IMPACT OF NITROGEN DEPOSITION ON NITROGEN CYCLING IN FORESTS - A SYNTHESIS OF NITREX DATA

Impact of nitrogen (N) deposition was studied by comparing N fluxes, N concentrations and N pool sizes in vegetation and soil in five conife rous forest stands at the NITREX sites: Gardsjon (GD), Sweden, Kloster hede (KH), Denmark, Aber (AB), Wales, UK, Speuld (SP), the Netherlands , and Ysselsteyn (YS), the Netherlands. The sites span a N-deposition gradient from 13 to 59 kg N ha(-1) yr(-1). Measurements of soil N tran sformation rates by laboratory and field incubations were part of the site comparison. Further, results from 4-5 yr of NH4NO3, addition (35 kg N ha(-1) yr(-1)) at low deposition sites (GD, KH, AB) and 6 yr of N removal (roofs) at high deposition sites (SP, YS) were included in th e analysis. Significant correlations were found between a range of var iables including N concentrations in foliage and litter, soil N transf ormation rates and forest floor characteristics. Using the methods fro m principal component analysis (PCA) these variables were summarized t o an index of site N status that assigned the lowest N status to GD an d the highest to YS. Site N status increased with N deposition with th e exception that AB was naturally rich in N. Nitrate leaching was sign ificantly correlated with N status but not correlated with N depositio n. Forest floor mass and root biomass decreased with increased N statu s, Characteristics of the mineral soil were not correlated with vegeta tion and forest floor variables. High C/N ratios in the mineral soil a t the high-N deposition sites (SP, YS) suggest that the mineral soil p ool changes slowly and need not change for N saturation to occur. Nitr ogen transformation rates measured in laboratory incubations did not a gree well with rates measured in the field except for a good correlati on between 'gross' mineralization in the laboratory and 'net' minerali zation in the field. The changes in N concentrations and fluxes after manipulation of N input followed the direction expected from the site comparison: increases at N addition and decreases at N removal sites. Nitrate leaching responded within the first year of treatment at all s ites, whereas responses in vegetation and soil were delayed. Changes i n N status by the manipulation treatments were small compared to the d ifferences between sites. Changes in nitrate leaching were small at th e low-N status sites and substantial at the high-N status sites. Nitro gen-limited and N-saturated forest ecosystems could be characterized q uantitatively. (C) 1998 Elsevier Science B.V.