Dissimilatory nitrate reduction to ammonium (DNRA), not denitrification dominates nitrate reduction in subtropical pasture soils upon rewetting

This study combines the direct quantification of N2 and N2O with a numerical 15N tracing model to establish the relationship between denitrification and DNRA in pasture soils after wetting. Soil microcosms were fertilised with NH4NO3 (35 μg N g−1 soil) using a triple 15N labelling approach, wetted to four different water-filled pore space (WFPS) levels and incubated over two days. The abrupt increase in soil moisture triggered a burst of N2 and N2O emissions, with peak fluxes of N2 > 13.1 μg N g−1 soil day−1 at high soil moisture levels. At 95% and 80% WFPS, denitrification was dominated by N2 emissions, with the N2/(N2+N2O) ratio ranging from 0.5 to 0.9. At 60% and 40% WFPS, the N2/(N2+N2O) ratio ranged from 0.2 to 0.3, showing N2O as the main product of denitrification. The wetting of dry pasture soils resulted in increased DNRA rates across soils and WFPS. Both denitrification and DNRA increased exponentially with WFPS and responded to NO3− availability, demonstrating both processes as N-substrate driven. The labile C/NO3− ratio was not correlated to DNRA rates and as such did not explain NO3− partitioning between denitrification and DNRA, likely due to the high C availability in the pasture soils. Increasing labile C availability stimulated heterotrophic soil respiration, which had no effect on denitrification rates, but increased DNRA. Increased soil respiration is likely to have lowered the soil redox potential, promoting a shift of NO3âˆ...
Source: Soil Biology and Biochemistry - Category: Biology Source Type: research