Depth-dependent effects of Ericoid Mycorrhizal shrubs on soil carbon and nitrogen pools are accentuated under Arbuscular Mycorrhizal Trees

Plant mycorrhizal associations influence the accumulation and persistence of soil organic matter and could therefore shape ecosystem biogeochemical responses to global changes that are altering forest composition. For instance, arbuscular mycorrhizal (AM) tree dominance is increasing in temperate forests, and ericoid mycorrhizal (ErM) shrubs can respond positively to canopy disturbances. Yet how shifts in the co-occurrence of trees and shrubs with different mycorrhizal associations will affect soil organic matter pools remains largely unknown. We examine the effects of ErM shrubs on soil carbon and nitrogen stocks and indicators of microbial activity at different depths across gradients of AM versus ectomycorrhizal (EcM) tree dominance in three temperate forest sites. We find that ErM shrubs strongly modulate tree mycorrhizal dominance effects. In surface soils, ErM shrubs increase particulate organic matter accumulation and weaken the positive relationship between soil organic matter stocks and indicators of microbial activity. These effects are strongest under AM trees that lack fungal symbionts that can degrade organic matter. In subsurface soil organic matter pools, by contrast, tree mycorrhizal dominance effects are stronger than those of ErM shrubs. Ectomycorrhizal tree dominance has a negative influence on particulate and mineral-associated soil organic matter pools, and these effects are stronger for nitrogen than for carbon stocks. Our findings suggest that increasing co-occurrence of ErM shrubs and AM trees will enhance particulate organic matter accumulation in surface soils by suppressing microbial activity while having little influence on mineral-associated organic matter in subsurface soils. Our study highlights the importance of considering interactions between co-occurring plant mycorrhizal types, as well as their depth-dependent effects, for projecting changes in soil carbon and nitrogen stocks in response to compositional shifts in temperate forests driven by disturbances and global change.

植物菌根共生关系可影响土壤有机质的积累与留存，进而调控全球变化背景下森林群落组成改变所引发的生态系统生物地球化学响应。例如，温带森林中丛枝菌根（arbuscular mycorrhizal, AM）树木的占比正逐步升高，而欧石楠类菌根（ericoid mycorrhizal, ErM）灌木可对林冠干扰产生正向响应。然而，具备不同菌根共生类型的树木与灌木的共存格局发生改变时，会如何影响土壤有机质库，目前仍尚不明确。本研究依托3个温带森林样地，沿着丛枝菌根与外生菌根（ectomycorrhizal, EcM）树木占比的梯度，探究了欧石楠类菌根灌木对不同深度土层的土壤碳、氮储量及微生物活性指标的影响。研究发现，欧石楠类菌根灌木可显著调控树木菌根占比的效应。在表层土壤中，欧石楠类菌根灌木会促进颗粒态有机质的积累，并削弱土壤有机质储量与微生物活性指标间的正向关联；该效应在缺乏可降解有机质的真菌共生体的丛枝菌根树木林下表现最为显著。与之相反，在亚表层土壤有机质库中，树木菌根占比的效应强于欧石楠类菌根灌木。外生菌根树木占比对颗粒态与矿质结合态土壤有机质库均产生负向影响，且该效应对氮储量的作用强度高于碳储量。本研究结果表明，欧石楠类菌根灌木与丛枝菌根树木的共存度提升，会通过抑制微生物活性促进表层土壤的颗粒态有机质积累，但对亚表层土壤的矿质结合态有机质几乎无影响。本研究强调，在预测扰动与全球变化驱动的温带森林群落组成改变所引发的土壤碳、氮储量变化时，需同时考虑共存植物菌根类型间的相互作用及其深度依赖性效应，这一维度具有重要研究价值。