On China's Qinghai-Tibetan Plateau, duration of grazing exclosure alters R: S ratio, root morphology and attending root biomass

Unsustainable human actions and climate change have threatened global grasslands and lead to biodiversity loss and compromised ecosystem functions. Widely considered to be an effective approach to restore degraded grasslands, grazing exclosure (GE) likely affects biomass allocation and root morphology in the long-term, although their impacts are yet to be documented. The objective of this study was to investigate the effects of grazing exclosure durations of 5, 22, and 39 years (GE(5), GE(22), GE(39), respectively) on root:shoot biomass allocation (R:S ratio) and root morphology ? including root average diameter (RAD), root density (RD), specific root length (SRL), specific root area (SRA), and root length density (RLD) - in an alpine grassland on the Qinghai Tibetan Plateau. Seasonal grazing (SG) land served as a control. In the 0-0.30 m soil layer, the R:S ratios of the SG (1.27) and GE(5) sites (0.90) were significantly lower than that of the GE(39) site (2.14). However, there was no difference between the GE(22) (1.83) and GE(39) sites. The R:S ratio of the 0-0.15 m soil layer showed an increasing trend with duration of grazing exclosure and had a significantly greater value under GE(39) (1.89) than under GE(5) (0.76). However, the R:S ratios for the 0.15-0.30 m soil layer (0.14, 0.28 and 0.25 for GE(5), GE(22), and GE(39), respectively) showed no significant changes with grazing exclosure duration. In the 0-0.30 m soil layer across sampled sites, RAD remained constant, RD and RLD exhibited an increasing trend, while SRA and SRL displayed a slightly decreasing trend with grazing exclosure duration. Below-ground biomass (BGB) was most strongly related to RLD across all sites; to RD and RLD at the GE(5) and GE(22) sites, and to RAD and RLD at the GE(39) site. Both R:S ratio and root morphology were mainly influenced by soil nitrate nitrogen. Although long-term grazing exclosure enhanced BGB and R:S ratio, soil nutrients declined with increasing exclosure duration. Therefore, to achieve improved ecosystem functions, grazing exclosure should be used with caution in this area or other similar regions around the world.