Effects of natural grassland enclosure on carbon and nitrogen storage in deep soil in the Loess Plateau of China

Grassland enclosure and establishing are important measures to recovery and reclamation of degraded grassland vegetation. Accurate assessment of dynamics of soil carbon and nitrogen storage has important ecological significance. The present study focuses on dynamic of organic carbon (SOC) and total nitrogen (STN) in deep soil (0-500 cm) in 30 years enclosure grassland and overgrazing grassland in semi-arid region of Guyuan city, Ningxia Province. The results showed that SOC content in 0-440 cm soil of 30 years enclosure grassland was significantly higher (P<0.05) than that in overgrazing grassland. There was no critical change layer of SOC in enclosure grassland, however, there was critical change layer of SOC located at 130 cm soil layer in overgrazing grassland. There was critical change layer of STN located at 130 cm soil layer in both enclosure and overgrazing grassland. The accumulation of STN occurred in 60-320 cm soil layers with very less accumulation topsoil layers (0-60 cm) and deeper layers (>320 cm). SOC storage in 30 years enclosure grassland was stable in 340-500 cm depth soil with 11.5 mg·hm-2 of each 20 cm soil layers, however, SOC storage in overgrazing grassland was stable in 140-500 cm depth soil with 8 mg·hm-2 of each 20 cm soil layers. SOC and STN storage in 0-100 cm soil in 30 years enclosure grassland accounted for 44.8% and 27.2% of total SOC and STN storage in 0-500 cm soil, respectively, those in 0-200 cm soil accounted for 72.2% and 38.5%, respectively, those in 0-300 cm accounted for 83.5% and 64%, respectively, those in 0-400 cm accounted for 92.2% and 80.1%, respectively. These results showed that there was much carbon and nitrogen information in the deep soil which was necessary to study dynamic of SOC and STN in the deep soil.