of older or younger SOM and interacting effects of soil water. This uncertainty is further complicated by the effect of temperature on microbial communities. The objective of this research was to assess the effect of temperature and soil water on soil microbial respiration, community structure and vulnerability of SOM. The soil was a Kennebec silt loam previously planted to C3 crops prior to cultivation of continuous corn (Zea mays L.) established in 1990. The experiment design was a complete factorial with three temperatures (15°C, 25°C, 35°C) and three soil water potential levels (-1 MPa, -0.03 MPa and -0.01 MPa) with four replications. Measurements included respiration, permanganate oxidizable C, microbial community using PLFA monitored over 120 days incubation. Cumulative respired CO2 data were fitted closely to a first-order kinetic model. There was significant interaction between temperature and soil water on cumulative respired CO2-C which was highest at 35°C for 0.01 MPa increasing from 35ug C g-1 soil on day 3 to 1341 ug g-1 soil for day 120. It was least at 15°C for -1 MPa soil water, increasing from 5 ug C g-1 soil on day3 to 193 ug g-1 soil for day 120. The mineralization rate (K) was lowest for -1 MPa but not significantly different for -0.01 and -0.03 MPa. Results of delta 13C-CO2 varied over time but indicate initial depletion, which was highest for the -0.01 MPa at 25°C from -13.4 to -20.45‰. Permanganate oxidizable C fluctuated significantly with time for all treatments over the course of incubation. Therefore, rising temperature will increases soil C mineralization, but the magnitude depends on soil water content.