Forest fragmentation effects on soil microbial processes along an urbanization gradient

Sarah M. Garvey and Lucy R. Hutyra, Department of Earth & Environment, Boston University, Boston, MA, Pamela H. Templer and Jennifer M. Bhatnagar, Department of Biology, Boston University, Boston, MA

Background/Question/Methods
Widespread forest fragmentation and urbanization have complex effects on soils and their microbial communities that are not fully understood. Soils at the forest edge are exposed to conditions distinct from the forest interior including increased temperatures, aridity, and atmospheric nitrogen deposition. The distinct microenvironment at a forest edge is shaped by the surrounding land use, and soil behavior varies with urbanization intensity. Recent work suggests that urban soil processes are less sensitive to rising temperatures than rural soils. Soils at rural forest edges experience increased carbon losses, but soil respiration is suppressed at urban forest edges, which may indicate unaccounted-for carbon storage. The objective of this research was to characterize the underlying drivers of observed patterns in soil respiration and to provide a mechanistic understanding of the effects of forest fragmentation and urbanization. We established eight field sites across an urban to rural gradient in Massachusetts and collected growing season soil samples in 2018 and 2019. We assayed soil extracellular enzyme activity, as it mediates decomposition, and quantified other soil characteristics along transects that spanned the forest edge to 90m into the interior.
Results/Conclusions
Our results suggest that patterns of soil respiration at the forest edge may be explained by variations in soil extracellular enzyme activity per gram soil organic matter (SOM). In general, microbial enzyme activity per gram SOM was elevated at rural forest edges relative to the forest interior. Betaglucosidase activity in 2018, a key cellulose-degrading enzyme, was more than 2.3 times greater per gram SOM at the rural forest edge than the interior. In urban forest fragments, enzymatic activity per gram SOM was not significantly different from edge to interior. Soil characteristics such as %SOM, pH and texture did not explain the variation in soil respiration at forest edges across the urbanization gradient. In both rural and urban forests, %SOM decreased from nearly 60% in the forest interior to 28.0±7% at the edge, while soil pH increased from interior (4.1± 0.1) to edge (5.0± 0.2). Forest edge soils were sandier than those in the interior, but this trend was driven by edge sites near roadways, independent of urbanization category. These results suggest that while soil processes at the forest edge were driven in part by short-term responses to abiotic variables (e.g., temperature), long-term changes to soil temperatures and other conditions at the edge may lead to shifts in soil decomposition dynamics that are observable across scales and processes.