Nitrogen availability in some Wisconsin forests: comparisons of resin bags and on-site incubations

Estimates of ammonium and nitrate availability in conifer and hardwood forests using an ion exchange resin (IER) bag method and with on-site incubations of soil cores in buried bags were compared. Correlations between the two methods were generally high. Correlation coefficients (r) between IER nitrate and buried-bag mineralized nitrate ranged from 0.87 to 0.92. Both methods also correlated well with aboveground net primary production, litter fall N content, and fine root biomass. The major differences between the methods related to the relative importances of ammonium and nitrate forms of available N. The IER method indicated that both ammonium and nitrate were important on all sites, with nitrate predominating in most soils. The buried-bag results indicated that available N was primarily in the form of nitrate (all ammonium was oxidized), but that nitrate was insignificant on infertile sites.