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).