Composition of soil recalcitrant C regulates nitrification rates in acidic soils

In acidic soils heterotrophic nitrification is considered to be an important NO₃⁻ production process. Previous studies have demonstrated that the content of soil C and soil C:N ratio was positively correlated with heterotrophic nitrification. However, relatively high heterotrophic nitrification rates also observed in cultivated black soils, with relatively low soil organic C content and soil C:N ratio compared to forest soils. To obtain a mechanistic understanding of heterotrophic nitrification the simultaneously occurring gross N transformations were quantified in five cultivated soils (SC) and six forest soils (SF) in subtropical region, ten cultivated soils (TC) and seven forest soils (TF) in temperate region subsequently analysed with respect to the soil physicochemical properties (pH, C:N ratio, SOC, TN), the chemical compositions of soil organic C (recalcitrant and labile organic C) and microbial communities (abundance of fungi, bacteria, amoA gene of AOB and AOA). The results showed that a significantly higher heterotrophic nitrification rate was observed in TC (0.52 mg N kg⁻¹ d⁻¹) compared to the other soils. The total content of soil C and soil C:N ratio could not explain the occurrence of heterotrophic nitrification but recalcitrant C fractions of carbonyl C and aromatic C, and the ratio of recalcitrant C to labile C (R:L ratio) was significantly positively correlated with heterotrophic nitrification rates (P < 0.01). This was further confirmed by structural equation modeling (SEM) that the R:L ratio was the most important indicator, suggesting that chemical composition of SOC is likely to be key to explain and predict heterotrophic nitrification rates across a large range of soils including soils high in black carbon such as chernozems, old grassland soils and/or anthropogenic soils such as anthrosols (Terra preta). In addition, heterotrophic nitrification rates were significantly positively correlated with fungal gene copy numbers (P < 0.05), providing a strong indication that fungi might be the predominant organisms carrying out heterotrophic nitrification.