SPATIAL PATTERNS IN STREAM NITRATE CONCENTRATIONS IN UPLAND WALES IN RELATION TO CATCHMENT FOREST COVER AND FOREST AGE

Data on nitrate nitrogen were collected weekly during 1984 from 136 si tes on streams in upland Wales. Mean nitrate concentrations in summer (<0.02 to 1.05 mg litre(-1)) were significantly lower (P<0.001) than i n winter (<0.02 to 1.26 mg/litre(-1)), particularly at sites with matu re conifers (>30 years old). Mean concentrations increased significant ly with the average age of conifers on each catchment (P<0.001), and w ith increasing areal cover by trees over 30 years old (P<0.001). Nitra te concentrations increased significantly with stream total hardness ( P<0.001), possibly reflecting nitrogen mineralisation in soils of high er base status. Concentrations also increased with stream chloride (P< 0.001), which is predominantly atmospherically derived, implying that increased nitrate occurred where general atmospheric inputs of solutes were increased. After accounting for variation in hardness, residual nitrate concentrations still increased with the average age of the con ifers (P<0.001), and with catchment cover by mature trees (P<0.001). W e infer that some additional nitrate under older conifers is thus inde pendent of catchment sources associated with increasing hardness. Two possibilities are increased inputs and decreased retention of nitrogen within the ecosystem of maturing conifer forest. Residual nitrate aft er accounting for variations in chloride also increased significantly with conifer age (P<0.01) and cover (P<0.01), a pattern implying that some sources of nitrate may also be independent of increased sea-salt deposition. We allude to the possibility that additional nitrogen depo sition adds to nitrogen throughputs from maturing forests, and we disc uss the potential ecological role of additional nitrogen in runoff