BEF-China

Nutrient resorption and litter decomposition recycle about 90% of nitrogen (N) for plant growth. Plants benefit from mycorrhizal symbiosis to enhance their nutrient access. When competition intensifies, whether mycorrhizal plants can adjust the trade-off between nutrient resorption and litter decomposition to optimize their N supply remains unresolved. We studied nutrient strategies of arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) trees in a subtropical forest diversity experiment, where the two mycorrhizal types were evenly planted in mixtures of 2, 4, 8 or 16 tree species. We found that net primary production (NPP) of EcM sub-communities declined but NPP of AM sub-communities increased with tree species richness. Tree species richness reduced nutrient resorption efficiency (NRE) of EcM trees, which increased leaf litter N concentration and thus its decomposition. However, by reducing the lignin concentration of leaf litter and increasing the richness of saprophytic fungi, tree species richness promoted NRE and litter decomposition for AM sub-communities. This demonstrates that AM trees are more effective in maintaining their N supply than EcM trees with increasing diversity, suggesting a new mechanism to explain the dominance of AM trees in high-diversity subtropical forests.养分重吸收（nutrient resorption）与枯落物分解（litter decomposition）过程可循环约90%的氮（N）用于植物生长。植物可通过菌根共生（mycorrhizal symbiosis）提升自身的养分获取能力。当种间竞争加剧时，菌根植物能否通过调节养分重吸收与枯落物分解之间的权衡关系以优化氮供应，目前仍未有定论。本研究依托一项亚热带森林多样性实验，探究了丛枝菌根（arbuscular mycorrhizal, AM）与外生菌根（ectomycorrhizal, EcM）树木的养分策略；该实验中，两种菌根类型的树木被均匀种植于包含2、4、8或16个树种的混交林样地内。研究发现，随着树种丰富度提升，外生菌根亚群落的净初级生产力（net primary production, NPP）呈下降趋势，而丛枝菌根亚群落的净初级生产力则显著上升。树种丰富度降低了外生菌根树木的养分重吸收效率（nutrient resorption efficiency, NRE），这一变化提升了其叶片枯落物的氮浓度，进而加速了枯落物分解过程。与之相对，对于丛枝菌根亚群落，树种丰富度通过降低叶片枯落物的木质素浓度、增加腐生真菌（saprophytic fungi）的丰富度，同时促进了养分重吸收效率与枯落物分解过程。本研究表明，随着群落多样性增加，丛枝菌根树木在维持自身氮供应方面的能力优于外生菌根树木，这为解释亚热带高多样性森林中丛枝菌根树木的优势地位提供了新的机制。