4. Discussion
4.1 Effect of grassland managements on plant biomass
In this study, both aboveground and root biomass were significantly increased after 17 years of GE, which is in agreement with many other studies conducted in alpine meadows (Xiong, Shi, Sun, Wu, & Zhang, 2014) and semiarid grasslands of China (An & Li, 2015; Yu, Sun, & Huang, 2021), highland grasslands of Argentina (Vaieretti et al., 2021), semi-arid grasslands of South Africa (Snyman, 2005), degraded loess farmlands of Israel (Leu, Ben-Eli, & Mor-Mussery, 2021), and temperate grasslands of India (Husain, Geelani, & Bhat, 2021). GE can relieve livestock damage to grassland vegetation, increase vegetation height and cover, alter plant functional groups, and improve photosynthetic material partitioning between aboveground and belowground biomass (Lei Deng, Zhang, & Shangguan, 2014; Gao et al., 2008; Xiong et al., 2014), thus promoting increased productivity of grassland vegetation. Moreover, GE improves soil textures, such as soil water content, bulk density, and erosion (Lei Deng et al., 2014; Feyisa et al., 2017; Y. Li et al., 2012), Yu et al. (2021) found that the increases in AGB and litter were driven by the direct effects of GE and its indirect effects mediated by soil water content. However, few studies have also reported that GE had no effect on total biomass in alpine grasslands (Lu et al., 2015), belowground biomass in desert steppe (Niu et al., 2011), and cause a slight decrease in the aboveground biomass in temperate grasslands (Bork et al., 2019). The mechanism of these differences is not well understood; they may however, occur partly because of the variation in fencing years (Y. Li et al., 2012). It has been shown that aboveground biomass and root biomass tend to increase with increasing years (7, 12, and 25 years) of enclosure (Y. Li et al., 2012), and that grassland species richness index, diversity index, and plant cover reach a maximum after 20 years of GE (Ghorbani et al., 2021).
Grazing has a highly complex effect on plant growth because plant characteristics such as life history, plant height, and life type are significantly responsive to grazing on a global scale (Díaz et al., 2007). Overall, there are direct effects of trampling and feeding on plant leaves and stems and indirect effects of returning livestock manure into the soil, thus interfering with the synthesis and supply of carbohydrates and the accumulation of nutrients, which in turn affect grassland productivity (Dai, Fu, et al., 2021; Snyman, 2005; Wu, Wang, & Sun, 2021). In the present study, all the grazing methods significantly decreased both aboveground and root biomass, which is in contrast to GE. This finding is consistent with the majority of previous studies conducted across grasslands worldwide (Díaz et al., 2007; Yan et al., 2020). Despite the decreased plant biomass observed after implementing all grazing methods, we found that the aboveground biomass was similar for the three grazing methods; however, the root biomass of CG was significantly higher than that of MG and RG. This may partly be because compensatory plant growth depends on the net effect between promotion and inhibition, which is closely related to the grazing method (Dai et al., 2019). Plant growth in CG may not fully compensate for the biomass because the aboveground parts are eaten and trampled consecutively (Schönbach et al., 2011). We found that the root/shoot ratio of CG was significantly higher than that of MG and RG, and a previous study reported that the root/shoot ratio of plants increased significantly with increasing grazing pressure (L. Deng, Sweeney, & Shangguan, 2014; Yan et al., 2020). An increase in the root/shoot ratio implies an increase in the uptake of nutrient elements from the soil (Kiær, Weisbach, & Weiner, 2013). Plants need to absorb more nutrients and water to resume growth of aboveground parts under long-term sustained grazing pressure, and therefore allocate photosynthetic products to the roots for storage and utilisation as much as possible (Bai et al., 2015; Dong, Wu, Zhu, & Shi, 2014; Hafner et al., 2012), leading to an increase in belowground biomass and root/shoot ratio in CG (J. Sun et al., 2014).