Acidity and Inorganic Nitrogen Concentrations in Soil Solution in Short-term Response to N Addition in Subtropical Forests

The effects of simulated N deposition on acidity and inorganic N concentrations in soil solution collected at 20 cm soil depth under pine forest, needle broadleaved mixed forest, and mature monsoon evergreen broadleaved forest in subtropical China were studied. Additional N (NH4NO3 solution) treatments were established for mature forest: Low N (50 kg N hm-2 a-1), Medium N (100 kg N hm-2 a-1) and High N (150 kg N hm-2 a-1), and Low N and Medium N treatments were established for mixed and pine forests. pH values in soil solution in the control plots of the 3 forests ranged from 3.82 to 4.24, but pH values decreased by 0.08-0.18 in plots treated by spraying additional nitrogen under the forests for 5 months. NH4+-N concentrations in soil solution increased averagely by 74.0%, 355.6% and 57.9% in low N treatment and by 68.4%, 93.2%, and 188.8% in medium N treatment for pine forest, mixed forest, and mature forest, respectively. As in high N treatment it increased by 1254.7% for mature forest. The increases of NO3--N concentrations were by 56.4%-88.5% in low N treatment, by 65.6%-132.2% in medium N treatment, and by 85.5% in high N treatment. The results showed that additional N deposition increased the potential of inorganic N leaching losses and enhanced soil acidity.