The distribution, effectiveness and environmental threshold of soil aggregate phosphorus fractions in the sub-alpine region of Southwest China

This article is a study of total phosphorus (TP), available phosphorus (AP) and phosphorus (P) fractions of soil aggregates along an altitude gradient in the sub-alpine region of Southwest China. We conclude that soil aggregate TP and AP contents are higher at 3980 m and 3347 m; there are no significant differences in soil aggregate TP at any altitude (p > 0.05); the AP and phosphorus activation coefficient (PAC) are higher in 0.25-0.5 mm aggregate. Soil aggregates TPi > TPo at 3009 m and TPo > TPi at 3347, 3654 and 3980 m. Using multiple stepwise regression analysis, we conclude that NaOH-Pi is the key fraction of AP supply, NaHCO3-Pi and NaOH-Pi are important fractions of CaCl2-P. Through piecewise linear fitting of soil aggregate AP and CaCl2-P, the soil P environmental threshold obtained is 27.766 mg kg(-1) in this region. Also we got <0.25 mm, 0.25-0.5 mm aggregates at 3980 m which have high soil P loss risk. Our study fills in gap about the distribution of P fraction in aggregates and effects of soil P fraction for AP and CaCl2-P in high-altitude areas. Our study provides a reference and scientific basis for understanding the soil aggregate P storage capacity and the biological effectiveness of P fractions at high altitudes. Our study on the P environmental threshold provides a reference for soil P storage capacity. Our study provides a reference value for the soil P loss risk management, and sustainable development of soil ecosystems in the sub-alpine region of Southwest China.