University of Vienna, Department of Chemical Ecology and Ecosystem Research, Althanstrasse 14, A-1090 Vienna, Austria.
Ecology. 2011 May;92(5):1036-51. doi: 10.1890/10-1011.1.
Soil microbes in temperate forest ecosystems are able to cycle several hundreds of kilograms of N per hectare per year and are therefore of paramount importance for N retention. Belowground C allocation by trees is an important driver of seasonal microbial dynamics and may thus directly affect N transformation processes over the course of the year. Our study aimed at unraveling plant controls on soil N cycling in a temperate beech forest at a high temporal resolution over a time period of two years, by investigating the effects of tree girdling on microbial N turnover. In both years of the experiment, we discovered (1) a summer N mineralization phase (between July and August) and (2) a winter N immobilization phase (November-February). The summer mineralization phase was characterized by a high N mineralization activity, low microbial N uptake, and a subsequent high N availability in the soil. During the autumn/winter N immobilization phase, gross N mineralization rates were low, and microbial N uptake exceeded microbial N mineralization, which led to high levels of N in the microbial biomass and low N availability in the soil. The observed immobilization phase during the winter may play a crucial role for ecosystem functioning, since it could protect dissolved N that is produced by autumn litter degradation from being lost from the ecosystem during the phase when plants are mostly inactive. The difference between microbial biomass N levels in winter and spring equals 38 kg N/ha and may thus account for almost one-third of the annual plant N demand. Tree girdling strongly affected annual N cycling: the winter N immobilization phase disappeared in girdled plots (microbial N uptake and microbial biomass N were significantly reduced, while the amount of available N in the soil solution was enhanced). This was correlated to a reduced fungal abundance in autumn in girdled plots. By releasing recently fixed photosynthates to the soil, plants may thus actively control the annual microbial N cycle. Tree belowground C allocation increases N accumulation in microorganisms during the winter which may ultimately feed back on plant N availability in the following growing season.
温带森林生态系统中的土壤微生物每年能够循环数百公斤的氮，因此对氮的保留至关重要。树木的地下碳分配是季节性微生物动态的重要驱动因素，因此可能直接影响一年中氮转化过程。我们的研究旨在通过调查树木环割对微生物氮周转的影响，在两年的时间内以高时间分辨率揭示温带山毛榉林中植物对土壤氮循环的控制作用。在实验的两年中，我们发现了（1）夏季氮矿化阶段（7 月至 8 月之间）和（2）冬季氮固定阶段（11 月至 2 月）。夏季矿化阶段的特点是氮矿化活性高、微生物氮吸收低，随后土壤中氮的可用性高。在秋季/冬季氮固定阶段，总氮矿化率低，微生物氮吸收超过微生物氮矿化，导致微生物生物量中氮含量高，土壤中氮的可用性低。冬季观察到的固定阶段可能对生态系统功能至关重要，因为它可以防止秋季凋落物降解产生的溶解态氮在植物大部分不活跃的阶段从生态系统中流失。冬季和春季微生物生物量氮水平之间的差异为 38 公斤/公顷，因此几乎占植物年氮需求的三分之一。树木环割强烈影响了年氮循环：环割样地中冬季氮固定阶段消失（微生物氮吸收和微生物生物量氮显著减少，而土壤溶液中有效氮的量增加）。这与环割样地中秋季真菌丰度降低有关。通过将最近固定的光合产物释放到土壤中，植物可能会主动控制年微生物氮循环。树木地下碳分配在冬季增加了微生物中氮的积累，这可能最终反馈到下一个生长季植物氮的可利用性上。
