Vegetation restoration drives dynamics of soil nitrogen content and availability in the subtropics

• Contents of soil total nitrogen (N) and N forms increased with vegetation restoration. • The availability of soil N decreased as vegetation restoration proceeded. • Soil N cycle was still closed when forests restored to climax communities. • Soil N availability was co-regulated by plant diversity index and soil available N pools. Nitrogen (N) is an essential element for the functioning of terrestrial ecosystems and is often the primary limiting resource for the productivity of many terrestrial ecosystems. Existing knowledge on the effect of vegetation restoration on soil N dynamics is inconsistent. The aim of this study was to investigate the dynamics of soil N contents and availability with vegetation restoration and other influencing factors. Soil samples were collected to a depth of 40 cm from four distinct plant communities along a restoration gradient from scrub-grassland (4–5 years), shrub (10–12 years), coniferous and broadleaved mixed forest (45–46 years) to the evergreen broadleaved forest (>90 years). Total N (TN), soluble organic N (SON), microbial biomass N (MBN), inorganic N (IN, including ammonium (NH 4 + ), nitrate (NO 3 – )), IN/SON, NO 3 – /NH 4 + , and MBN/IN were compared across vegetation factors, soil physicochemical properties and microbial activities. TN, SON, MBN, and NH 4 + contents increased remarkably with vegetation restoration, while NO 3 – decreased from scrub-grassland to shrub and then increased. The dominant influencing factors of soil N forms were fine root biomass, soil organic carbon, C/N ratio, pH, microbial biomass carbon, and ureases, which indicated that vegetation factors, soil physicochemical properties, and microbial activities collectively control soil N accumulation. IN/SON and NO 3 – /NH 4 + ratios were < 1 and decreased with vegetation restoration, while the MBN/IN ratio increased. The results implied that soil N availability reduced with vegetation restoration and N cycling remained closed, although it has been restored to the broadleaved forest stage. The close correlation between plant diversity index and soil N availability indicated that soil N availability reduces largely due to increasing plant N demands induced by the improvement in plant species diversity. Therefore, vegetation restoration facilitates long-term soil N accumulation and retention in this region.