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

Abstract

The enrichment of organic matter, total nitrogen, phosphorus, and exchangeable calcium, magnesium, sodium, and potassium in runoff sediment was studied by means of sediment traps in two steep slopes (25–34°) for a period of 18 months after site preparation (scalping and down-slope ripping) for Pinus radiata plantation. Total nitrogen concentration was significantly higher in the source soils than in their sediments. Phosphorus, and exchangeable calcium, magnesium, and potassium concentrations were higher in the sediments, although not significantly. No relation was found between sediment 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 nonselective displacement of soil particles. In spite of this lack of enrichment of the sediments, surface soil samples taken 18 months after site preparation showed significantly lower concentrations of organic matter, total nitrogen, phosphorus, and exchangeable magnesium, sodium, potassium, and aluminium, and higher aluminium saturation of the exchange complex in relation to samples taken before site preparation. Removal of the nutrient-rich organic and mineral surface horizons by scalping, rather than soil water erosion taking place afterwards, is the process responsible for most of the decrease in soil nutrient concentration occurring as a result of site preparation.