Carbon limitation of sediment bacterial production and denitrification in high nitrate low carbon systems

The concentrations and ratios of carbon and nitrogen may affect bacterial denitrification through changes in enzyme or bacterial production, but short-term laboratory and field assays may not account for changes over longer periods. Drainage from agricultural landscapes can have different ratios of C:N depending on the water source, i.e., surface runoff has elevated C: N relative to agriculture tile drainage water. We hypothesized that differences in C: N ratios limit bacterial production and reduce denitrification rates. Sediment cores collected from surface or tile water fed wetlands were incubated in low C: N (1: 1) and high C: N (4: 1) ratios keeping nitrogen concentrations at 10 mg L-1 NO3-N. We used replicated laboratory incubations that were destructively sampled on days 0, 1, 3, 5, 10, and 20 to represent pulsed water inputs to wetlands to test for potential increases in denitrification over time with elevated C: N (4: 1) ratios. Bacterial production and denitrification were estimated using [H-3] leucine incorporation and the acetylene block method. Bacterial production and denitrification increased (p < 0.0001) in response to elevated C: N (4: 1). Carbon was likely preferentially allocated for bacterial production at low C: N as explained by the lack of change in denitrification at the 1: 1 C: N ratio, but facilitated a significant increase in bacterial production throughout the 20 day incubation. The results suggest that if labile carbon were to be increased in subsurface tile drainage, prolonged seasonal water inputs would likely increase nitrate removal as shown by the increased denitrification throughout the 20 day study period. New agricultural conservation practices such as the implementation of cover crops have the indirect benefit of restoring soil organic matter and may improve allochthonous inputs of labile carbon.