The effect of C:N ratio on heterotrophic nitrification in acidic soils

Generally, a high carbon to nitrogen (C:N) ratio is considered an important factor inducing heterotrophic nitrification in acidic soils; however, few studies have investigated the regulation of C:N ratio on heterotrophic nitrification. In the present study, a C:N ratio gradient (C:N 23, C:N 19, C:N 15, C:N 10) was regulated in acidic forest soil (SF) and acidic agricultural soil (SC) under 24 h and 30 days incubation, and N-15-labeling techniques combined with acetylene inhibition were used to distinguish between heterotrophic and autotrophic nitrification. Gene copy abundance and soil fungi and bacteria composition were also determined under 30 days C:N ratio regulation. The results showed that N input in C-abundant soils (e.g. SF in this investigation) and C input in N-abundant soil (e.g. SC in this investigation), instead of high C:N ratios, may stimulate soil heterotrophic nitrification. The stimulatory effect of acetic acid on heterotrophic nitrification in SC rather than SF further confirmed our conclusion. Soil fungal populations were significantly and positively correlated with gross heterotrophic nitrification rates and the ratio of heterotrophic nitrification rate to total nitrification rate, in addition to the contribution of heterotrophic NO3- production to total NO3- production in both the N-15-NH4+ and N-15-Glycine treatments, indicating soil fungi could play the key role in driving heterotrophic nitrification. In addition, our results suggested that Mortierella and Trichodenna were the potential fungal species driving heterotrophic nitrification in acidic forest soils, while Exophiala and Acidomelania seemed to be the fungal species driving heterotrophic nitrification in acidic agricultural soils according to their high abundance in C-abundant treatments. Consequently, initial soil C and soil N status should be taken into account when predicting the responses of soil heterotrophic nitrification to C or N soil inputs.