Significance of litter- and humus-layer quality for rates and forms of N cycling in moder- to rawhumus-moder profiles under Scots pine (Pinus sylvestris L.)

Abstract

In three Scots pine stands in northeastern Germany along a gradient of air pollutant deposition loads (particularly sulfate and alkaline dust), the net N release by decomposition, net N immobilization by microorganisms, net N in- and output via soil solution, net N uptake by plants and net N exudation were studied utilizing the soil core method. Apart from NH4-N and NO3-N dissolved organic nitrogen (DON) was measured. The chemical properties of the organic horizons are altered with respect to what is expected from the classification of the moder and rawhumus-moder profiles. These changes are due to former air pollution and site management impacts. Lowest pH values (3.8 in H2O) and widest C:N ratios (27) are found in the moder at the background site Neuglobsow. High concentrations of total extractable N (250 mg N kg−1 d.m.) at this site are explained by the fairly high proportion of extracted DON (64%). The high DON concentrations are correlated to the low pH as well as to high concentrations of total N. Both conditions may stimulate specific microbial activity, known to control the mobilization of soluble organic matter. A shift of the dominant N form to mineral N, especially to NO3-N is observed at the former pollution site Rosa, where pH values are at about 5.2 (H2O). At Rosa the annual net N turnover (189 kg N ha−1 yr−1) is by far higher than at the two other sites (Neuglobsow: 45 and Taura: 70 kg ha−1 yr−1). This holds also true for the annual net N losses (Rosa: 29 kg N ha−1 yr−1, Taura: 9 kg N ha−1 yr−1). At Neuglobsow, instead, net N input of 9 kg ha−1 yr−1 was observed. At the site with the lowest N-input via deposition/fertilization, i.e. Neuglobsow, DON plays an important role for the N fluxes, especially for the N uptake by plants.