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.