Impacts of Shrub Encroachment on Soil Properties in Global Alpine Grasslands: A Meta-Analysis

Shrub encroachment in alpine grasslands alters soil biogeochemical processes, but reported soil responses remain inconsistent and conflicting among studies. We performed a systematic review and meta-analysis to evaluate ten physical and chemical alongside biological soil properties. Evaluated indicators covered soil water content (SWC) and bulk density (BD) alongside soil organic carbon (SOC) and total nitrogen (TN). They also included ammonium (NH4+-N) and total phosphorus (TP) and pH alongside microbial biomass carbon (MBC) and nitrogen (MBN) and phosphorus (MBP). In total, 66 peer-reviewed articles provided 175 paired observations across four continental alpine regions, with most studies conducted in High Mountain Asia. Most compiled studies evaluated topsoil chemical properties, while non-nitrogen-fixing shrubs dominated the dataset. Shrub encroachment significantly stimulated MBC (+7.59%) and MBN (+5.44%) across both functional groups. TN increased significantly (+2.08%) driven entirely by nitrogen-fixing shrubs (+7.57%). Nitrogen-fixing shrubs also induced rhizosphere acidification (−1.87%) and increased MBP (+14.41%). Non-nitrogen-fixing shrubs significantly reduced BD (−5.67%), while bulk SOC (+0.15%) and TP (−0.31%) showed no pooled change. SOC responses diverged sharply at the genus level, increasing under Dasiphora (+6.58%) but declining under Salix (−6.88%) and Vaccinium (−11.62%). Soil sampling depth and shrub genus identity exerted primary controls on soil carbon and nitrogen dynamics. Current evidence concentrates heavily on Qinghai–Tibetan Plateau topsoils, leaving critical geographic and subsoil data gaps across global cold alpine biomes. Given functional divergence and depth stratification, sustainable alpine management must adopt shrub regulation differentiated by functional traits, linking microscale microbial processes with whole ecosystem carbon and nitrogen cycling.