1. Introduction
Grasslands are one of the major terrestrial ecosystems and account for 40% of the global land surface (Abdalla et al., 2018). As an important part of the global carbon and nitrogen cycle, grasslands accounted for 34% of carbon reserves and 30% of nitrogen reserves in terrestrial ecosystems (Eze, Palmer, & Chapman, 2018; Xu, He, & Yu, 2019). The grazing regime is one of the most widespread management practices in grasslands and is an important factor affecting ecosystem carbon and nitrogen cycling (Abdalla et al., 2018; G. Zhou et al., 2017). However, overgrazing and poor management have detrimental effects on grasslands, such as degradation of plant cover and species diversity (Louhaichi, Ghassali, Salkini, & Petersen, 2012) and changes in plant productivity and nutrient cycles (Ghosh et al., 2022; Wen Li et al., 2017), which in turn affect grassland carbon and nitrogen allocation (Ferlan et al., 2011). Thus, there has been great concern over ways to maintain and improve the ability of grasslands to sequester carbon and nitrogen through reasonable management practices.
Grassland regimes, such as grazing imposition or exclusion, significantly influence grassland carbon and nitrogen storage (G. Zhou et al., 2017). Many studies have been conducted on the effects of management practices on grassland carbon and nitrogen pools, among which the overall major decrease in global grassland carbon stocks is mainly attributed to grazing (Eze et al., 2018). For instance, Soil organic carbon (SOC), total nitrogen(TN), and phosphorus (TP) stocks declined in a five-year grazing period, and an increased stocking rate reduced the SOC content and storage in the Tibetan alpine meadow (D. S. Sun et al., 2011); grazing with moderate to high stocking rates significantly reduced SOC and nitrogen in a semiarid tropical inceptisol (Ghosh et al., 2022). However, optimised grazing has synergistic benefits on soil carbon and nitrogen sequestration in terms of reduced interference with plant-insect interactions, water depletion, and improvement of aboveground biomass (Bossio et al., 2020; Dai, Fu, et al., 2021; de Vries et al., 2012). For example, rotational grazing (RG) could increase soil carbon and nitrogen contents compared to those of hayed pastures (Contosta et al., 2021); soil carbon and nitrogen contents were 13% and 9% higher, respectively, after RG compared to those observed after conventional grazing at the three farms in the northeastern United States (Mosier et al., 2021). However, some studies have reported that heavy grazing has a significant positive effect on soil carbon and nitrogen pools, which are positively correlated with increases in below-ground biomass allocation. Moreover, higher grazing intensity may have a potential positive effect on increasing soil carbon and nitrogen storage (W. Li, Huang, Zhang, & Wu, 2011). Therefore, the effects of grazing practices on grassland carbon and nitrogen pools remain inconclusive.
Grazing exclusion (GE) has been recommended as an effective measure for restoring degraded grasslands (Dai, Fu, et al., 2021). Generally, grassland enclosures can enhance plant production and soil carbon pools worldwide (Su & Xu, 2021). GE facilitates the restoration of soil carbon and nitrogen by increasing the productivity of plants, decreasing the accumulation of surface litter in the soil, and thus minimising the disturbance of vegetation, which in turn increases carbon and nitrogen accumulation (Y. Li et al., 2012). Compared with grazing, soil organic carbon and nitrogen stocks in the 0–100 cm soil layer increased after 25 years of GE (Y. Li et al., 2012), and total soil carbon stocks and nitrogen content were higher after 10 years of GE than those observed after five years of GE and free grazing (Gebregergs, Tessema, Solomon, & Birhane, 2019). However, other studies have arrived at a contrary conclusion; that is, no significant difference is observed in soil organic carbon and nitrogen stocks after GE and light grazing, while markedly higher SOC is observed in areas of heavy grazing than that detected after GE (Reeder, Schuman, Morgan, & Lecain, 2004). Another study (Cui, Dong, Liu, & Sun, 2021) reported that 4–7 years of GE decreased SOC content and SOC stock compared to that obtained after free grazing. These results indicate that the effects of fencing on grassland carbon and nitrogen pools remain uncertain. The different responses to fenced grazing prohibition may result from the diversity of the research regions, topography, vegetation types, grazing histories, enclosure times, and their interactions (Gong et al., 2014; Vivanco & Austin, 2006; Zhang et al., 2018).
Many studies have focused on the effects of grazing management practices on grassland carbon and nitrogen pools in temperate and high-altitude regions, such as the Tibetan, Loess, and Inner Mongolian Plateau located in Northern China (Gong et al., 2014; Tserang Donko Mipam, Chen, Liu, Miehe, & Tian, 2021; Yan et al., 2020). However, grasslands undergoing the transition to forests at the hills and slopes of southwest China have received little attention; these grasslands cover an area of 35.58×106 ha and account for 32.59% of the total area of this region (Huangfu, Mao, & Lu, 2012). This situation is more pronounced in case of grasslands in typical karst landscapes. The response of karst grasslands to grazing management practices may not be in line with other regions because of the characteristics of high rock exposure rate, sparse and shallow scattered soils, weak resistance to disturbance, and poor stability (Huang, Cai, & Xing, 2008). To elucidate the responses of plant productivity and soil nutrient sequestration to long-term grazing, it is necessary to evaluate the implications of management practices on the stability of ecosystem carbon and nitrogen storage.
We conducted in situ experiments in an alpine grassland of the Yunnan-Guizhou Plateau after 17 years of grazing impositions and exclusion. The objectives were (1) to assess the effects of long-term GE on the vertical distribution of plant biomass and soil carbon and nitrogen, (2) compare the variations in ecosystem carbon and nitrogen storage under different grazing conditions, and (3) explore the main factors affecting soil carbon and nitrogen accumulation.