矿质养分输入对森林生物固氮的影响

Effects of the addition of mineral nutrients on biological nitrogen fixation in forest ecosystems

生物固氮是森林生态系统重要的氮素来源,并且在全球氮循环中占有重要的地位。近代以来,因人类活动加剧而导致氮沉降的增加以及其它矿质养分元素(如磷、钼、铁等)输入的改变已成为影响森林生态系统生物固氮的重要因素之一,并引起了学术界的普遍关注。综述了国内外关于森林生物固氮对矿质养分输入的响应及机理。主要内容包括:(1)森林生物固氮的概念及主要的测定方法;(2)矿质养分输入对森林生物固氮的影响。整体上讲,氮素输入抑制了森林生物固氮,磷和其他营养元素输入则表现为促进作用。氮和磷、磷和微量元素同时添加均提高了森林的固氮量;(3)矿质养分改变森林生物固氮的机理。包括生物作用机制(如改变地表层固氮菌的数量或群落丰度、改变结瘤植物的根瘤生物量和附生植物的丰度或盖度)和环境作用机制(如引起土壤酸化、改变碳源物质的含量);(4)探讨了矿质养分输入对森林生物固氮影响研究中所存在的问题,并对未来该领域的研究提出建议。

Biological nitrogen fixation （BNF） in forest ecosystems plays a very important role in global nitrogen （N） cycling, because forest ecosystems cover 30% of the global land area and represent nearly half of the N fixation that occurs in terrestrial ecosystems. Additionally, these ecosystems harbor abundant N-fixation microbes, including both symbiotic and free-living N-fixation bacteria. It is generally believed that environmental conditions （e.g. temperature, moisture, and light intensity） and nutrient availability are the most important factors regulating BNF in ecosystems. Among these factors, the effects of nutrient availability have recently received increasing attention. In the last several decades, anthropogenic activities have greatly modified the global biogeochemical cycles of N and other mineral nutrients. For example, the quantity of global N deposition increased from 41 Tg N/a in 1950 to 103 Tg N/a in 2000. Long-term N deposition may exert direct effects on forest BNF by reducing the competitive advantage of free-living N-fixation bacteria or by reducing the energy allocated to symbiotic N-fixation bacteria. N deposition may also lead to imbalances in mineral nutrient proportions that play vital roles in the growth of N-fixation bacteria. Phosphorus （P） , for example, is thought to be a major component of microbial cell membranes and also to be directly related to the formation of adenosine triphosphate （ATP） , which is veryimportant for nitrogenase function. In addition, many rock-derived nutrients （e.g. Ca, K, Mo, and Fe） are depleted and their availability becomes poor during forest succession. Deficiency of these mineral nutrients may consequently influence forest BNF by limiting the growth of N-fixation bacteria. In recent decades, mineral nutrient addition experiments （ in the lab or in situ） have been widely used in Europe and North America to investigate how nutrient availability influences forest BNF. However, little information on how forest BNF responds to nutrient addition is available from China, although N deposition is projected to increase with the rapid growth of the Chinese economy in the future. In this paper, we review the impacts and underlying mechanisms of mineral nutrient addition on forest BNF, based on the available literature. The objectives were to enhance our understanding of how mineral nutrient addition regulates forest BNF and to provide scientific data for sustainable forest management. This review focuses on the following four aspects ： （ 1 ） The concept of forest BNF and the principal measuring methods; （2） The effects of mineral nutrient addition on forest BNF, including the N fixation by free-living organisms occurring in the upper soil, the lichens and mosses growing on the litter layer, and symbiotic N fixation via relationships with root nodule plants （e.g., legumes）. In general, N addition decreases forest BNF, whereas addition of P or other nutrients has a positive impact. In addition, the combined addition of N and P, or P and other mineral nutrients, increases forest BNF; （3） The mechanisms underlying forest BNF can be divided into biotic （e.g., variance in the abundance or richness of N-fixing bacteria, the cover or richness of epiphytes, and the biomass of root nodule plants） and abiotic mechanisms （e.g., soil acidification and changes in the available carbon content） ; （4） The limitations of the current research and our suggestions for further research on forest BNF.