磷的生物有效性对山地生态系统的影响

磷(P)的生物有效性对山地生态系统的发育和稳定至关重要。由于大气CO_2浓度升高和N沉降增加,生态系统C、N和P的化学计量比失衡,P的生物有效性受到更多关注。近年来山地系统中P的研究不断深入,2004—2013年间ISI Web of Knowledge中相关研究论文几乎是此前近百年的3倍。总结了山地生态系统中P的生物有效性的特点及其对植物物种多样性和初级生产力的影响。山地生态系统P的生物有效性因垂直高差和地形梯度空间变异明显,快速物质运移和生物过程是控制山地生态系统P的生物有效性的关键因素。P的生物有效性可以影响山地生态系统物种多样性和初级生产力,其对初级生产力的限制存在于全球范围内的山地生态系统。当P的生物有效性发生改变时,山地生态系统的结构越复杂,其植物物种多样性和初级生产力的响应可能会越平缓。全球变化的重要驱动因子(如增温和N沉降增加)可以直接或间接地改变山地生态系统P的生物有效性,因此需要在山地生态系统中加强长期监测和养分控制实验,并结合新型P分析技术,以期认识山地生态系统P的生物有效性的现状、变化趋势和对生态系统的影响,从而为适应全球变化背景下山地生态系统养分状况的改变提供依据。

Phosphorus bioavailability in mountain ecosystems: characteristics and ecological roles

Phosphorus (P) is an essential element for sustainable development of terrestrial ecosystems. Mountains cover approximately 20% of the land area. The biogeochemical cycle of P in the mountain ecosystems differs from that in lowland ecosystems because of the huge spatial heterogeneity, fast transport of materials, and relatively rich biodiversity. As global changes, the increase in temperature, atmospheric CO₂ concentration, and N deposition have disrupted the stoichiometric equilibrium of C, N, and P. In mountain ecosystems, especially alpine regions that are sensitive to global changes, P bioavailability has become a hot area in the field of biogeochemical-cycle research. Nearly three times as many articles on the biogeochemical cycle of P in mountain ecosystems were published in the last decade in comparison with the century before. This review summarizes the characteristics of P bioavailability in mountain ecosystems; its effects on plant biodiversity and primary productivity are comprehensively explored. The P bioavailability in mountain ecosystems differs from that in lowland ecosystems in three ways: (1) there is obvious spatial heterogeneity in P bioavailability (because of steep elevation and topographic gradients), which exists on different spatial scales and in various forms depending on the factors influencing the biogeochemical cycle of P; (2) the P bioavailability in the mountains is significantly affected by the fast P transport, because hydrologic and gravitational processes are accelerated by the steep elevation gradient; and (3) biological processes are the key factors influencing the P bioavailability in the mountains. Plants and microorganisms act both as active pools and regulators in the P cycle. It is generally accepted that diversity of plant species reaches its maximum at moderate P bioavailability. Under conditions of low and high P bioavailability, P limitations and competitive exclusion may suppress the diversity of plant species. Nevertheless, in mountain ecosystems with complex structures, some mechanisms (including disturbance, niche separation, and rich biodiversity) can alleviate this suppression; thus, plant species diversity may be affected to a lesser degree by the changes in P bioavailability. In terms of the primary productivity, limitations on P bioavailability can be found in mountain ecosystems worldwide according to field surveys and nutrient manipulation experiments. In contrast to the croplands featured by single species, the responses of primary productivity in the mountain ecosystems to P bioavailability can be affected by composition of the set of plant species. When P bioavailability decreases, species with high P utilization efficiency can become more dominant; thus, the primary production may not decrease substantially. In addition, environmental factors such as temperature, water, wind, and radiation in mountain ecosystems may further weaken the responses of primary productivity to P bioavailability. Therefore, just as the plant species diversity, the primary productivity in mountain ecosystems with complex structure may be less sensitive to the changes in P bioavailability. The P bioavailability in the mountains is more sensitive to global changes (e.g., an increase in temperature and N deposition) because these regions generally have low temperature and N limitations. An increase in temperature and N deposition can alter the P bioavailability in mountain ecosystems directly or indirectly. Meanwhile, P bioavailability can, to a certain degree, affect the responses of mountain ecosystems to global changes. Long-term field observation and nutrient manipulation experiments, together with the use of new P analysis techniques such as X-ray absorption near-edge structure (XANES) spectroscopy and nuclear magnetic resonance (NMR) open up new opportunities to elucidate the P status, its changing trends, and its influences on mountain ecosystems. On the basis of this knowledge, it is possible to achieve better adaptation to the changing nutrient status in mountain ecosystems.