Soil Carbon at Forest Edges along an Urban to Rural Gradient in Massachusetts since 2018

Global proliferation of forest edges through anthropogenic land-use change and forest fragmentation is well documented, and while forest fragmentation has clear consequences for soil carbon (C) cycling, underlying drivers of belowground activity at the forest edge remain poorly understood. Increasing soil C losses via respiration have been observed at rural forest edges, but this process was suppressed at urban forest edges. We offer a comprehensive, coupled investigation of abiotic soil conditions and biotic soil activity from forest edge to interior at eight sites along an urbanization gradient to elucidate how environmental stressors are linked to soil C cycling at the forest edge. Despite significant diverging trends in edge soil C losses between urban and rural sites, we did not find comparable differences in soil % C or microbial enzyme activity, suggesting an unexpected decoupling of soil C fluxes and pools at forest edges. We demonstrate that across site types, soils at forest edges were less acidic than the forest interior (p less than 0.0001), and soil pH was positively correlated with soil calcium, magnesium and sodium content (adj R2 = 0.37), which were also elevated at the edge. Compared to forest interior, forest edge soils exhibited a 17.8% increase in sand content and elevated freeze-thaw frequency with probable downstream effects on root turnover and decomposition. Using these and other novel forest edge data, we demonstrate that significant variation in edge soil respiration (adj R2 = 0.46; p = 0.0002) and C content (adj R2 = 0.86; p less than 0.0001) can be explained using soil parameters often mediated by human activity (e.g., soil pH, trace metal and cation concentrations, soil temperature), and we emphasize the complex influence of multiple, simultaneous global change drivers at forest edges. Forest edge soils reflect legacies of anthropogenic land-use and modern human management, and this must be accounted for to understand soil activity and C cycling across fragmented landscapes.

人为土地利用变化与森林破碎化导致森林边缘在全球范围内扩散的现象已得到充分记录，尽管森林破碎化对土壤碳（C）循环的影响明确，但森林边缘地下活动的潜在驱动机制仍知之甚少。在乡村森林边缘，已观察到通过呼吸作用导致的土壤碳流失增加，但这一过程在城市森林边缘受到抑制。我们在城市化梯度上的8个研究点开展了从森林边缘到内部的非生物土壤条件与生物土壤活性的综合耦合研究，旨在阐明环境胁迫因子与森林边缘土壤碳循环之间的关联机制。尽管城市与乡村研究点的边缘土壤碳流失趋势存在显著差异，但我们并未发现土壤碳含量百分比或微生物酶活性存在相应差异，这表明森林边缘的土壤碳通量与碳库之间存在意外的解耦现象。我们发现，在所有研究点类型中，森林边缘土壤的酸性低于森林内部（p < 0.0001），且土壤pH值与土壤钙、镁、钠含量呈正相关（调整后R²=0.37），而这些元素在边缘土壤中的含量同样较高。与森林内部相比，边缘土壤的砂含量增加了17.8%，冻融频率也有所升高，这可能对根系周转和分解产生后续影响。利用这些以及其他新颖的森林边缘数据，我们证明边缘土壤呼吸（调整后R²=0.46；p=0.0002）和碳含量（调整后R²=0.86；p < 0.0001）的显著变异可通过常受人类活动介导的土壤参数（如土壤pH值、微量金属与阳离子浓度、土壤温度）来解释；同时我们强调，多种全球变化驱动因子的同步作用对森林边缘具有复杂影响。森林边缘土壤反映了人为土地利用和现代人类管理的遗留效应，要理解破碎化景观中的土壤活性与碳循环，必须考虑这一因素。