Diverging patterns at the forest edge: Soil respiration dynamics of fragmented forests in urban and rural areas

Abstract

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 temperatureand moisture-sensitivity modeling shows 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 the 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 >350,000 Mg C. This difference is almost 2.5 times that for rural soils in the analogous comparison (underestimate of <143,000 Mg C), 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. K E Y W O R D S carbon, CO 2 , forest edge, forest fragmentation, rural, soil respiration, soil temperature, urban | 3095 GARVEY Et Al.critical to investigate how soil carbon storage will change in the face of climbing global temperatures and pervasive land-use change, such as forest fragmentation and urbanization (Bradford et al., 2016;

Urbanization and fragmentation have opposing effects on soil nitrogen availability in temperate forest ecosystems

“…Soil N cycling rates are well known to be associated positively with soil properties such as moisture (Guntiñas et al, 2012), pH (Cheng et al, 2013), and organic matter content (Cookson et al, 2007), but negatively associated with soil bulk density (Tan & Chang, 2007) and electrical conductivity (Green & Cresser, 2008). Among these factors, soil moisture has been shown to be lower in urban forests and at forest edges, including at the forested sites in this study (Garvey et al, 2022), but the combined effects of all these soil properties on soil N cycling are not well quantified in urban and fragmented temperate forests.…”

“…This lack of consistency in the literature regarding which aspects of urbanization most strongly affect nutrient cycles is likely due to the heterogeneity of urban environments. Garvey et al (2022), for example, found variation in soil respiration at the same eight study sites was more strongly related to %ISA than to distance to Boston. As such, we conclude that it is important to consider multiple metrics of urbanization, and how they interact with each other, when assessing the impacts of cities on nearby forests.…”

“…The difference in net nitrification dynamics at rural compared to urban forest edges, despite both having similar soil pH, may indicate that soil acidity was not the dominant influencing factor on nitrification rates. We were surprised by the higher rates of net nitrification in rural forest edges than interiors given their lower soil moisture (Garvey et al, 2022) and because soil moisture is typically positively related to net nitrification rates (Schmidt, 1982). However, in our sites gravimetric soil moisture did not appear to be a major predictor of net nitrification at rural edges despite the significant relationship between nitrification and moisture at rural sites across the edge to interior gradient.…”

“…Soil and plant samples were collected as part of the Urban New England project with eight sites along a 120 km urban to rural gradient from Boston to central MA. The same sites were used to measure the combined effects urbanization and forest fragmentation on rates of soil respiration and a suite of other properties (Garvey et al, 2022). These sites include a mix of public and privately owned land with at least 200 × 200 m intact forest area.…”

“…These sites include a mix of public and privately owned land with at least 200 × 200 m intact forest area. A detailed site map is included in Garvey et al (2022) and additional topographical and land‐use information about the area adjacent to each forest edge is shown in Table 1. All forested sites, including urban and rural forests, were chosen to comply with our minimum threshold of 200 × 200 m size.…”