The Hulunber grasslands in Inner Mongolia cover an area of up to 9.97×10⁶ km². Within this area, the Hulunber Leymus chinensis meadow steppe is located in the eastern part of the Eurasian continent grassland region. It is a very important example of a temperate grassland ecosystem. Grazing is the most common land use in the Hulunber meadow grasslands, therefore, understanding the changes of soil carbon, soil nitrogen and carbon sequestration under different grazing gradients will provide an important scientific basis for grassland carbon sink management in Inner Mongolia. Six stocking rate treatments (0, 0.23, 0.34, 0.46, 0.69 and 0.92 Au/hm²) repeated three times for each treatment, were established, with soil samples collected in early August 2011. The experimental results showed that the grazing gradient has a distinct effect on the change of soil carbon and nitrogen content and soil carbon and nitrogen storage. Soil carbon and nitrogen storage and carbon sequestration potential in the top 0-30 cm soil layers had a significant linear downward trend as the grazing gradient increased.
Compared with non-grazed grassland G0.00 (107.1 t/hm²), the soil carbon sequestration potentials are 108.61-109.67 t/hm², 107.06 t/hm², 95.13-100.14 t/hm² in light grazing, moderate grazing and heavy grazing grassland, respectively. For the soil nitrogen, compared with non-grazed grassland G0.00 (9.22 t/hm²), soil nitrogen sequestration potentials were 8.37-8.65 t/hm², 8.20 t/hm², 6.74-7.35 t/hm² in light grazing, moderate grazing and heavy grazing grassland, respectively. Light grazing improved the soil carbon storage and carbon sequestration potential. With an extension of the grazing time, the soil carbon and nitrogen content, carbon and nitrogen storage, and carbon and nitrogen sequestration potential in soils of heavily grazed grasslands decreased significantly.
There were obvious vertical distributions in the soil carbon and nitrogen content and carbon and nitrogen storage under different grazing gradients. The change of soil carbon and nitrogen was higher in the 0-10 cm soil layer under different grazing gradients while soil carbon and nitrogen content and carbon and nitrogen storage significantly decreased with the increase in soil depths in different grazing gradients. About 42.26% of the soil organic carbon was distributed in the top 0-10 cm soil layer, 30.67% in the 10-20 cm layer, and 27.07% in the 20-30 cm layer. For the 0-10 cm, 10-20 cm and 20-30 cm soil layers, the total soil contents were 44.39%、30.13% and 25.48%, respectively; the soil carbon storage were 39.60%, 31.26% and 29.14%, respectively; the soil nitrogen storage were 41.57%, 30.97% and 27.46%, respectively. The soil carbon and nitrogen content and soil carbon and nitrogen storage for the 0-10 cm soil layer were significantly higher than those for the 10-20 cm and 20-30 cm soil layer.
We found that there was an underlying transformation from soil carbon sequestration under light grazing gradient to soil carbon loss under a heavy grazing gradient, and this transformation threshold was found to be 0.46 Au/hm². In short, the Hulunber meadow steppe of Inner Mongolia has a good capacity to sequester carbon in soil, and that can increase the soil carbon sequestration potential through rational control of grazing gradients to achieve carbon sinks. We can increase the soil carbon sequestration potential to achieve carbon sinks through controlling grazing gradients.