Increasing urbanization and widespread forest fragmentation will affect soil carbon pools and soil respiration processes, but it is unclear how. Forest edges host distinct conditions that affect soil microbial communities. Edges tend to be hotter and drier than the forest interior due to increased sunlight and wind. Elevated soil temperatures can cause increased microbial activity, leading to soil carbon losses through heightened soil respiration. Forest edges also experience increased atmospheric nitrogen deposition, which can dampen soil respiration via changes in microbial community composition and activity, potentially sequestering carbon in soil. These complex responses suggest that forest edge dynamics and soil carbon responses vary based on proximity to urban areas where atmospheric deposition, temperature, air quality, and other conditions are directly modified through human activities. The objective of this study is to provide a mechanistic understanding of forest fragmentation and urbanization effects on belowground microbial processes and carbon fluxes at both forest edges and the interior. We established eight field sites across an urban to rural gradient in Massachusetts. We collected data during the 2018 and 2019 growing seasons to examine soil respiration and potential abiotic drivers from the forest edge to 90 m into the interior.
Results/Conclusions
Our results demonstrate a divergent response from soils near forest edges, where soil respiration was 27% higher at rural forest edges compared to 30 m into the interior, but was 25% lower at the forest edge near urban areas. In rural forests, the observed soil respiration response was likely driven by elevated temperatures at the edge, and we observed a mean Q10 of 1.58 ± 0.11. The Q10 was significantly lower in urban plots at 1.37 ± 0.09. This result demonstrates that urban soil respiration is less sensitive to increases in soil temperature compared to rural forest soils, and this could be explained the unique conditions at urban forest edges. While soil temperatures were higher at the forest edge at both urban and rural forested sites, urban edge soils were 3.0˚C warmer and 45% drier on average than rural edge soils. These extreme temperatures coupled with other anthropogenic effects can directly or indirectly decrease microbial community activity and reduce microbial biomass. These results suggest that while a changing climate may stimulate carbon losses from soils at forest edges of rural environments, urban forests may experience enhanced soil carbon sequestration at the forest edge.