Nitrogen and Carbon Transformations

Net N-mineralisation was studied using three different in situ methods (buriedbag method, tube method, field leaching method) at the three beech forest ecosystems. The highest rate was measured with the in situ buried-bag method fallowed by in situ tube method and the field leaching method. Of the three methods, the in situ tube method provided N-values which came close to theexpected ones when the litter N-input by vegetation, input/output balances and inventories were considered. Net C-mineralisation was measured with the leaching method and was lower than the input of C by plant litter suggesting that litter
from fine roots and herbs which was not included in the field leaching method might have underestimated the C (and N) mineralisation of soils.
Proportions of net N-mineralisation and net C-mineralisation in the acid moderhumus from the Solling site were similar to those measured in the surface mineral soil of the less acid soil at the Go¨ttinger Wald site. High soil acidity at the Solling site has reduced the earthworm and microbial activity in its mineral soil and has shifted the microbial activity to the surface organic layer.
Lower C/N ratios of the mineralisation products than those of the soil organic matter suggest that a fraction of soil organic matter in the surface organic layer of the Solling soil with a high N concentration was preferential substrate for microbial respiration. This fraction might derive from immobilisation of atmospheric nitrogen in the surface organic layer.
The acid soil of the Solling site contains a high fraction of heterotrophic nitrifiers causing low relative nitrification rates between 12 and 51% of the mineral N produced. However, autotrophic nitrifiers are active in the less acid soil at the Go¨ttinger Wald site and are responsible for the transformation of more than 87%of net N-mineralisation to nitrate. Heterotrophic nitrifiers at the Solling site are sensitive to temperature and soil moisture changes resulting in low nitrification at low soil temperatures and under dry soil conditions.