Nitrogen deposition has a negative effect on the richness of seed bank in Tan sheep grazing typical steppe on Loess Plateau

Background/Question/Methods

Grasslands have suffered from grazing and nitrogen deposition all over the world, which accounts for over 50% of the total global land (Wang et al., 2018). The soil seed bank of grasslands is the basis of population settlement, survival, reproduction, and expansion (Hu et al., 2019). In spite of wide reports on the effect of grazing or N deposition on soil seed bank, the effects of their interactions were unclear. Hereby, we estimated that abundance and richness of soil seed bank in a grazing typical steppe with four stocking rates (0, 2.67, 5.33, 8.67 sheep ha^-1) and three gradients of simulated N deposition (0, 10, and 20 g m^-2 yr^-1).

Results/Conclusions

Our results showed that the abundance of soil seed bank was negatively affected by stocking rate and the interaction of stocking rate and N addition, and the richness was negatively affected by N addition and the interaction of stocking rate and N addition. The abundance and richness of functional group Leguminosae were negatively affected by N addition. The abundance of functional group of Gramineae was positively affected by the interaction of the two factors, and the richness of functional group of Gramineae was significantly affected by N addition (positive) and the interaction of stocking rate and N addition (negative). The abundance of soil seed bank decreased 38.4 per m² by increased stocking rate of 1 sheep ha^-1 in grazing areas without nitrogen addition. But, the abundance of soil seed bank first increased and then decreased at stocking rate of 8.67 sheep ha^-1 in the N addition areas with 10 g m^-2 yr^-1. In contrast, the abundance of soil seed bank increased by 21.2 per m² for each increase in nitrogen addition by 1 g m^-2 yr^-1 in no grazing areas. But the abundance of soil seed bank increased first and then decreased with the increased N addition. The N addition has no effect on the richness of soil seed bank in no grazing plots, but has negative effect on the richness of seed bank in the grazing plots. The richness of soil seed bank increased by 0.3 species per 0.12 m² with increased stocking rate by one sheep ha^-1 in grazing plots without N addition. Over all, The interaction of moderate grazing and nitrogen deposition can increase the abundance of soil seed bank, but it is not negligible that they significantly reduce the richness of species, especially the legumes.