Soil phosphorus retention in a Mediterranean pasture subjected to differential management

The objective of this study was to test a simple input-output model to estimate soil phosphorus (P) retention over 12 years in a Mediterranean pasture. A 6-ha permanent pasture field in Southern Portugal was subjected to different management systems in terms of grazing and fertilizer application. The soil and pasture were monitored and samples were geo-referenced with RTK GPS. The model was developed to explain the effect of P fertilizer, based on the premise that P added through fertilizer and amounts removed in harvested plant products are the main fluxes affecting available P status, and was used to estimate the soil P retention. The results show that P fertilizer addition to Mediterranean grazed pastures on shallow soils influences several P dynamics under different grassland management systems. A positive linear relationship between the P budgets and the changes in measured P was demonstrated over the study periods. This work highlights the difficulty associated with the creation of recommendation maps for differentiated P fertilizer application in pastoral systems and indicates that before any accurate decision making, the historic knowledge of changes in the concentrations of P in the soil should be considered, as well as a prediction of the residual effects of applied nutrient. The importance of this knowledge increases when the aim is to use techniques and technologies for variable-rate fertilizer application and implementation of site-specific management strategies in the context of precision agriculture.