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Harvard Forest Symposium Abstract 2019

• Title: Urbanization and forest fragmentation effects on soil respiration and other microbial processes
• Primary Author: Sarah Garvey (Boston University)
• Additional Authors: Lucy Hutyra (Boston University); Erin Pierce (Boston University); Andrew Reinmann (Boston University); Ian Smith (Boston University); Pamela Templer (Boston University); Jonathan Thompson (Harvard Forest)
• Abstract:

  Increasing urbanization and widespread forest fragmentation will almost certainly 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 wind and light exposure. Elevated soil temperatures can lead to increased soil respiration and carbon losses as carbon dioxide due to increased microbial community activity. Previous research at the Harvard Forest showed average soil respiration rates 25% higher at the forest edge compared to the interior, with no significant difference in the sensitivity of soil respiration to temperature between edge and interior. Edges also experience increased atmospheric nitrogen deposition, which can dampen soil respiration via changes in microbial community composition and activity, thus stimulating carbon sequestration 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, 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 forest interior. We established eight field sites across an urban to rural gradient in Massachusetts and collected samples between July and October 2018 to examine soil respiration and potential abiotic drivers from the forest edge to 90 m into the interior.
  
  Our results from Boston to the Harvard Forest demonstrate that in rural forests, mean soil respiration was 32% higher at the edge relative to 30 m into the interior. Conversely, in forest plots in urban areas, mean respiration at the edge decreased 37% compared to the interior. In rural forests, the observed soil respiration response was likely driven by elevated temperatures at the edge, and we observed a mean Q10 of 2.9 ± 0.5. The Q10 was significantly lower in urban plots at 1.8 ± 0.4. This result demonstrates that urban soil respiration rates are less sensitive to changes in soil temperature compared to rural forest soils. The urban forest sites experienced greater rates of atmospheric nitrogen deposition and heightened concentrations of ground-level ozone at the forest edge, possibly explaining the pattern of lower rates of soil respiration at the forest edge in urban sites, since greater nitrogen availability in soils can decrease microbial biomass, and elevated ground-level ozone has phytotoxic effects of on vegetation. 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.

• Research Category: Soil Carbon and Nitrogen Dynamics
  Forest-Atmosphere Exchange