Data from: Anthropogenic n deposition increases soil c storage by reducing the relative abundance of lignolytic fungi

Atmospheric nitrogen (N) deposition has increased dramatically since preindustrial times and continues to increase across many regions of the Earth. In temperate forests, this agent of global change has increased soil carbon (C) storage, but the mechanisms underlying this response are not understood. One long-standing hypothesis proposed to explain the accumulation of soil C proposes that higher inorganic N availability may suppress both the activity and abundance of fungi which decay lignin and other polyphenols in soil. In field studies, elevated rates of N deposition have reduced the activity of enzymes mediating lignin decay, but a decline in the abundance of lignolytic fungi has not been definitively documented to date. Here, we tested the hypothesis that elevated rates of anthropogenic N deposition reduce the abundance of lignolytic fungi. We conducted a field experiment in which we compared fungal communities colonizing low-lignin, high-lignin, and wood substrates in a northern hardwood forest that is part of a long-term N deposition experiment. We reasoned that, if lignolytic fungi decline under experimental N deposition, this effect should be most evident among fungi colonizing high-lignin and wood substrates. Using molecular approaches, we provide evidence that anthropogenic N deposition reduces the relative abundance of lignolytic fungi on both wood and a high-lignin substrate. Furthermore, experimental N deposition increased total fungal abundance on a low-lignin substrate, reduced fungal abundance on wood, and had no significant effect on fungal abundance on a high-lignin substrate. We simultaneously examined these responses in the surrounding soil and forest floor, in which we did not observe significant reductions in the relative abundance of lignolytic fungi or in the size of the fungal community; however, we did detect a change in community composition in the forest floor that appears to be driven by a shift away from lignolytic fungi and towards cellulolytic fungi. Our results provide direct evidence that reductions in the abundance of lignolytic fungi are part of the mechanism by which anthropogenic N deposition increases soil C storage.

自工业革命前以来，大气氮（N）沉降量已大幅攀升，且在全球诸多区域仍处于持续增长态势。在温带森林生态系统中，这一全球变化驱动因子已被证实可提升土壤碳（C）储量，但其背后的核心作用机制尚未完全明晰。长期以来，学界提出了一项用以解释土壤碳积累的假说，该假说认为更高的无机氮有效性可能会抑制土壤中降解木质素与其他多酚类物质的真菌的活性与种群丰度。已有野外研究显示，高氮沉降速率会降低介导木质素降解的酶的活性，但截至目前，尚未有确凿研究证据证实木质素分解真菌（lignolytic fungi）的丰度出现了明确下降。本研究针对“人为氮沉降速率升高会降低木质素分解真菌的丰度”这一科学假说开展了实验检验。我们依托一项长期氮沉降试验中的北方硬木林设置了野外操控实验，对比了定殖于低木质素基质、高木质素基质以及木材基质的真菌群落组成。我们推测，若实验性氮沉降会导致木质素分解真菌丰度下降，该效应在定殖于高木质素基质与木材基质的真菌群落中应最为显著。通过分子生物学分析手段，我们证实人为氮沉降显著降低了木材与高木质素基质上木质素分解真菌的相对丰度。此外，实验性氮沉降提升了低木质素基质上的总真菌丰度，降低了木材基质上的真菌丰度，而对高木质素基质上的真菌丰度无显著影响。我们同时对实验样地周边的土壤与森林地表层开展了同步观测，未发现木质素分解真菌相对丰度或真菌群落总规模出现显著下降，但我们确实检测到森林地表层的真菌群落组成发生了显著变化，这一变化似乎由木质素分解真菌占比降低、纤维分解真菌（cellulolytic fungi）占比上升所驱动。本研究结果直接证实，木质素分解真菌丰度的降低，正是人为氮沉降提升土壤碳储量的关键作用机制之一。