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

An in situ N-15 tracing study was conducted to investigate the characteristics of soil mineral nitrogen (N) production and conservation in acid forest soils of subtropical 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 mu g N g(-1) soil day(-1), and immobilization of NH (4) (+) increased logarithmically with the increase in total gross N mineralization. Oxidation rates of ammonium (NH (4) (+) ) in the acidic forest soils (pH from 4.1 to 4.7) were low, ranging from 0.12 to 0.65 mu g N g(-1) day(-1). The oxidation of organic N, that is the heterotrophic nitrification, was an important nitrate (NO (3) (-) ) production process and approximately 17.2 % to 74.9 % of total NO (3) (-) 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 increase in the soil C/N ratio (R (2) = 0.71, p < 0.05) and heterotrophic nitrification rate increased significantly with the increase in soil C/N ratio (R (2) = 0.92, p < 0.01). Significant positive correlations were also observed between the NO (3) (-) 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 immobilization of NO (3) (-) may be important N transformation pathways affecting ecosystem productivity.