Abstract:

There are substantial carbon exchange fluxes among soil, vegetation and atmosphere in the terrestrial ecosystems, which are highly relevant to global climate changes. Mycorrhizal fungi can form symbiotic associations with most terrestrial plants, linking the above- and below-ground ecosystems through mineral nutrient-carbon exchange; thus, mycorrhizal fungi play crucial roles in terrestrial carbon cycling. This review summarized the involvements of mycorrhizal fungi in the terrestrial carbon cycling processes, including the carbon input, and formation, stabilization, and decomposition of soil organic matter. Studies have demonstrated that mycorrhizal fungi markedly influence the terrestrial carbon input processes by alleviating plant nutrient deficiencies, improving plant stress resistance, influencing plant photosynthesis, and regulating plant diversity-productivity relationships, subsequently sustaining or improving primary productivity of terrestrial vegetation. A considerable proportion of photosynthetic carbon is channeled directly into the soil matrix via the fungal mycelial network, where it is partly converted into microbial-derived organic carbon, further changes the composition of soil organic carbon, and be stabilized through association with minerals and/or forming soil aggregates. Mycorrhizal fungi can affect the decomposition and transformation of soil organic matter mainly through two mechanisms: the rhizosphere priming effects and/or hyphosphere biogeochemical processes. These mechanisms involve the secretion of specific extracellular enzymes, shaping hyphosphere microbial communities, induction of chemical oxidation, and competition for limited resources (e.g., nutrients and water) with free-living saprotrophs. Considering the sensitivity of mycorrhizal fungi to environmental and climate changes, we also discuss the impact of global change factors on soil carbon cycling mediated by mycorrhizal fungi. Finally, we proposed future research directions, emphasizing a need for in-depth studies on the role of mycorrhizal fungi in terrestrial carbon cycling and their environmental dependence based on network experiments in typical ecosystems. Quantitative studies should be strengthened to integrate mycorrhizal fungi into ecosystem carbon cycling models, and mycorrhizal technologies should be developed and practiced in ecological restoration and agriculture to facilitate terrestrial carbon sequestration for achieving the national carbon neutrality goals and combating climate changes.

Key words: terrestrial ecosystem, carbon cycling, global changes, arbuscular mycorrhizal fungi, ectomycorrhizal fungi, soil organic matter