Plant and microbial communities follow fast-to-slow strategies in response to grazing in an arid rangeland

Biotic and abiotic resources in arid area have been threatened by the disturbance of human activities. It remains unclear what life strategies plant and microorganisms use to cope with livestock grazing, and how these strategies are associated with soil metabolic compounds. Based on a 22-year grazing experiment in an arid rangeland on the Loess Plateau, we investigated changes in the resource strategies of plant and soil bacterial communities in response to grazing pressure by examining plant morphological traits and bacterial ribosomal RNA gene operon (rrn) copy number, as well as changes in their associated soil metabolites. With increasing grazing pressure, both plant and soil bacterial communities followed a fast-to-slow resource acquisition strategy. Plant traits related to fast-growing strategy were replaced by traits related to slow-growing strategy, shifting from higher specific leaf area, specific root length, and plant height to higher root and leaf tissue density. Bacterial rrn copy number also decreased with increased grazing intensity, indicating that the soil bacterial community tends to change from r-strategy to K-strategy. A small portion (7/144) of soil metabolites played a significant role in the plant fast-slow gradient, and their abundances decreased with increasing grazing pressure. These metabolites were positively correlated with plant slow-growing strategy and bacterial rrn copy number, but negatively correlated with plant fast-growing strategy. Our work indicates a unified fast-to-slow strategy that above- and below-ground organisms apply to cope with grazing intensification, highlighting that plant fast-growing strategy may rely on soil metabolic activities. This contributes to the monitoring and sustainable management of intensively grazed arid rangelands.