Soluble reactive phosphorus levels in rainfall, cloud water, throughfall, stemflow, soil waters, stream waters and groundwaters for the Upper River Severn area, Plynlimon, mid Wales

Abstract

Soluble reactive phosphorus (SRP) data are presented for rainfall, cloud water, soil waters, stream waters and groundwaters at the Plynlimon catchments in mid Wales to examine the hydrochemical functioning of inorganic phosphorus for an acidic and acid sensitive area characteristic of much of the UK uplands. In general, stream water concentrations are low compared to lowland areas. Average concentrations of SRP in rainfall and cloud water (0.3 and 0.9 μM l −1, respectively) are higher than in stream water with wider ranges in concentration (0–19.3 and 0–20.9 μM l −1, respectively). Throughfall and stemflow is enriched in SRP compared to rain and cloud water by a factor of approximately twofold and sixfold, respectively: the average concentrations and ranges are 0.73 and 0–6.61 μM l −1 for throughfall and 2.12 and 0–18.61 μM l −1 for stemflow. Soil water SRP concentrations measured in the surface layers of representative areas of podzol and gley soils, are further enriched with respect to inputs. Average concentrations and ranges for the L/F and Oh horizons in the podzols are 3.1 μM l −1 (range: 0.03–17.2 μM l −1) and 0.75 μM l −1 (range: 0.03–2.64 μM l −1), respectively. Correspondingly, the average values and ranges for the L/F and Oh horizons in the gley are 2 μM l −1 (range: 0.03–16.65 μM l −1) and 0.4 μM l −1 (range: 0.03–8.61 μM l −1). SRP concentrations in stream and ground water are lower than in atmospheric inputs and surface soil waters and show marked spatial variability. This variability is linked to three catchment features. (1) For streams draining podzolic soils, most of the SRP is retained by the catchment. For this situation, stream and ground waters have average concentrations of approximately 0.05 μM l −1 with a range of 0–1.47 μM l −1. There is no clear stream or groundwater SRP response to felling despite a large release of SRP from felling debris (brash) and the forest floor (L/F horizon) with average post-felling concentrations of 11.02 μM l −1 (0.40–155.0 μM l −1) and 23.60 μM l −1 (0.26–172.23 μM l −1), respectively. (2) For forested catchments with gley soils, stream water SRP concentrations are more variable with, in one case, much higher concentrations than for the podzol counterparts (range in average 0.05–0.46 μM l −1). (3) For the streams draining gley soils, felling results in a mixed SRP response. At the local scale (ditch drainage), there is a marked enrichment in SRP concentration (average concentrations increase from 0.05 to 1.31 μM l −1, with a peak concentration of 4.0 μM l −1). This response is consistent with the observed mobilisation of SRP from brash and forest floor material (post-felling mean concentrations of 9.39 and 11.94 μM l −1, respectively). However, stream water concentrations are an order of magnitude lower than observed in the soil waters implying considerable immobilisation of SRP between the soils and the stream. At the larger catchment scale, no discernable enrichment in SRP is observed following felling. The results are related to input–output budgets and the findings interpreted in terms of the dominant hydrogeochemical processes operative and environmental management issues.