4.4 Way of degradation influence AGB of grasses
Degradation significantly decreased AGB of grasses both through direct way and indirect way in those two types of grasslands. Degradation declined AGB of grasses directly and through decreased Pn indirectly in native grasslands, and declined AGB of grasses directly and through decreased Pn, plant N and P indirectly in active restoration grasslands. So, plant Pn plays important role in determining AGB of grasses both in native grasslands and active restoration grasslands (Xu et al., 2018; Shen et al., 2019, 2021). For the direct way, land degeneration decreased biomass accumulation through restricting plant abundance, declining plant individual height and plant richness (Xu et al., 2015, 2018; Li et al., 2018; Guo et al., 2019). For the indirect way, in native grasslands, land degeneration first decreased plant N concentration (Elser et al., 2007; Güsewell and Gessner, 2009; Dlamini et al., 2014; Peńuelas et al., 2015) and thus to decrease photosynthetic activity (Wright et al., 2004, 2010; Güsewell, 2010; Sundqvist et al., 2014; Castellano et al., 2016). In active restoration grasslands, the main reason for the indirect way was that land degradation declined soil nutrient content such as C, N and P and the gs and Ci of grasses, the former declined plant N and P content and the later inhibited plant photosynthetic rate (Sundqvist et al., 2014; Zhou et al., 2021; Shen et al., 2019).
Conclusions
In conclusion, land degeneration both the native and active restoration grasslands significantly influenced AGB, photosynthetic capacity and nutrient concentration of grasses. In native grasslands, the decline of Pn mainly caused by non-stoma limitation, and in active restoration grasslands, the decline of Pn mainly caused by stoma limitation. Plant photosynthetic properties and N and P concentration in active restoration grasslands shows stronger association with its AGB than native grasslands. Land degradation decreased AGB of grasses through both direct way and indirect way in those two types of grassland. Plant N and P are the two key nodes that matter in the way of declining AGB of grasses and can indicate the of degradation level of active restoration grasslands.