Soil Respiration at Forest Edges along an Urban to Rural Gradient in Massachusetts 2018-2019

As urbanization and forest fragmentation increase around the globe, it is critical to understand how rates of respiration and carbon losses from soil carbon pools are affected by these processes. This study characterizes soils in fragmented forests along an urban to rural gradient, evaluating the sensitivity of soil respiration to changes in soil temperature and moisture near the forest edge. While previous studies found elevated rates of soil respiration at temperate forest edges in rural areas compared to the forest interior, we find that soil respiration is suppressed at the forest edge in urban areas. At urban sites, respiration rates are 25% lower at the forest edge relative to the interior, likely due to high temperature and aridity conditions near urban edges. While rural soils continue to respire with increasing temperatures, urban soil respiration rates asymptote as temperatures climb and soils dry. Soil temperature- and moisture-sensitivity modeling show that respiration rates in urban soils are less sensitive to rising temperatures than those in rural soils. Scaling these results to Massachusetts (MA), which encompasses 0.25 Mha of urban forest, we find that failure to account for decreases in soil respiration rates near urban forest edges leads to an overestimate of growing-season soil carbon fluxes of greater than 350,000 MgC. This difference is almost 2.5 times that for rural soils in the analogous comparison (underestimate of less than 143,000 MgC), even though rural forest area is more than four times greater than urban forest area in MA. While a changing climate may stimulate carbon losses from rural forest edge soils, urban forests may experience enhanced soil carbon sequestration near the forest edge. These findings highlight the need to capture the effects of forest fragmentation and land use context when making projections about soil behavior and carbon cycling in a warming and increasingly urbanized world. We provide soil respiration, soil temperature and soil volumetric water content (VWC) data collected from forest edge soils at 8 field sites in Massachusetts.

随着全球范围内城市化进程与森林破碎化程度持续加深，明确这两类过程如何影响土壤碳库的呼吸速率与碳流失量，已成为一项关键研究课题。本研究沿城市-乡村梯度对破碎化森林的土壤开展表征分析，评估森林边缘附近的土壤呼吸（soil respiration）对土壤温度与湿度变化的敏感性。过往研究发现，乡村地区温带森林边缘的土壤呼吸速率高于林内，但本研究发现城市区域的森林边缘土壤呼吸反而受到抑制。在城市样地中，森林边缘的呼吸速率较林内低25%，这一现象可能源于城市边缘区域的高温与干旱环境。乡村土壤的呼吸速率随温度升高持续上升，而城市土壤的呼吸速率则随温度升高与土壤干燥逐渐趋于平缓。土壤温度与湿度敏感性模型结果显示，城市土壤的呼吸速率对温度升高的敏感性低于乡村土壤。将本研究结果外推至拥有0.25百万公顷（Mha）城市森林的马萨诸塞州（Massachusetts，MA）后，我们发现，若未考虑城市森林边缘附近土壤呼吸速率的下降，会将生长季土壤碳通量高估超过350,000兆克碳（MgC）。尽管马萨诸塞州的乡村森林面积是城市森林的四倍以上，但该高估量几乎是同类对比中乡村土壤碳通量估算误差（低估量不足143,000 MgC）的2.5倍。尽管气候变化可能会加剧乡村森林边缘土壤的碳流失，但城市森林的边缘区域反而可能增强土壤碳固存（soil carbon sequestration）。本研究结果表明，在气候变暖、城市化进程不断加快的全球背景下，预测土壤动态与碳循环时，亟需纳入森林破碎化与土地利用背景的影响效应。本研究公开了从马萨诸塞州8个野外样地的森林边缘土壤中采集的土壤呼吸、土壤温度与土壤体积含水量（volumetric water content, VWC）数据。