Nitrate Reduction to Ammonium in Anaerobic Soil

Abstract

AbstractKnowledge regarding the pathway and soil conditions necessary for NO3‐ Conversion to NH4+ and organic N, a process which conserves soil N, is limited. The influence of rice straw (2.5 mg C/g soil), methanol (1.0 mg C/g soil), and glucose (1.0 mg C/g soil) on the fate of 100 µg/g 15N‐labelled NO3‐‐N was studied in Crowley silt loam. The fraction of the added NO3‐‐N in the NO3‐, N2, NH4+, and organic N forms was determined after 4‐day incubations under an Ar atmosphere at 30°C. In glucose‐amended soil 9 and 19% of the applied NO3‐‐N was recovered as NH4+‐N and organic N, respectively. A 1‐day preincubation with glucose (0.5 mg C/g soil) before the 4‐day incubation with added NO3‐ and glucose increased labelled NH4+‐N and organic N recovery to 36 and 34% of the added NO3‐‐N, respectively. The corresponding values for rice straw‐ and methanol‐treated soil and soil containing no added energy source were each less than 5%.Nitrate‐N reduction to NH4+‐N increased to 20.5% after a 4‐day incubation when soil with no C amendment was incubated under Ar for 20 days before NO3‐ addition. The redox potential was −260 mV upon NO3‐ additionTransformation of significant amounts of NO3‐ to NH4+ and organic N required intensely reduced soil conditions. The reaction was apparently not suppressed by NH4+. Evidence indicated that NO3‐ was reduced to NH4+ by a nonassimilatory pathway in which NO3‐ functioned as a terminal electron acceptor.