Grazing and selenium addition interactively affect plant productivity and species diversity in an alpine meadow at the Qinghai−Tibet plateau

Qinghai−Tibetan Plateau (QTP) is the largest contiguous grazing area in the world, while its soil has a low soil selenium (Se) content, an essential rare element for human health. Grazing and Se addition are two common management practices in the regional grasslands, but their interactions with the vegetation growth of alpine meadows in the QTP remain largely unknown. Here, we conducted a two−year field experiment (2017 and 2018) to explore the effects of grazing [two stocking rates (0 and 6 sheep months ha⁻¹)] and Se addition [six Se addition levels (0, 5, 10, 20, 40 and 80 g ha⁻¹)] on above−ground biomass (AGB), below−ground biomass (BGB), root−to−shoot ratio (R/S), species richness and Shannon−Wiener's diversity index of an alpine meadow at the Maqu county in the Eastern QTP. The results indicate that AGB peaked at a Se addition rate of 21 g ha⁻¹ combined with no grazing (i.e., exclosure) and at a rate of 20 g ha⁻¹ Se addition with a stocking rate of 6 sheep months ha⁻¹; BGB was highest under a Se addition of 23.5 g ha⁻¹ (under exclosure) and 23 g ha⁻¹ (under grazing). After Se addition, the species richness and Shannon−Wiener index of the alpine meadow under the grazing treatment were significantly lower than those in the exclosure treatment, which was mainly driven by the reduction of sedges and forbs. The structural equation model showed that grazing reduced the biomass of sedge, gramineae, and forbs, while Se addition increased the soil available nitrogen and phosphorus content and promoted the growth of sedge, gramineae, and forbs. Our results suggested that moderate grazing combined with Se addition is a suitable management method for maintaining alpine meadow productivity.