Assessment of the importance of dissimilatory nitrate reduction to ammonium for the terrestrial nitrogen cycle

The nitrogen (N) cycle contains two different processes of dissimilatory nitrate (NO(3)(-)) reduction, denitrification and dissimilatory NO(3)(-) reduction to ammonium (DNRA). While there is general agreement that the denitrification process takes place in many soils, the occurrence and importance of DNRA is generally not considered. Two approaches have been used to investigate DNRA in soil, (1) microbiological techniques to identify soil microorganisms capable of DNRA and (2) (15)N tracing to elucidate the occurrence of DNRA and to quantify gross DNRA rates. There is evidence that many soil bacteria and fungi have the ability to perform DNRA. Redox status and C/NO(3)(-) ratio have been identified as the most important factors regulating DNRA in soil. (15)N tracing studies have shown that gross DNRA rates can be a significant or even a dominant NO(3)(-) consumption process in some ecosystems. Moreover, a link between heterotrophic nitrification and DNRA provides an alternative pathway of ammonium (NH(4)(+)) production to mineralisation. Numerical (15)N tracing models are particularly useful when investigating DNRA in the context of other N cycling processes. The results of correlation and regression analyses show that highest gross DNRA rates can be expected in soils with high organic matter content in humid regions, while its relative importance is higher in temperate climates. With this review we summarise the importance and current knowledge of this often overlooked NO(3)(-) consumption process within the terrestrial N cycle. We strongly encourage considering DNRA as a relevant process in future soil N cycling investigations