Grazing regulates soil water-holding functions via altering plant functional groups in the southern Qilian Mountains

Abstract

The mattic epipedon (ME) is typically composed of abundant grassroots, organic matter and fine particles, and serves as a vital protective layer for alpine meadows. Overgrazing and human activities have greatly affected the water-holding capacity of ME. However, few studies have focused on the effect mechanism of grazing intensity on the soil water holding function of ME in alpine meadows. In this study, we examined the variations in alpine meadow vegetation and soil physicochemical properties within 0–10 cm depth under four grazing intensities: light grazing (LG, 2.7 sheep units/hm²), moderate grazing (MG, 4.5 sheep units/hm²), heavy grazing (HG, 9 sheep units/hm²), and severe grazing (SG, 18 sheep units/hm²). The results showed: 1) As the grazing intensity increased, the dominant vegetation functional group in the alpine meadows transitioned from grasses with a small mean root diameter (0.34 cm) to forbs with a larger mean root diameter (1.36 cm), and the thickness of ME reached its highest value (4.89 cm) under HG; 2) grazing altered soil physicochemical properties, particularly physical structure parameters such as porosity and bulk density, as well as chemical properties like soil organic carbon; 3) the HG grazing level was an important threshold in terms of soil saturated water holding capacity (143.74 %) and saturated hydraulic conductivity (12.63 mm·min⁻¹), with both parameters reaching their maximum values under HG; 4) correlation analysis results revealed that the proportion of plant functional groups was significantly correlated (P ≤ 0.05) with soil total porosity (TPO) and belowground biomass (BGB). Soil factors, represented by TPO, were involved in the direct and dominant pathways (path coefficients ≥0.50) affecting the SWC of the alpine meadow. This study revealed the effect mechanisms of different grazing intensities on soil water holding function, which are valuable for the development of scientific and rational grazing measures.