Sediment enrichment ratios after mechanical site preparation for Pinus radiata plantation in the Basque Country

The enrichment of organic matter, total nitrogen, phosphorus, and exchangea ble calcium, magnesium, sodium, and potassium in runoff sediment was studie d by means of sediment traps in two sleep slopes (25-34 degrees) for a peri od of 18 months after site preparation (scalping and down-slope ripping) fo r Pinus radiata plantation. Total nitrogen concentration was significantly higher in the source soils than in their sediments. Phosphorus, and exchang eable calcium, magnesium, and potassium concentrations were higher in the s ediments, although not significantly. No relation was found between sedimen t enrichment ratios and total rainfall and rainfall intensity. Steep slopes and hydraulic soil-surface conditions favourable to runoff production are suggested to govern the high erosive power of runoff, which results in a no nselective displacement of soil particles. In spite of this lack of enrichm ent of the sediments, surface soil samples taken 18 months after site prepa ration showed significantly lower concentrations of organic matter, total n itrogen, phosphorus, and exchangeable magnesium, sodium, potassium, and alu minium, and higher aluminium saturation of the exchange complex in relation to samples taken before site preparation. Removal of the nutrient-rich org anic and mineral surface horizons by scalping, rather than soil water erosi on taking place afterwards, is the process responsible for most of the decr ease in soil nutrient concentration occurring as a result of sire preparati on. (C) 1999 Elsevier Science B.V. All rights reserved.