Nitrate production and availability in residential soils

The rapid increase in residential land area in the United States has raised concern about water pollution associated with nitrogen fertilizers. Nitrate (NO3-) is the form of reactive N that is most susceptible to leaching and runoff; thus, a more thorough understanding of nitrification and NO3- availability is needed if we are to accurately predict the consequences of residential expansion for water quality. In particular, there have been few assessments of how the land use history, housing density, and age of residential soils influence NO3- pools and fluxes, especially at depth. In this study, we used 1 m deep soil cores to evaluate potential net nitrification and mineralization, microbial respiration and biomass, and soil NO3 and NH4+ pools in 32 residential home lawns that differed by previous land use and age, but had similar soil types. These were compared to eight forested reference sites with similar soils. Our results suggest that a change to residential land use has increased pools and production of reactive N, which has clear implications for water quality in the region. However, the results contradict the common assumption that NO3- production and availability is dramatically higher in residential soils than in forests in general. While net nitrification (128.6 +/- 15.5 mg.m(-2).d(-1) vs. 4.7 +/- 2.3 mg.m(-2).d(-1); mean +/- SE) and exchangeable NO3- (3.8 +/- 0.5 g/m(2) vs. 0.7 +/- 0.3 g/m(2)) were significantly higher in residential soils than in forest soils in this study, these measures of NO3- production and availability were still notably low, comparable to deciduous forest stands in other studies. A second unexpected result was that current homeowner management practices were not predictive of NO3- availability or production. This may reflect the transient availability of inorganic N after fertilizer application. Higher housing density and a history of agricultural land use were predictors of greater NO3- availability in residential soils. If these factors are good predictors across a wider range of sites, they may be useful indicators of NO3- availability and leaching and runoff potential at the landscape scale.