The controlling factors and the role of soil heterotrophic nitrification from a global review

Nitrification is a key process in the nitrogen cycle, converting the most reduced N compounds to the most oxidized ones. In general, soil NO3− is predominantly produced by two pathways: i) oxidation of NH4+ via nitrite (NO2−) to nitrate (NO3−) (autotrophic nitrification, An) and ii) oxidation of NH4+ and organic N to NO3− (heterotrophic nitrification, Hn). While, much research has gone into the study of An and much less is known about Hn, although a growing number of studies demonstrate the importance of Hn to N cycling in many terrestrial ecosystems. In this paper, we collected 491 observations from 84 publications (covering cropland, forest and grassland ecosystem) that also considered Hn. Results show that Hn accounts for 24 %, 32 % and 43 % of NO3− production in forest, cropland and grassland soils, respectively. The gross rates and the contribution of Hn to total nitrification are significantly and negatively affected by temperature (P < 0.05) rather than moisture. Although soil pH and C:N ratio was reported to affect Hn, only pH has a significant negative effect (P < 0.05). And Hn is reported to occur over a wide pH range (pH 3.6–7.0). Neither the gross rate nor the contribution of Hn is significantly related to the C and N additions (P > 0.05). Thus, the effect of soil C and N substrates needs further investigation. Interestingly, Hn is significantly related to other heterotrophic processes, such as dissimilatory nitrate reduction to ammonia (DNRA), NO3− immobilization (INO3) and organic N mineralization (Morg) (P < 0.05). These correlations provide potential insights that the occurrence of Hn probably contributes to the maintenance of inorganic N balance in soils, since the growth of heterotrophs was proposed independent of their nitrification. Conclusively, Hn occurs in cropland, forest and grassland ecosystems, which is much more widespread than previously thought. Its occurrence is significantly related with temperature and soil pH, while its correlation with soil C and N substrates and other N transformation processes needs further investigation.