Nitrogen mineralization, immobilization turnover, heterotrophic nitrification, and microbial groups in acid forest soils of subtropical China

An in situ 15 N tracing study was conducted to investigate the characteristics of soil mineral nitrogen (N) production and conservation in acid forest soils of subtrop- ical China. The six experimental soils were strongly acidic (pH ranged from 4.1 to 4.7), except for one soil, from an orange orchard which had a higher pH (5.7) due to lime application. Total gross N mineralization rates ranged from 2.30 to 9.20 μ gN g �1 soil day �1 , and immobilization of NH4 + increased logarithmically with the increase in total gross N mineralization. Oxidation rates of ammonium (NH4 + ) in the acidic forest soils (pH from 4.1 to 4.7) were low, ranging from 0.12 to 0.65 μ gN g �1 day �1 . The oxida- tion of organic N, that is the heterotrophic nitrification, was an important nitrate (NO3 � ) production process and approx- imately 17.2 % to 74.9 % of total NO3 � production was immobilized by soil microbes in these acidic forest soils. The multiple regression analysis showed that the total gross N mineralization rate decreased significantly with the in- crease in the soil C/N ratio (R 2 00.71, p<0.05) and hetero- trophic nitrification rate increased significantly with the increase in soil C/N ratio (R 2 00.92, p<0.01). Significant positive correlations were also observed between the NO3 � immobilization rate (p<0.05), heterotrophic nitrification rate (p<0.01), and fungal biomass. The soil C/N ratio, which depends on dominant vegetation and composition of soil microbial communities, can be an effective predictor of soil mineral N production and conservation in the acid forest soils of subtropical China. Heterotrophic nitrification and