Constraining the conditions conducive to dissimilatory nitrate reduction to ammonium in temperate arable soils

Here we offer the first assessment of conditions conducive to dissimilatory nitrate reduction to ammonium (DNRA) in temperate arable soils, through an examination of the potential for this process to occur in a range of soils of contrasting characteristics. NH₄¹⁵NO₃ (6.2 g N m^-2, 25 atom % excess ¹⁵N) was applied, and recovery of ¹⁵N in the NH4+ pool taken as indicative of occurrence of DNRA. Up to 5% of applied ¹⁵N was recovered in the NH4+ pool 2 d after addition of N, glucose (44.6 g C m^-2) and l-cysteine (7.7 g m^-2, 0.9 g N m^-2, 2.3 g C m^-2). N15-NH4+ concentrations were positively correlated with soil pH, C-to-NO3- ratio, bulk density, sand content and NO2- concentration, but negatively correlated with soil C and organic N content. Our results demonstrate the potential for DNRA to contribute to N cycling in temperate arable soils, but its detection and significance is likely to depend on the provision of a low molecular weight C source.