Microbially mediated dissimilatory nitrate reduction to ammonium (DNRA) plays a pivotal role in regulating ecosystem nitrogen (N) retention versus loss. By converting nitrate to ammonium, DNRA retains N in ecosystems to support primary productivity, reduces nitrate leaching to ground- and surface waters, and competes with denitrification to decrease gaseous dinitrogen and nitrous oxide losses. Global change factors or land management practices that affect DNRA will, therefore, have cascading effects through the ecosystem N cycle. Despite its importance, DNRA is generally disregarded in upland terrestrial ecosystems because of the misconception that the process is restricted to reducing conditions typically found in flooded environments. We conducted a study to determine the genetic and environmental potential for DNRA to occur in upland agricultural soils under a variety of management practices that may alter the soil microbial community and edaphic conditions. We collected soil samples from field trials in Urbana, Illinois throughout the growing season to capture variation in plant-microbe interactions and environmental conditions. We used 15N-based techniques to measures rates of DNRA and related N-cycling processes. We quantified N-cycling functional gene abundances using qPCR, and we characterized microbial community diversity and composition using Illumina sequencing of the functional genes and 16S rRNA.
Results/Conclusions
Soil samples from all agricultural field trials exhibited detectable DNRA rates, with rates as high as 4 µg-N g-1 dry soil h-1. Rates were not consistently affected by any specific management practice over the growing season. Instead, they showed a positive relationship with soil moisture early in the growing season when soil NO3- was high (P < 0.01), and a positive relationship with soil NO3- later in the growing season when soil NO3- was below 3 µg-N g-1 dry soil (P < 0.05). In contrast to the consistent rate response to NO3- and moisture across all treatments, DNA sequencing of the 16S rRNA gene region revealed strong management practice treatment effects on microbial community composition (i.e., crop perenniality, tillage, and crop residue management). However, these treatment differences were not reflected in DNRA response to environmental conditions, as all treatments exhibited the same relationship with soil NO3- and moisture regardless of community composition. This may be due to the homogeneity in community structure within functional groups, where no significant treatment differences were found in the nosZ, nirK, nirS, or nrfA genes. We conclude that upland agricultural soils have strong genetic potential for DNRA to occur, with environmental conditions regulating process rates.