Long-term natural succession improves nitrogen storage capacity of soil on the Loess Plateau, China

Land-use change resulting from natural succession enhances the nitrogen (N) accumulation capacity of terrestrial ecosystems. To explore those factors that foster changes in soil N storage under evolving conditions of vegetation succession, a study on N storage at differing stages along a 150-year chronosequence was conducted in the Ziwuling Forest Region in the central part of the Loess Plateau, China. A principal finding was the rapid increase in N storage in the 0-60cm soil layer, which achieves a stable value after the shrub community stage (∼50-60 years), leading to the overall long-term (∼150 years) accumulation of soil stored N in the post-abandonment secondary forest. Soil N accumulated mainly in the pioneer stage and showed a significant increase before the shrub community stage (P<0.05). The N storage in the 0-60cm soil layer changed from 5.8 to 8.4Mgha-1 during the transition from abandoned farmland (∼3-5 years) to climax community (Quercus liaotungensis Koidz forest) (∼150 years). The N storage values were higher in the upper (<20cm) than the deeper soil layers (>20cm). In the topsoil (0-20cm), N storage values showed a markedly positive correlation with soil organic carbon (SOC), total soil N and fine roots. In the deeper soil layers (20-40 and 40-60cm) there was a correlation only with TN. Soil bulk density, soil water content and soil pH were not the determining factors behind N storage values in the topsoil (0-20cm), although they did show negative, positive and negative correlations, respectively. In addition, they showed no consistent correlations in the lower soil layer (<20cm). The results suggest that changes to N storage values were the result of the accumulation of SOC, total N and primary productivity during the process of forest succession, and this capacity is positively related to post-abandonment forest succession on the Loess Plateau, China.