Tracing controls of autotrophic and heterotrophic nitrification in terrestrial soils

Abstract

The major mechanisms and drivers of autotrophic (ONH4) and heterotrophic (ONrec) nitrification in terrestrial soils are still not well understood. A synthetic analysis of the factors influencing ONH4 and ONrec in cropland, forest, and grassland soils was thus conducted to establish an evidence-based prediction after compiling data from 71 reports. This synthesis applied single-factor regression approach to highlight major relationships, with variable importance measures provided using random forest (RF) models. Empirical evidence suggests that N mineralization (Mtot) modulates ONH4 fluxes in croplands and grasslands. pH has more of an effect than Mtot on ONH4 fluxes in acidic cropland and forest soils. Fungal biomass can be predictive of ONrec fluxes in strongly acidic forest soils. The ratio of heterotrophic to gross nitrification (Rhn) might increase with forest soil C:N ratios. RF models revealed that soil C:N ratio, soil N (TN), ONH4, Mtot and N mineralization-immobilization turnover (MIT) are important regulators of ONrec fluxes in forests. pH is a significant factor affecting Rhn in forest soils. However, its influence is less than that of soil C:N ratio, TN and Mtot. There is little control from soil C–N status (e.g., TN and soil C:N ratio), microbial C–N status (e.g., Mtot and MIT), and soil pH status over ONrec and Rhn in croplands and grasslands. In summary, this study advances our understanding of determinants to ONH4 and ONrec in various soil habitats. It highlights that acidity-based mechanisms outweigh substrate-based mechanisms in mediating ONH4 in acidic cropland and forest soils. Soil and microbial N status are central to heterotrophic nitrifying activities in forest ecosystems.