Land-use change is considered to have a significant impact on the global C balance by profoundly altering biota, land cover, and biogeochemical cycles. Therefore, the impact of land-use change on the storage or sequestration of carbon (C) and nitrogen (N) is one of the most important domains to global change research. As a dominant form of landscape and an integral component of the Eurasian landmass, grasslands of northern China, which account for 40% of the land area in China, play important roles in servicing the ecological environment and in the socio-economy of the region. Furthermore, an increase in the soil C and N storage in the grasslands of northern China is anticipated with the implementation of measures aimed at encouraging grassland protection. However, there is little information regarding the potential of C and N storage (or C sequestration rate) due to the absence of long-term grazing exclusion plots in multiple sites.
In this present research, using pair-sampling methods, we investigated the C and N storage in aboveground biomass, litter, roots and soil organic matter in the 0-100 cm soil layer in the fenced grasslands and free-grazing grasslands in 4 sites. The main objectives of this study was to assess the effect of long-term grazing exclusion on the storage of C and N in temperate grasslands of northern China, and further determined the sequestration rates of C and N in fenced grasslands by comparing with the data of free grazing grasslands.The results showed that, compared with free-grazing grasslands, there were significant increases in the C and N storage in the fenced grasslands.
It was estimated that, after 3-decades grazing exclusion, the quantity of C sequestration ranged from 1401 to 2858 g C m^-2 in the 4 sites, averaging 2126 g C m^-2; and the rates of C sequestration in fenced grasslands ranged from 46.7 to 129.2 g C m^-2 a^-1, averaging 84.2 g C m^-2 a^-1. The rates of N sequestration in fenced grasslands varied from 2.8 to 14.7 g N m^-2 a^-1, averaging 7.3 g N m^-2 a^-1. From the view of C and N sequestration rates, there is a total trend as follows: Stipa grandis grassland < Leymus chinensis grassland < degraded L. Chinese grassland < Sowing Medicago falcata and L.chinensis grassland. Moreover, we found that the lower soil layers (40-100 cm) had also play an important role in the total C and N sequestered in fenced grasslands although sequestration rates were relative higher in the upper soil layer (0-40 cm). Together, the study demonstrated that temperate grasslands of northern China have the vast potential to increase C storage, and long-term grazing exclusion should be the most valuable and feasible approach to make true the C sequestration of temperate grasslands in future.