Influence of grazing disturbance on stoichiometric characteristics of alpine Rhododendron shrublands underground soil nutrient pool

Alpine range is sensitive to global climate change and artificial disturbances and generally adapts to changes in external environment by adjustments in chemical elements. Research on belowground ecosystems is critical for ecological restoration. A combination of field and lab experiments was conducted to study the nutrients characteristics of soils, roots of alpine shrublands in Qilian Mountains under different grazing disturbances. The dynamics of root system phytocoenosis in alpine regions were also analyzed. The results showed that underground biomass of the plant community decreased with increasing soil depth. About 60%~70% of root biomass was concentrated in the 0~10 cm soil layer. Also with increasing grazing intensity, living roots penetrated deeper into the soil. Total root biomass along with carbon, nitrogen and phosphorus contents decreased with increasing grazing intensity. Compared with lightly grazed areas, C, N and P contents in heavily grazed areas decreased by 26%, 17% and 27%, respectively. Per unit mass surface living roots and carbon were highest in lightly grazed areas, nitrogen highest in medium grazed areas and phosphorus highest in heavily grazed areas. Variations in dead root N and P contents were inversely correlated to those in living root N and P contents. With increasing grazing intensity, soil organic carbon, total nitrogen, and available N and P increased. However, the content of total phosphorus remained relatively stable even though surface total phosphorus content decreased. Under non-grazing treatment, root biomass increased, C/N and C/P ratios increased both in surface soils and roots, and N/P ratio decreased, compared with grazing treatments. This implied that roots were vital elements for both soil carbon accumulation and nutrient cycle. Non-grazing in vegetation growth seasons facilitated plant recovery and better growth and high plant nutrient efficiency. Non-grazing during vegetation growth seasons was an effective way of promoting recovery of fragile alpine shrub for high ecological functions and economic values.