The riparian buffer-zone is a management practice implemented by farmers to mediate excess nitrogen from agricultural fertilizer application. While the buffer zone’s efficiency to either retain or respire the nitrogen is affected by the interaction between plants and soil microbes, little is known about how plant variation and subsequent microbial communities impact the ecosystem function of the buffer zone. Our objective is to use PCR of diagnostic nitrogen-processing genes to evaluate the potential for microbe-mediated nitrogen-cycling in different cropping systems with plant species that can provide additional economic value to buffer zone.
We conducted a multi-site, multi-year field experiment. Two sites contained the full set of buffer zone planting treatments (bioenergy grass, forage grass, edible shrubs, floral shrubs) and the other two had locally-appropriate subsets (grasses or shrubs only). At all sites, we used a corn-soy rotation as a reference control. DNA was extracted from soil samples, and nitrogen cycling genes (archaeal amoA, bacterial amoA, nifH, nirK, nirS, nosZ) were quantified using fluidigm with microfluidic 96 X 96 well Fluidigm Gene Expression chip. The qPCR data set was analyzed using MANOVA on SAS and normality of the data were checked using the Shapiro-Wilk Normality test.
Results/Conclusions
For all nitrogen cycling gene we found that site and year were significant sources of variation (archaeal amoA (R2=0.384), bacterial amoA (R2=0.499), nifH (R2=0.361), nirK (R2=0.338), nirS (R2=0.48), nosZ (R2=0.413); p<0.0001 for all tests). Nevertheless, our main variable plant type was significant only for bacterial and archaeal amoA and nifH genes. The mean copy numbers of archaeal amoA genes were lower in bioenergy grass plots compared to our control. Since our quantification of each gene closely relates to the buffer zone’s potential to process nitrogen, the profile we see with bioenergy crop is a desirable trait, suggesting lower nitrification potential and potential higher retention of N in these buffer zones. We also found treatment forage grass plots had lower nifH copy numbers compared to control.
The result helps us to understand two important features about plant and microbial interaction at the buffer zone. Despite the significant variation across the samples due to site-specific features, we were able to use plant type to explain the nitrification potential of the buffer zone. On the other hand, it tells us that the denitrification potential will highly vary across location to location and season to season and plant type is not significant.