Farmland abandonment decreases soil bioavailable phosphorus but increases organic phosphorus in the mid-hills of Nepal

Abstract

Global land use changes (LUCs) have been comprehensively modulated by human activities, yet the direction and magnitude of the LUCs’ effects on soil phosphorus (P) cycling in mountain regions remain unclear. Here, the variations in the concentrations and stocks of soil bioavailable phosphorus (Bio-P) and its supply pathways were explored in four land uses (farmland, abandoned land, shrub and forest) from the mid-hills of Nepal. After farmland abandonment, the concentrations of soil Bio-P decreased significantly especially in the forest, while organic P presented an opposite case. Compared with the farmland, the stocks of soil Bio-P in the abandoned land, shrub and forest decreased 30.3%, 42.2% and 73.1%, respectively, and those of organic P in the abandoned land and forest increased 19.2% and 52.2%, respectively. This was mainly attributed to the increased aboveground biomass, while soil microorganisms had a limited role in organic matter decomposition because of the low soil moisture. Relative to the farmland affected by fertilization, more complex factors (e.g., pH, MBP, SOC and clay) regulated soil P fractions in other lands where Bio-P was mainly supplied by secondary mineral P, followed by primary mineral P and occluded P. Soil erosion caused an evident P loss in farmland, while forest had a great potential to conserve soil P. This study provides new insights into the cycling of soil P under the LUCs in mountain regions, which will serve as a foundation for the sustainable use of soil P resources for livelihood and land management.