Plant phosphorus demand stimulates rhizosphere phosphorus transition by root exudates and mycorrhizal fungi under different grazing intensities

Abstract

Soil erosion and phosphorus (P) removal due to livestock production have led to a grave depletion of P in grasslands. Accordingly, understanding how plants cope with such P limitation conditions and which P transition processes operate in the soil under different grazing intensities is imperative for better scientific management of grazed grasslands. Here, we conducted a field experiment that tested the impact of different grazing intensities (light, medium, and heavy) and a control (no grazing) on P-related dynamics in a typical temperate grassland site in Inner Mongolia, China. The increased N:P ratio in leaves of dominant plant species and higher annual primary production (APP) of vegetation that generally ensued under grazing indicated that plants’ compensatory growth in response to grazing pressure was a factor contributing to P limitation. Plants’ P requirement for growth stimulated P transition in the rhizosphere via the mobilization of low-molecular-weight organic acids (LMWOAs) and mycorrhizal fungi. Specifically, LMWOAs promoted the conversion of stable P into organic P, and mycorrhizal fungi converted that organic P into labile P in the rhizosphere. This P transition process is jointly affected by changes to key soil physiochemical properties and the altered P input from excrement caused by herbivores. Altogether, these results suggest that grazing increases the biological vitality of the soil and accelerates the transition of P in the rhizosphere; and a medium grazing intensity may be more suitable for agricultural production because it provides the highest APP and a more biologically dynamic soil environment.