4.2 Effect of grassland managements on carbon and nitrogen
stocks
The grazing regime alters ecosystem carbon and nitrogen cycles through
livestock feeding, trampling, and manure return in grassland ecosystems
(Tserang Donko Mipam et al., 2021; G. Zhou et al., 2017). We found that
GE significantly increased soil organic carbon storage in this study,
which is consistent with other studies in alpine meadows (Xiong et al.,
2014), desert steppe (Niu et al., 2011), and arid and semiarid
grasslands (Yu et al., 2021). A meta-analysis concluded that GE mostly
increased the soil C pool from 78 study sites in the Tibetan alpine
grassland (Yu, Chen, Sun, & Huang, 2019) and 164 sites across
grasslands worldwide (Abdalla et al., 2018). This positive effect may
result from the following mechanisms. First, we found that GE
significantly increased the aboveground and root biomass (Fig. 1). A
study (Xiong et al., 2014) indicated that the soil organic carbon and
nitrogen stocks of fencing grasslands were positively correlated with
plant root biomass. Positive feedback may exist between soil organic
carbon and plant biomass (Dai, Guo, et al., 2021), because most soil
organic carbon comes from root exudates and litter decomposition
(Kaiser, 2000; Xiong et al., 2014). Fencing-induced increases in root,
litter, and aboveground biomass subsequently increase organic carbon
input and promote the contribution of root-derived C to SOC by
increasing root carbon content (Su & Xu, 2021; Yang, Wang, & An,
2021), whereas, grazing decreases the organic matter in the soil by
consumption of the plant. This was supported by a significant positive
correlation between plant biomass and SOC stocks (Fig. 5). Second, soil
stoichiometry plays a critical role in the regulation of grassland
nutrient cycles (Chen, Wang, & Baoyin, 2021). A previous study
indicated that GE promoted nitrogen release from roots and litter,
thereby increasing organic carbon (Yu et al., 2019). We found that the
C/N ratio was significantly higher after years of GE than that observed
after different grazing methods (Table 1),
suggesting that the grassland may
have nitrogen restrictions without exogenous N input after years of
fencing, while a lower C/N ratio of the grazing methods can promote the
decomposition rate of soil microorganisms and thus reduce SOC (Yang et
al., 2021). Additionally, GE could facilitate soil moisture and
temperature and decrease soil degradation by increasing plant cover and
preventing livestock trampling, respectively, which in turn stimulates
soil microbial biomass C and plant growth and reduces soil carbon
leaching (Feyisa et al., 2017; Niu et al., 2011; Xiong et al., 2014).
Furthermore, grazing causes a
decrease in palatable species, which have faster litter decomposition
and nutrient release than unpalatable species, leading to a lower SOC
content than GE (Dai, Fu, et al., 2021). However, few studies have found
that GE decreases SOC accumulation and storage in subtropical (Wilson,
Strickland, Hutchings, Bianchi, & Flory, 2018), semiarid (Chen et al.,
2021), alpine grasslands (Wu et al., 2021) and causes no variation in
SOC of alpine meadows (Yuan & Jiang, 2021). These different conclusions
may be due to variations in belowground allocation, such as root
biomass, fine root exudates, and microbial biomass (Wu et al., 2021).
Contrary to SOC stocks, GE significantly decreased soil TN stocks
compared to grazing methods in the present study (Fig. 2b), indicating
that grazing improved the soil N content in this region. This result is
in agreement with other studies (Contosta et al., 2021; Wu et al., 2021;
Y. Zhou et al., 2020) and a meta-analysis showing that grazing led to a
decrease in SOC but an increase in TN stocks at a global scale (Abdalla
et al., 2018). Grazing results in higher levels of plant N, efficient
use of N uptake, and accelerated N release from roots and plankton,
leading to a significant increase in TN during grazing (Dai, Fu, et al.,
2021; Yu et al., 2019). A study by Zhu, Liu, Wang, Sun, & Han (2021)
found that grazing disturbances could change the activities of nitrogen
assimilation-related enzymes which are beneficial for nitrogen
assimilation by grassland plants. On the contrary, grazing could
increase soil N content by returning excreta from livestock into the
soil, as urine and faeces contain a large amount of available N for
uptake (Oenema, Oudendag, & Velthof, 2007). Meanwhile, the grazing
methods were implemented using 60 kg·ha-1 urea and 300
kg·ha-1 of superphosphate in late June and mid-to-late
October, respectively; this suggests that the grazing grassland contains
sufficient nitrogen but lacks C due to plant intake when compared to
grassland subjected to GE, which resulted in a lower C/N ratio and in
turn inhibited the decomposition of soil organic matter for nitrogen
utilisation (Kumar, Kundu, Ghorai, Mitra, & Singh, 2018). We found that
soil TN stock was significantly negatively correlated with the C/N ratio
(Fig. 4). Furthermore, soil N content may depend on stocking rates
because soil N sequestration potential tends to decrease with increasing
stocking rates, which is beneficial for soil N sequestration; however,
heavy grazing leads to lower N sequestration potential and N loss (Y. Li
et al., 2012). The stocking rate in this study was 10 sheep units /
hm2. However, some previous studies have reported that
GE increased SOC stocks and TN stocks in alpine meadows (W. Li et al.,
2011; Xiong et al., 2014). The meta-analysis of 32 studies reported that
grazing significantly decreased both SOC (-20%) and TN (-15%) in
alpine grasslands (Yan et al., 2020). As very few meta-analyses have
been conducted on the responses of soil SOC and TN stocks to grazing at
regional and global scales, the effects and related mechanisms have not
been consistently determined due to the complicated effects of grazing
on soil-plant systems depending on the regional climate (precipitation
gradient, mean annual temperature), altitude, soil depth, stocking rate,
fencing years, and preceding grazer
type to some degree (Abdalla et al.,
2018; Su & Xu, 2021; Yan et al., 2020; G. Zhou et al., 2017). For
instance, grassland SOC was enhanced by mean annual temperature, but
stocking rates had little effect, regardless of the mean annual
temperature (Throop, Munson, Hornslein, & McClaran, 2022).