The impact of cover crop and fertilizer application on the spatial patterns of denitrification genes nirK, nirS, and nosZ in soil
Denitrification is a microbially mediated, multi-step reduction process that converts nitrate (NO3) to nitrogen (N2) gas. This process is a key pathway for the loss of bioavailable nitrogen (N) from the soil, and can produce nitrous oxide (N2O), a potent greenhouse gas. Many studies that investigate denitrification, including those in agricultural ecosystems, fail to take into account the denitrifying community. Therefore, little is known about the impact of agricultural management practices on the ecology of the soil denitrifying community. The three objectives of this study are to 1) determine how two agricultural management practices, cover cropping and fertilizer placement, affect the abundance of nitrogen cycling genes, 2) map the spatial distribution of nitrogen cycling genes, and 3) determine the environmental drivers of these genes. Treatments were a factorial combination of two fertilizer applications (broadcast and subsurface banded) and two cover crops (vetch and rye) in a randomized split block design with three replicates. The abundance of bacteria (16S), as well as denitrification genes (nirK, nirS, and nosZ2) were measured with qPCR. Spatial patterns were mapped using distance weighted interpolation.
Each gene analyzed is this study had a unique spatial distribution in the soil profile and was differently affected by agricultural management factors. Overall bacterial abundance was not changed by cover crop or fertilizer, however, their spatial distribution in the soil were altered. The bacterial abundance peaked around the subsurface band of fertilizer and the corn root. nirK concentrations were significantly altered by cover crop with the highest abundances in the rye plots (4.27x107 genes/g soil). In contrast, nirS concentrations were significantly altered by fertilizer placement with the highest abundances in the broadcasted plots (2.51x107 genes/g soil). nosZ concentrations were significantly altered by both cover crop and fertilizer placement. Each gene was highly correlated with depth and nitrate. 16S and nirK were also correlated with soil moisture. Distance from the corn plant does not correlated with any of the genes. While the bacterial communities peaked around the banded poultry litter, there was no evidence for a “hotspot” of denitrifiers around the subsurface band of fertilizer. These results show that agricultural management affects both gene abundances and spatial distribution and can alter the community composition and structure, possibly leading to changes in the functionality of the nitrogen cycling microbial communities.