Environmental factors regulate soil microbial attributes and their response to drought in rangeland ecosystems.

In ecosystems, soil microbial variables characterization are used to determine soil biological health and the response of soils to environmental stress. Although there are strong associations between plants and soil microorganisms, they may respond asynchronously to environmental factors and severe droughts. We aimed to: I) evaluate the special variation of soil microbiome such as microbial biomass carbon (MBC) and nitrogen (MBN), soil basal respiration (SBR) and microbial indexes in eight rangeland sites located across an aridity gradient (distributed from arid to mesic climates); II) analyze the relative importance of main environmental factors (climate, soils, and plants) and their relationships with microbial variables in the rangelands; and III) assess the effect of drought on microbial and plant variables in field-based manipulative experiments. First, we found significant changes of microbial variables along a precipitation and temperature gradient. The responses of MBC and MBN were strongly dependent on soil pH, soil nitrogen (N), soil organic carbon (SOC), C:N ratio and vegetation cover. In contrast, SBR was influenced by the aridity index (AI), the mean annual precipitation (MAP), the soil pH and vegetation cover. MBC, MBN and SBR were negatively related with soil pH compared to the other factors (C, N, C:N, vegetation cover, MAP and AI) that had a positive relationship. Second, we found a stronger soil microbial variables response to drought in arid sites compared to humid rangelands. Third, the responses of MBC, MBN, and SBR to drought showed positive relationships with vegetation cover and aboveground biomass, but with different regression slopes, this suggest that plant and microbial communities responded differently to drought. The results from this study improve our understanding about the microbial response to drought in different rangelands, and may facilitate the development of predictive models on responses of soil microorganisms in carbon cycle under global change scenarios.

Richness, not evenness, varies across water availability gradients in grassy biomes on five continents.

We sought to understand the role that water availability (expressed as an aridity index) plays in determining regional and global patterns of richness and evenness, and in turn how these water availability-diversity relationships may result in different richness-evenness relationships at regional and global scales. We examined relationships between water availability, richness and evenness for eight grassy biomes spanning broad water availability gradients on five continents. Our study found that relationships between richness and water availability switched from positive for drier (South Africa, Tibet and USA) vs. negative for wetter (India) biomes, though were not significant for the remaining biomes. In contrast, only the India biome showed a significant relationship between water availability and evenness, which was negative. Globally, the richness-water availability relationship was hump-shaped, however, not significant for evenness. At the regional scale, a positive richness-evenness relationship was found for grassy biomes in India and Inner Mongolia, China. In contrast, this relationship was weakly concave-up globally. These results suggest that different, independent factors are determining patterns of species richness and evenness in grassy biomes, resulting in differing richness-evenness relationships at regional and global scales. As a consequence, richness and evenness may respond very differently across spatial gradients to anthropogenic changes, such as climate change.

Grasses and grazers in arid rangelands: Impact of sheep management on forage and non-forage grass populations.

Ecological modeling that includes plant population processes as a critical determinant of vegetation dynamics is useful for sustainable rangeland management. However, we know little about how long-term sheep grazing pressure drives the plant community structure through changes in different native grass species at both individual and population levels. In this study, we hypothesized that plant populations perform differently under different grazing management due to their specified preference by livestock animals. We also tested whether grazing-rest management, aimed at increasing long-term rangeland sustainability, improves the plant growth of forage grass species. We evaluated plant density, individual morphology and plant-size distribution of dominant grass populations in permanent exclosures and open fields under moderate and intensive grazing pressures in Patagonian steppes (South America). We also examined the effects of seasonal grazing-rest managements on the growth and tillering (asexual reproduction) of forage species plants, using temporary mobile exclosures. Grazing intensity changed population density and structure according to species. Compared to permanent exclosures, moderate grazing maintained the plant density of palatable species highly preferred by sheep, reduced the standing-dead biomass proportion of individual plants, and promoted the green biomass of tussocks. Conversely, intensive grazing (double stocking rates) decreased the plant density and individual size of species highly preferred by sheep, and increased the plant density of non-preferred species. Grazing-rest enhanced forage grass species growth and reproduction compared with year-round grazing management, especially during the growing season of a wet year. Our studies support that sheep can be managed to control the plant-size distribution of dominant grass species, their population dynamics, and thereby the overall forage availability at the community level. Both moderate grazing and grazing-rest management can improve the forage availability and preserve the dominant native grasses. We suggest applying a plant population dynamics perspective to facilitate sustainable management of global rangelands.