Plant cover and control of diffuse pollution from P in olive groves

Plant cover modifies the physicochemical properties of the soil surface, which results in an enrichment in organic matter and a greater infiltration. So, this study was conducted to determine if its establishment in olive groves was effective, compared to conventional tillage, for the reduction in pollution by soluble phosphorus (P). Surface runoff, soluble P and Olsen P losses in sediment were analyzed in three experiment fields of ecological olive trees in the province of Cordoba (Spain), from 1 June 2003 to 1 June 2005. The cover system reduced the total losses of both variables in the three plots (between 7.6% and 36.5% the dissolved P loss and between 16.3% and 56.4% that of the runoff), with a certain parallelism being observed in the time distribution of the losses in both soil management systems, with significant Spearman coefficients of correlation, ranging between 0.60 and 0.98. Over half the losses of the runoff and dissolved P were produced in two or three events for the two management systems. The establishment of plant cover altered the relative composition in the Olsen P loss (raising the proportion of soluble P) and usually increased the dissolved P rate. Finally, and although it is a positive technique and highly recommendable in the area's olive groves, it was not completely effective in controlling water pollution in relation to the soluble P concentration, since this was over 0.11 mg l(-1) in all the cases with a cover.