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.本研究针对3个山毛榉森林生态系统，采用3种不同的原位（in situ）方法——埋袋法（buried-bag method）、试管法（tube method）、野外淋溶法（field leaching method）——开展土壤氮净矿化（net N-mineralisation）研究。结果显示，原位埋袋法测得的矿化速率最高，其次为原位试管法与野外淋溶法。在3种方法中，当结合植被枯落物氮输入、输入输出平衡与土壤储量调查数据进行校验时，原位试管法得到的氮数值最接近预期值。 本研究采用淋溶法测定了土壤碳净矿化（net C-mineralisation）量，其结果低于植物枯落物的碳输入量，这表明野外淋溶法未纳入细根与草本植物枯落物，可能低估了土壤碳（及氮）的矿化水平。 索林（Solling）样地酸性粗腐殖质（acid moderhumus）中的氮、碳净矿化占比，与哥廷根林（Göttinger Wald）样地弱酸性土壤的表层矿质土壤（mineral soil）中测得的占比相近。索林样地的高土壤酸度抑制了其矿质土壤中的蚯蚓与微生物活性，并将微生物活性转移至地表有机层（surface organic layer）。 矿化产物的碳氮比（C/N ratios）低于土壤有机质（soil organic matter）的碳氮比，这说明索林土壤地表有机层中，一部分氮浓度较高的土壤有机质组分可作为微生物呼吸（microbial respiration）的优先底物。该组分可能来源于大气氮在地表有机层中的固持（immobilisation）作用。 索林样地的酸性土壤中含有大量异养硝化菌（heterotrophic nitrifiers），导致其相对硝化率较低，仅为所产生矿质氮（mineral N）的12%~51%。而哥廷根林样地的弱酸性土壤中存在活跃的自养硝化菌（autotrophic nitrifiers），可将超过87%的氮净矿化产物转化为硝酸盐（nitrate）。索林样地的异养硝化菌对温度与土壤湿度变化敏感，在低温与干旱土壤条件下硝化作用较弱。