Overgrazing, not haying, decreases grassland topsoil organic carbon by decreasing plant species richness along an aridity gradient in Northern China

We examine how plant species richness, plant biomass, and soil C concentrations respond to a spatial gradient of aridity and land-use at 701 sites across China. We hypothesized that (1) aboveground biomass and SOC would increase with increasing plant species richness, and species richness would have greater effects in arid regions, (2) aridity and land-use (grazing and haying) will reduce plant diversity and consequently decrease aboveground biomass and SOC. Over the period of the three summers (during the season of peak biomass, from late July to early August) of 2013-2015, a total of 701 field sites were surveyed across this region. At each site, one large plot (50 m × 50 m) was established and five quadrats (0.5 m × 0.5 m) were selected within each large plot (located at the center and corners along a diagonal line within the plot). Vegetation was sampled in each quadrat, and all vascular plant species and their cover and density were recorded. All the plants in each quadrat were clipped to ground level, oven-dried to constant weight at 65°C for 48 h, and then weighed in the laboratory to calculate aboveground biomass. Species richness was defined as the sum of distinct species in the five quadrats per plot. Soil samples were collected from three randomly assigned quadrats at each site, and a composite sample consisting of these three soil cores (3-cm diameter, 10-cm depth) was collected from each quadrat (carefully removing the surface organic materials). After field collection, soil samples were taken to the laboratory, sieved, air-dried for one month, and stored for laboratory analyses. Soil organic carbon (SOC) was determined with a total organic C analyzer (vario Toc, Elementar, Germany). The aridity index (AI, the ratio of precipitation to potential evapotranspiration) of each site was calculated using data from WorldClim (https://worldclim.org/) (Hijmans et al. 2005), including annual mean precipitation and temperature. We found that aridity and grazing reduced SOC through decreasing plant species richness, but did not significantly affect above-ground biomass. Notably, we observed strong negative synergistic effects of aridity and grazing, suggesting that soil carbon content was particularly threatened by grazing in arid environments. By contrast, haying reduced above-ground biomass and had no significant effect on SOC, although it increased plant species richness. Plant species richness had greater positive effects on SOC than on above-ground b