Nitrogen and phosphorus in soil solutions and drainage streams in Upper Teesdale, northern England: implications of organic compounds for biological nutrient limitation

The availability of nitrogen (N) and phosphorus (P) to organisms is a fundamental control on the ecology of uplands in the British Isles, yet there has been relatively little consideration of the role of organic forms of these nutrients. N and P fractions in soil solutions and drainage streams were determined at monthly intervals over 1 year from Widdybank Fell, Upper Teesdale National Nature Reserve, an upland area of northern England. Nitrogen was present in inorganic and organic forms at relatively high concentrations, whereas P was mainly organic, with inorganic P concentrations near the detection limit (1 μg P l−1). The highest concentrations of organic P occurred in ‘pulses’ during the spring. These pulses may have originated in the soil following drying and rewetting cycles, and appear to demonstrate hydrochemical connectivity between soils and streams. Seasonal changes in N:P ratio were estimated in various ways to assess when the availability of these elements was likely to influence organisms. Based on inorganic nutrients alone, marked P limitation would be expected throughout the year, but inclusion of organic nutrients suggests that some communities may become N-limited during the spring. This agrees with known seasonal changes in the biology of the cyanobacterium Rivularia, which dominates the main drainage streams. Our data highlight the importance of including organic nutrients when assessing biological nutrient limitation.