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

Abstract

Nitrogen (N) availability relative to plant demand has been declining in recent years in terrestrial ecosystems throughout the world, a phenomenon known as N oligotrophication. The temperate forests of the northeastern U.S. have experienced a particularly steep decline in bioavailable N, which is expected to be exacerbated by climate change. This region has also experienced rapid urban expansion in recent decades that leads to forest fragmentation, and it is unknown whether and how these changes affect N availability and uptake by forest trees. Many studies have examined the impact of either urbanization or forest fragmentation on nitrogen (N) cycling, but none to our knowledge have focused on the combined effects of these co‐occurring environmental changes. We examined the effects of urbanization and fragmentation on oak‐dominated (Quercus spp.) forests along an urban to rural gradient from Boston to central Massachusetts (MA). At eight study sites along the urbanization gradient, plant and soil measurements were made along a 90 m transect from a developed edge to an intact forest interior. Rates of net ammonification, net mineralization, and foliar N concentrations were significantly higher in urban than rural sites, while net nitrification and foliar C:N were not different between urban and rural forests. At urban sites, foliar N and net ammonification and mineralization were higher at forest interiors compared to edges, while net nitrification and foliar C:N were higher at rural forest edges than interiors. These results indicate that urban forests in the northeastern U.S. have greater soil N availability and N uptake by trees compared to rural forests, counteracting the trend for widespread N oligotrophication in temperate forests around the globe. Such increases in available N are diminished at forest edges, however, demonstrating that forest fragmentation has the opposite effect of urbanization on coupled N availability and demand by trees.

Effects of Urbanization and Forest Fragmentation on Atmospheric Nitrogen Inputs and Ambient Nitrogen Oxide and Ozone Concentrations in Mixed Temperate Forests

“…(2022) and Caron et al. (2023) were the same as the classifications of our sites when we applied only % ISA. We highlight the importance of examining and selecting the optimal metrics for urbanization metric in nutrient input and ambient pollutant studies such as ours.…”

“…Harvard Forest 6, 4, and 5 correspond to Harvard Forest 1, 2, and 3, respectively, in Caron et al. (2023).…”

“…We deployed mixed anion-cation-exchange resin throughfall collectors to quantify rates of atmospheric N deposition in throughfall (beneath canopy) inputs at each of the seven sites. Our past field measurements of atmospheric Harvard Forest 6, 4, and 5 correspond to Harvard Forest 1, 2, and 3, respectively, in Caron et al (2023).…”

“…The patterns of higher throughfall N inputs and ambient NO x and O 3 concentrations at forest edges compared to forest interiors were generally stronger when using % ISA as an urbanization metric than when measured using distance from Boston. Two companion studies in our larger Urban New England project utilized the same sites and accounted for multiple urbanization metrics to categorize forests by "urban" or "rural," including % ISA, population density, and distance from Boston (Caron et al, 2023;Garvey et al, 2022). The urbanization classifications used in Garvey et al (2022) and Caron et al (2023) were the same as the classifications of our sites when we applied only % ISA.…”

A distinctive latitudinal trend of nitrogen isotope signature across urban forests in eastern China

“…Ecosystem 14 N loss decreases with higher mean annual precipitation (MAP), causing a reduction of soil δ 15 N (Rivero‐Villar et al, 2021; Wang et al, 2014). Edaphic and vegetation drivers can also affect soil δ 15 N by regulating N cycling (Caron et al, 2023; Wu et al, 2019). Specifically, soil acidity influences soil ammonia (NH 3 ) volatilization (Cameron et al, 2013) as well as emissions of soil nitrous oxide (N 2 O) and dinitrogen (N 2 ; Liu, Lin, et al, 2017; Zhang et al, 2022), which result in a large isotopic fractionation and thus alter soil δ 15 N (Denk et al, 2017; Robinson, 2001).…”