Sources of nitrite in a permanent grassland soil

Summary The objective of this study was to separate the observed nitrite (NO 2 – ) concentration in a permanent grassland soil into process‐specific subpools. A laboratory experiment was carried out where either the nitrate and/or ammonium pool was labelled with 15 N at 60 atom % excess. The main N transformations that occurred in this experiment were analysed with a 15 N tracing model extended with a NO 2 – submodel. Techniques that have been used to date have been only able to identify NO 2 – subpools related to nitrification (NO 2 – nit ) and denitrification (NO 2 – den ). With the analysis presented here, we were able to quantify the size of an additional NO 2 – pool in the soil related to organic N turnover (NO 2 – org ). All transformations related to NO 2 – turnover of the three subpools occurred simultaneously. After non‐linear parameter optimization the model predicted that on average NO 2 – den , NO 2 – nit and NO 2 – org pools contributed 57, 33 and 10% to the total soil NO 2 – concentration. The finding that heterotrophic processes can contribute to the NO 2 – dynamics in permanent grassland soils might also have important implications for the understanding of gaseous N production that are tightly linked to NO 2 – turnover. Further work is needed to find out how important the conversion of organic N to NO 2 – is in other soil‐based ecosystems and to identify the microbial groups responsible for this process.