Heterotrophic nitrification of organic nitrogen in soils: process, regulation, and ecological significance.

Heterotrophic nitrification is regarded as an eternal mystery in the nitrogen (N) cycle, although it was first reported more than 100 years ago. In this review, we discuss microbial mechanisms driving heterotrophic nitrification of organic N (OHORG) in soil and their modulations by pH, and carbon (C) and N contents. In acidic and oligotrophic soils, OHORG may occur as endogenous respiration or during oxidation of recalcitrant organics and cell lysis. Likely in soils with a low C:N ratio and a high pH, C limitation of N immobilization creates conditions for the transformation of dissolved organic N to nitrate (NO3−). Fungi with a deficiency in ammonia mono-oxygenase can drive OHORG. Heterotrophic nitrifiers include not only acid-tolerant, nitrophobic species, but also acid-sensitive, nitrotolerant species. There is concern about OHORG in soil because of its contribution to NO3− production and nitrous oxide (N2O) emissions. Therefore, reliable measurements of soil heterotrophic nitrifying activity are needed. We propose more biochemical surveys to understand the pathway for OHORG in ecologically relevant species, considering the challenges we are facing in managing heterotrophically derived nitrification to prevent N losses from cropland, forest, and grassland soils. [ABSTRACT FROM AUTHOR]