典型草原非生长季土壤微生物氮固持对不同功能群植物氮吸收的贡献

In most terrestrial ecosystems, plant growth is N-limited. Soil nitrogen (N) accumulation in non-growing season is vital for plant N uptake in early spring. A theory of seasonal succession of N cycles in high-latitude systems: in this concept soil microbes immobilize N in the absence of plant N uptake during autumn and winter, and release N at a time of maximum plant N demand. Such a concept of seasonal succession of N cycles has, to our knowledge, not yet been tested for temperate grasslands and the major driver mechanism to regulate soil microbial N immobilization is uncertainty. To examine these issues, we conducted a field experiment in a temperate steppe of northern China. We used 15N tracer, phospholipid fatty acid and DNA analysis methods to elucidate the pattern of seasonal microbial N cycle, explore major determinants of the pattern of non-growing season microbial N immobilization, and investigate the proportion of nitrogen uptake by different functional groups of plants during the microbial dieback in early spring. These issues will improve our understanding on predictions of plant community composition and ecosystem productivity, and the response of them to the climate change.

在很多陆地生态系统中，植物的生长受到氮素条件的限制，非生长季土壤氮素的积累量对植物的养分吸收十分关键。基于高纬度地区的研究结果所提出的土壤微生物氮循环季节演替模型认为，秋冬季土壤微生物固持的氮是春季植物生长的主要营养源。然而，该模型在温带草原生态系统中的适用性及非生长季微生物氮固持的机制和主要驱动因素尚未探究；非生长季微生物固持的氮对不同功能群植物氮吸收的贡献仍不确定。本研究以温带典型草原羊草草地为研究对象，采用同位素示踪、磷脂脂肪酸和核酸分析等方法阐明土壤微生物氮循环的季节性格局，揭示非生长季微生物氮固持的机制和主要驱动因素，量化春季氮释放被不同功能群植物所吸收利用的比例，这些研究对于理解和预测植被生产力和植物群落组成变化及其对未来气候变化的响应具有重要的意义，为陆地生态系统氮循环模型提供充足和有力的参数依据。

在大多陆地生态系统中，植物的生长受到氮素的限制，非生长季土壤微生物固持的氮素对植物的氮吸收十分关键。本研究以我国北方典型草原羊草草地为研究对象，通过野外控制实验和实验室分析，开展持续两年土壤微生物氮循环的季节性格局，非生长季微生物氮固持的机制和主要驱动因素，春季氮释放对不同功能群植物氮吸收贡献的研究。结果表明，与高纬度地区不同，温带典型草原土壤微生物无冬季氮固持的现象，植物光合产物分配不是驱动微生物冬季氮固持现象的主因。不同功能型植物冬春过渡期吸收氮素的时间和量不尽相同。丛生禾草在融雪前开始吸收氮素，根茎禾草和多年生杂类草从融雪时开始吸收氮素，而灌木和豆科植物几乎未吸收冬季固持的氮素，其中多年生杂类草利用最多。本研究结果对于理解和预测植被生产力及其对未来气候变化的响应具有重要的意义，为陆地生态系统氮循环模型提供充足和有力的参数依据。