Gaseous N emission during simultaneous nitrification–denitrification associated with mineral N fertilization to a grassland soil under field conditions

Gaseous emission of N from soil is essentially related to microbial activity, which includes nitrification and denitrification. In grassland soils subjected to high annual rainfall and intensive grazing, aerobic and anaerobic zones can develop in close proximity in the upper few centimeters of the soil, hence nitrification and denitrification can occur concurrently and adjacently. The objective of this study was to demonstrate the occurrence of simultaneous nitrification and denitrification following the addition of NO3− and NH4+ fertilizers to a grassland soil under field conditions. After applying 100 kg NO3−–N ha−1, ca. 25–75 kg ha−1 of the added N disappeared from the mineral N pool in 7 days. Emission of N2O and total denitrification was substantial, and 5–22 kg ha−1 of the added N was evolved as gaseous N. In the soil where NH4+–N was added, almost 50% of the N that disappeared from the mineral pool could not be accounted for. A substantial proportion of the applied N (7 kg ha−1) was evolved as gaseous N. The rate and amount of N loss and fluxes of N2O from both NO3− and NH4+ sources were greater in soils at 84% water-filled pore space (WFPS) compared with 71% and 63% WFPS. Emission of N2O from soil following NO3− addition can therefore be attributed to denitrification. In the soils to which NH4+ was added, accumulation of NO3−–N was greatest at low moisture content (63% WFPS), while the gaseous emissions were greatest at the highest WFPS. The study demonstrated that nitrification and denitrification occur simultaneously in compacted silty grassland soils at moisture conditions close to field capacity.