Rapid recovery of nitrogen retention capacity in a subtropical acidic soil following afforestation

Understanding soil nitrogen (N) dynamic and availability during afforestation (the conversion of cropland to forest plantations) is critical to maintain forest growth and long-term productivity, especially in rainfall-rich, subtropical region. However, only few studies have investigated the inherent N transformation processes involved in N availability in subtropical acidic soils. In a 15N tracing study, nine soils from croplands, 10-y and 50-y afforested woodlands were sampled to investigate the changes in soil gross N transformation rates in humid subtropical China. Gross N transformation rates were not significantly different in soils under 10- and 50-y after afforestation. Compared to cropland, however, afforestation stimulated the rates of mineralization, microbial NH4+ immobilization and adsorption of NH4+, leading to a faster turnover of NH4+ pool in afforested soils. Moreover, afforestation inhibited autotrophic nitrification and resulted in NO3− production dominated by heterotrophic nitrification. Furthermore, afforestation significantly enhanced NO3− consumption mainly through the increase in microbial NO3− immobilization rather than dissimilatory NO3− reduction to NH4+ in soil. These differences in gross N transformation rates resulted in low net NO3− production and strong NO3− retention capacity in afforested soils, similar to that found in undisturbed natural forest. Our results suggest a rapid recovery (several years) of soil N retention following afforestation. Soil NO3− retention capacity was correlated positively with TOC, TN, WHC, CEC, Al, free Al oxide and exchangeable Al3+, but negatively with pH, Ca and exchangeable Ca2+, indicating that the absence of agricultural management (e.g., N fertilizer and liming) and alteration in soil environment by tree establishment are responsible for the recovery of N retention capacity in subtropical acidic soils during afforestation.