Effects of land use changes on the potential for soil to contribute phosphorus loads in watersheds

The long-term implications of rapid urbanization on water and soil quality have raised concern in recent years. The expansion of urban occupation in riparian zones contributes to the accumula-tion of phosphorus (P) in soil. This is even more worrying when it comes to regions that live with water scarcity, such as the Brazilian semiarid region. Increasing the risk of P transport to water bodies and can accelerate eutrophication, threatening water security. To understand the effects of land use changes on the potential of soil to contribute to overall watershed P loads, soil P availability, including its relationship to physical and chemical attributes in areas with distinct land uses, was assessed. A total of 43 sample plots distributed among natural 50 vegetation, agriculture, and exposed soil of the riparian zones of seven Brazilian semi-arid reservoirs we evaluated: Armando Ribeiro Goncalves; Boqueira similar to o de Parelhas; Gargalheiras; Dourado; Cruzeta; Itans; and Passagem das Trairas. Here, we quantified soil phosphorus enrichment caused by land use changes and found that exposed soil had the highest soil-available phosphorus contents, representing scenarios with the greatest potential to contribute to overall P loads. Agricultural soils in riparian zones were statistically grouped with natural vegetation soils for having the lowest available phosphorus contents, forming a group of soils at low risk of P transport. The highest bulk density and the lower soil organic matter, soil carbon storage capacity and potential acidity favor the maintenance of high available P contents and the persistence of the risk of diffuse P losses from the soil if more appropriate management measures are not adopted. Our data provide unreleased and useful information for monitoring global watershed P loads not only in the study area, but also in watersheds under similar tropical conditions.