丛枝菌根真菌在土壤氮素循环中的作用

作为植物需求量最大的营养元素,氮素是陆地生态系统初级生产力的主要限制因子。丛枝菌根真菌能与地球上80%以上的陆生植物形成菌根共生体,帮助宿主植物吸收土壤中的P、N等矿质养分。目前,丛枝菌根真菌与氮素循环相关研究侧重于真菌对氮素的吸收形态以及共生体中氮的传输代谢机制,却忽略了丛枝菌根真菌在固氮过程、矿化与吸收过程、硝化过程、反硝化过程以及氮素淋洗过程等土壤氮素循环过程中所起到的潜在作用,并且越来越多的证据也表明丛枝菌根真菌是影响土壤氮素循环过程的重要因子。总结了丛枝菌根真菌可利用的氮素形态及真菌的氮代谢转运相关基因的研究现状;重点分析了丛枝菌根真菌在调控土壤氮素循环过程中的潜在作用以及在生态系统中的重要生态学意义,同时提出了丛枝菌根真菌在土壤氮素循环过程中一些需要深入研究的问题。

The role of arbuscular mycorrhizal fungi in soil nitrogen cycling

As one of the essential elements for plant, nitrogen (N) plays an important role in predicting the primary productivity in terrestrial ecosystems. On the other hand, arbuscular mycorrhizal fungi (AMF), a group of ubiquitous soil fungi that form symbiotic association with the majority of the terrestrial vascular plants, play a vital role in plant growth by providing their host plants with mineral nutrients such as phosphorus (P) and N, and trace elements. AMF can also protect plants from drought stress, pathogen infections and heavy metal contaminations, and improve soil structure by influencing soil aggregation dynamics. Recent studies on AMF and N cycling were mainly focused on the N forms absorbed by AMF and the N metabolism and translocation in the symbiosis. It has been well demonstrated that AMF can take up different N forms such as inorganic N, amino acids, and even complex organic N. AMF prefer to assimilate ammonium than nitrate in most circumstances. The N transfer and translocation via AMF has also been extensively studied. Various N sources are firstly incorporated into arginine (Arg) through the urea cycle in the extraradical mycelium (ERM), and then transported to the intraradical mycelium (IRM) possibly with polyphosphate (PolyP). Finally, Arg is catabolized through the catabolic arm of the urea cycle in the IRM, releasing NH₃/NH₄⁺ into arbuscules. In contrast to good knowledge of N metabolism in the mycorrhizal symbiosis, the possible involvements of AMF in soil N cycling processes such as mineralization of organic N, N fixation, nitrification, denitrification and leaching has been largely overlooked. However, AMF may mediate the soil N cycling process via different pathways from the smallest to the largest spatio-temporal scale, and much attention has been paid to the involvement of AMF in soil N cycling in recent years. The interaction of AMF and other functional microbial groups responsible for N cycling has been particularly studied. It has been well demonstrated that AMF had remarkable effects on diazotrophic, nitrifying and denitrifying microbial communities in soil microcosms or under field conditions. AMF were also shown to reduce soil inorganic N loss via leaching in microcosm-based studies. Taken together, AMF can serve as an important influencing factor for individual soil N cycling processes. Furthermore, the belowground common mycorrhizal networks could essentially affect N transfer and re-allocation among different plants in ecosystems, and thus have important ecological impacts on aboveground plant community and ecosystem stability. Moreover, all AMF hyphae have a high N content, thus the N pool in AMF mycelia could be similar in magnitude to that in roots considering their ubiquity in terrestrial ecosystems. The fungal hyphae also have a rapid turnover rate, indicating that AMF play an unappreciated role in global N cycling. This paper summarized the recent research progresses in N metabolism in AM symbiosis, including N species assimilated by AMF, and the N metabolism genes in AMF. Simultaneously, the potential role of AMF in mediating the soil N cycling processes and its ecological significances in ecosystems were discussed in details. Some important issues in relation to the involvement of AMF in soil N cycling were also proposed for further research.