Fate of C and N during Switchgrass Root Decomposition: Linking Trace Gas Emission, Microbial Biomass, and Extracellular Enzyme Activity

Abstract:

Treatments and measurements

We explored the fate of C and N during the switchgrass root decomposition under contrasting soil pore size distribution (dominant pore Ø> 30um and < 10um) and soil moisture content (40% and 70% Water-filled-pore-space, WFPS). Our unique experimental setting allowed to incubate in-situ grown roots as labeled sources of ¹³C and ¹⁵N. We conducted regular and isotopic analyses of gases (CO₂, N₂O, and N₂), total C/N, dissolved organic C/N, and microbial biomass C/N, to explore the fate of root-derived C and N during the decomposition process. In addition, chitinase activity was measured during the 21 days of incubation, using zymography, a 2-dimensional mapping technique for extracellular enzymes. This allowed us to locate and quantify the chitinase activity in roots and bulk soil separately.

Featured results

Root-derived N₂O emission was greater in large-pore dominated soil (Ø> 30um) compared to small-pore dominated soil (< 10um), and also greater at 70% than 40% WFPS. This trend was consistent with the activity of chitinase on the surfaces of decomposing roots. Interestingly, the influence of root chitinase activity in bulk soil was greater in large-pore dominated soil, compared to small-pore dominated soil. Greater influence of root residue on the enzyme activity in surrounding bulk soil might have led to the overall greater N₂O emission in large-pore soils. Root-derived CO₂ emission and microbial biomass N also tended to be greater in large-pore soils, but greater at 40% rather than 70% WFPS. More aerobic conditions at 40% than 70% WFPS seem to have led to greater microbial respiration due to ample supply of O₂. Also, it indicates that a larger portion of N derived from switchgrass roots goes to N₂O emission at 70% WFPS, while more N is assimilated into microbial biomass at 40% WFPS.