Differences of Soil Microbial Biomass and Nitrogen Transformation under Two Forest Types in Central Germany

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In order to observe the tree species effect on soil N status, soil microbial biomass C and N (Cmic, Nmic), potential N mineralization and potential nitrification (under laboratory incubation conditions, 22 °C) in different subhorizons (LOf1, Of2, Oh and mineral soil at 0–10 cm depth) were determined at three forest sites in central Germany. At each site, two contrasting stands (Beech, Norway spruce or Scots pine) were selected, where the initial soil conditions were similar. Three sampling dates that represented different stages of tree growth were selected: growing season - August, dormant season - November, after budbreak – April. In organic layers, Cmic-to-total C (Ct) ratios under beech and under conifer were 0.72–4.74% and 0.34–2.11%, respectively. Nmic-to-total N (Nt) ratios were 2.47–11.61% and 0.71–5.77%, respectively. Both concentrations of Cmic and Nmic were significantly affected by the stand type and sampling time. Potential N mineralization rates, ranging from 3.7 to 19.7 mg N kg−1 d−1, showed no clear pattern in relation to stand type. However, potential nitrification rates were mostly significantly higher under beech than under contrasting conifer. In mineral soils, concentrations of Cmic and Nmic showed a clear temporal pattern in the order: August>November>April. The average Nmic and Nmic-to-Nt were higher in soils from beech than conifer, while Cmic and Cmic-to-Ct ratios were similar between the two forest types. In organic layers, the highest values of Cmic-to-Nmic ratio and Cmic were found in November samples, especially under beech. By contrast, in mineral soils the highest value of Cmic-to-Nmic ratios were found in April samples, and at that time the Cmic concentrations were the lowest, especially under conifer. These results revealed the differences in microbial growth form and survival strategy associated with different tree species and soil layers.