Dissimilatory nitrate reduction to ammonium (DNRA) is a microbially-mediated process instrumental to the regulation of 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. In a preliminary study, we found that upland agricultural soils exhibited DNRA rates high enough to represent an important N-retention process in all management practices tested, regardless of differences in microbial community composition. To characterize the role of environmental conditions on regulating these rates, we conducted a study using soil from field trials in Urbana, Illinois, and used lab manipulations to achieve dual moisture and nitrate gradients. We used 15N-based techniques to measure rates of DNRA and related N-cycling processes.
Results/Conclusions
DNRA rates exhibited a pronounced bimodal response across the moisture gradient, peaking at 8 ugN/g/d at 18% gravimetric soil moisture, dropping back to near zero between 22 and 28%, and peaking again at 30% soil moisture. Rates increased across the nitrate gradient, reaching their maximum at 20 ugN/g. There was a strong significant interaction between these two soil characteristics, wherein increased nitrate at a given moisture level enhanced DNRA rates. Nitrous oxide (N2O) flux from denitrification increased exponentially as soil moisture rose from 21%. The soil moisture range between 22 and 28%--where DNRA was repressed--coincided with the first appearance of denitrification N2O flux. The occurrence of DNRA at soil moistures low enough to completely prevent denitrification suggests that DNRA acts as an alternative nitrite reduction pathway when reduction via denitrification has been inhibited by the presence of oxygen. Meanwhile, higher soil moistures that inhibit oxygen diffusion into soil yet promote nitrite diffusion through pore water could be conducive for the co-occurrence of denitrification and DNRA. Our findings demonstrate that DNRA can occur both within both the conventionally recognized reducing conditions characteristic of flooded ecosystems, as well as unsaturated soils of upland ecosystems.