Nitrogen deposition and climate effects on soil nitrogen availability: influences of habitat type and soil characteristics.

The amount of plant-available nitrogen (N) in soil is an important indicator of eutrophication of semi-natural habitats, but previous studies have shown contrasting effects of N deposition on mineralisable N in different habitats. The stock of readily mineralisable N (N(rm)) was measured in 665 locations across Britain from a range of intensively and extensively managed habitats, allowing N availability to be studied in relation to soil and vegetation type, and also to variation in climate and in reactive N deposition from the atmosphere. Mineralisable N contents were correlated with deposition in extensively managed habitats but not in intensively managed habitats. The following statements apply only to extensively managed habitats. All habitats showed a similar increase in N(rm) with N deposition. However, soil characteristics affected the relationship, and soil carbon content in particular was a major control on mineralisation. The N(rm) stock increased more with N deposition in organic than in mineral soils. The nitrate proportion of N(rm) also increased with N deposition but, conversely, this increase was greater in mineral than in organic soils. The measurements could be used as indicators of eutrophication, e.g. deposition rates of over 20 kg N ha(-1) y(-1) are associated with nitrate proportions of >41% in a mineral soil (2% carbon), and with N(rm) stocks of over 4.8 kg N ha(-1) in an organic soil (55% carbon). Both N(rm) and nitrate proportion increased with mean annual temperature of the sampling location, despite consistent incubation temperature, suggesting that increasing temperatures are likely to increase the eutrophying effects of N pollution on semi-natural ecosystems.