The effect of afforestation on water recharge and nitrogen leaching in the Netherlands

Water balances and nitrogen budgets were studied in two chronosequences of oak and spruce on former arable land. Quantity and quality of rainfall and throughfall, soil water contents and concentrations in the soil solution were measured during a period of 1¿2 years. Hydrological fluxes were calculated using the soil hydrological model SWAP. Nitrogen leaching fluxes were based on monthly measured concentrations and simulated hydrological fluxes. Results showed that water recharge declined from approximately 485 mm/yr in arable land to 172 mm in the 18-year old oak stand and approximately 100 mm in the 13 and 14-year old spruce stands. For both chronosequences the decline in water recharge upon afforestation can be described by a power function; the exponent being ¿0.22 for oak and somewhat higher, ¿0.31, for spruce. Nitrogen leaching fluxes were negligible in the spruce stands and declined with age in the oak stands, from 16 kg/ha/yr at the youngest stand to 8 kg/ha/yr at the 18-year old stand. The nitrogen budget for the four oak stands increased with age. An explanation for this unexpected result may be the declining release of nitrogen by mineralization of organic matter present in the (former) agricultural soil. The data provide valuable information for the validation of simulation models and decision support systems used for policy decisions.