Soil N chemistry in oak forests along a nitrogen deposition gradient

Anthropogenic N deposition may change soil conditions in forest ecosystems as demonstrated in many studies of coniferous forests, whereas results from deciduous forests are relatively scarce. Therefore the influence of N deposition on several variables was studied in situ in 45 oak-dominated deciduous forests along a N deposition gradient in southern Sweden, where the deposition ranged from 10 to 20 kg N ha(-1) year(-1). Locally estimated NO3- deposition, as measured with ion-exchange resins (IER) on the soil surface, and grass N concentration (%) were positively correlated with earlier modelled regional N deposition. Furthermore, the delta N-15 values of grass and uppermost soil layers were negatively correlated with earlier modelled N deposition. The data on soil NO3-, measured with IER in the soil, and grass N concentration suggest increased soil N availability as a result of N deposition. The delta N-15 values of grass and uppermost soil layers indicate increased nitrification rates in high N deposition sites, but no large downward movements of NO3- in these soils. Only a few sites had NO3- concentrations exceeding 1 mg N 1(-1) in soil solution at 50 cm depth, which showed that N deposition to these acid oak-dominated forests has not yet resulted in extensive leaching of N. The d15N enrichment factor was the variable best correlated with NO3- concentrations at 50 cm and is thus a variable that potentially may be used to predict leaching of NO3- from forest soils.