Nutrient losses by runoff and sediment from the taluses of orchard terraces

The nutrient fluxes of nitrate, ammonium, phosphorus and potassium in runoff and sediments were evaluated over a two-year period ( 1999 - 2000) on the taluses of terraces, in a zone of intense subtropical orchard cultivation ( SE Spain). The erosion plots were located on a terrace of 214% (65degrees) slope, at 180 m in altitude and with 16 m(2) ( 4 x 4 m) in area. Shrubby covers were tested for effectiveness in controlling the nutrient fluxes caused by runoff and sediments. Covers of Thymus serpylloides Bory sbsp. Gadorensis and Salvia officinalis L. reduced the NO3- runoff losses by 53 and 48%, with respect to the bare soil without natural vegetation, the NH4+ 61 and 56%; the PO4-3 65 and 56%; and K+ 58 and 46%, respectively. A greater proportion of NO3-, NH4+ and K+ were transported in runoff than in sediments. Thyme and sage with respect to the control reduced NO3- loss in sediments by 74 and 65%, NH4+ by 71 and 62%, P by 72 and 67%, and K by 69 and 61%, respectively. The total loss ( runoff and sediments) in the bare-soil plot for NPK was 260, 39 and 888 mg m(-2) yr(-1), in the sage plot 119, 15 and 460 mg m(-2) yr(-1), and in the thyme plot 105, 12 and 360 mg m(-2) yr(-1), respectively. The results show the importance of the plant covers in soil conservation and in the recycling of nutrients on terrace slopes. This has far-reaching implications in the sense that the control of pollution from erosion is vital in reducing the eutrophication of both surface waters and groundwater located in lowlands.