Land-use effects on phosphorus fractions in Indo-Gangetic alluvial soils

Phosphorus (P) in soil exists both in organic and inorganic forms and their relative abundance could determine P supplying capacity of soil. Differential input of exogenous and plant-mediated phosphorus and carbon in soil under different land-uses could influence P availability and fertilizer P management. While the effect of land-use on soil organic carbon (SOC) is fairly well-documented, its effect on soil P fractions is relatively less known. We investigated the effect of different land-uses including rice–wheat, maize–wheat, cotton–wheat cropping systems and poplar-based agroforestry systems on soil P fractions and organic carbon accrual in soils. Total P concentration was the highest under agroforestry (569 mg P kg^?1) and the lowest under maize–wheat (449 mg P kg^?1) cropping systems. On the contrary, soils under rice–wheat had significantly higher available P concentration than the agroforestry systems, probably because of higher fertilizer P application in rice–wheat and prevailing wetland conditions during rice growth. In soils under sole cropping systems viz. rice–wheat, maize–wheat and cotton–wheat, inorganic P was the dominant fraction and accounted for 92.2–94.6% of total P. However, the soils under agroforestry had smaller proportion (73%) of total P existing as inorganic P. Among soil P fractions, water soluble inorganic P (0.13–0.26%) represented the smallest proportion inorganic P in soils under different land-uses. Agroforestry showed significantly (p < 0.05) higher concentrations of SOC than the other land-uses. Soil organic C was significantly correlated with soil P fractions. It was concluded that poplar-based agroforestry systems besides leading to C accrual in soil result in build-up of organic P and the P supplying capacity of soil.