Impact of sewage sludge application on the long-term nutrient balance in acid soils of Scots pine (Pinus sylvestris, L.) forests

The objective of the investigation was to determine the effects of sewage sludge application on nutrient concentrations in soil and plant biomass fractions in Scots pine forests (Pinus sylvestris, L.), situated on sandy soils with low pH, in a south to north temperature gradient in Sweden. Twenty tons dw ha(-)1 of sewage sludge was applied in 50 to 60 yr old pine forests at four sites from Brosarp in South Sweden to Jukkasjarvi in the northern parts of the country. Application of 20 ton dw ha(-1) of sewage sludge significantly increased the concentrations of extractable N, P, K, Ca, Mg and Na, in both the mor layer and in the upper 10 cm of the mineral soil. Three years after sludge application K concentrations were only significantly increased in the upper 10 cm of the mineral soil. After 11 yr the concentrations of P were still at the same level in the mor layer as after three years. The concentrations of Ca, Mg and Na had slightly decreased only in the mor layer. There was, in most cases, a statistically significant positive correlation between the amount of applied sludge and nutrient concentrations in the soil, as well as in pine needles and in leaves of Vaccinium vitis-idaea. In all sites, Mg concentrations in the mor layer was positively and significantly correlated with Mg concentrations in current-year pine needles. Similarly, concentrations of Ca, Mg, and P in the mor layer were correlated with concentrations of these elements in current-year shorts of Vaccinium vitis-idaea.