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 (Nrm) 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 Nrm with N deposition. However, soil characteristics affected the relationship, and soil carbon content in particular was a major control on mineralisation. The Nrm stock increased more with N deposition in organic than in mineral soils. The nitrate proportion of Nrm 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 Nrm stocks of over 4.8 kg N ha− 1 in an organic soil (55% carbon). Both Nrm 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.