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Impact of nitrogen deposition on nitrogen cycling in forests: a synthesis of NITREX data

1998; Elsevier BV; Volume: 101; Issue: 1-3 Linguagem: Inglês

10.1016/s0378-1127(97)00124-2

1872-7042

Per Gundersen, Bridget A. Emmett, O. Janne Kjønaas, C. J. Koopmans, Albert Tietema,

Peatlands and Wetlands Ecology

Impact of nitrogen (N) deposition was studied by comparing N fluxes, N concentrations and N pool sizes in vegetation and soil in five coniferous forest stands at the NITREX sites: Gårdsjön (GD), Sweden, Klosterhede (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 transformation 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 the analysis. Significant correlations were found between a range of variables including N concentrations in foliage and litter, soil N transformation rates and forest floor characteristics. Using the methods from principal component analysis (PCA) these variables were summarized to an index of site N status that assigned the lowest N status to GD and the highest to YS. Site N status increased with N deposition with the exception that AB was naturally rich in N. Nitrate leaching was significantly correlated with N status but not correlated with N deposition. Forest floor mass and root biomass decreased with increased N status. Characteristics of the mineral soil were not correlated with vegetation and forest floor variables. High CN ratios in the mineral soil at the high-N deposition sites (SP, YS) suggest that the mineral soil pool changes slowly and need not change for N saturation to occur. Nitrogen transformation rates measured in laboratory incubations did not agree well with rates measured in the field except for a good correlation between 'gross' mineralization in the laboratory and 'net' mineralization 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 sites, whereas responses in vegetation and soil were delayed. Changes in N status by the manipulation treatments were small compared to the differences between sites. Changes in nitrate leaching were small at the low-N status sites and substantial at the high-N status sites. Nitrogen-limited and N-saturated forest ecosystems could be characterized quantitatively.