Housing Age and Affluence Influence Plant and Soil Nitrogen and Carbon Cycles in Two Semiarid Cities

Abstract

While human activities have altered the urban nitrogen (N) and carbon (C) cycles, the relationships between social and biophysical processes in cities are not well understood. Here we evaluated relationships between sociodemographic variables (median household income and housing age) and N and C contents and stable isotope ratios of vegetation in the Los Angeles Metropolitan Area (LA), California, and the Salt Lake Valley, Utah. We hypothesized that (1) N content and stable isotope ratios would be negatively related to income via increased N deposition in lower-income areas; (2) N and C content and N stable isotope ratios would have a positive relationship with age due to soil organic matter accumulation and increased exposure to N losses over time, respectively; and (3) δ 13 C would increase with income as a result of increased fossil fuel-derived CO 2 concentrations in lower-income areas. We found that δ 15 N values decreased with median household income in both cities and N content decreased with income in LA. In addition, atmospheric NO 2 concentrations decreased with income in LA. Soil N and C content increased with housing age. However, δ 15 N had opposing relationships with age in the two cities. Finally, foliar δ 13 C values were more isotopically depleted with both increasing household income and increasing NO 2 concentrations in LA. These results show that urban foliar and soil isotopic composition are associated with sociodemographic variables and that affluence, as well as housing age, influences urban plant and soil function.Urban areas also have highly altered carbon (C) cycles that are affected by a myriad of ecological and sociological factors. For example, urbanized areas have elevated CO 2 concentrations that may exceed 500 ppm

Using Traffic Density and Foliar Chemistry Variables to Understand Interactions Between Air Pollution and Household Income

“…We measured atmospheric pollutant (NO x and O 3 ) concentrations and collected C. arvensis leaf samples along high‐ and low‐traffic density roads in three high‐income and three low‐income neighborhoods in the Salt Lake Valley (SLV), Utah, USA, a region that has been extensively monitored for pollutant concentrations and foliar chemistry (Bares et al., 2018; Bush et al., 2015; Cobley et al., 2018; Cobley & Pataki, 2019; Lin et al., 2018; Mitchell et al., 2018; Pataki et al., 2007). The SLV has a semi‐arid, continental climate that is characterized by hot and dry summers and cold winters.…”

“…The SLV is bounded by the Great Salt Lake to the north, the Wasatch mountains to the east, Oquirrh mountains to the west and Transverse Mountains to the south (Alder et al., 1998; Bush et al., 2008; Hall et al., 2016). Elevation on the valley floor ranges between 1,285 and 2,304 m above sea level (Cobley et al., 2018). Before settlement by Mormon pioneers in 1847, the SLV consisted of mostly grasslands and sagebrush, with trees confined to the riparian corridor (Avolio et al., 2018; Ball & Brotherson, 1999; Cottom & Evans, 1945; Smith et al., 2018).…”

“…Cobley et al. (2018) found that NO x concentrations decreased with increasing median household income in Los Angeles, CA. O 3 concentrations are negatively correlated with metrics of neighborhood quality, which includes socioeconomic factors (Wilhelm et al., 2009).…”

“…Cobley et al. (2018) observed more enriched plant and soil nitrogen isotope ratios (δ 15 N) with lower median household income in Los Angeles, California and the Salt Lake Valley, Utah. Foliar δ 15 N is an indicator of different nitrogen (N) sources and enriched δ 15 N is often associated with increased N deposition from vehicle emissions (Cape et al., 2004; Cobley & Pataki, 2019; Kenkel et al., 2016; Pearson et al., 2000; Redling et al., 2013; Truscott et al., 2005).…”

“…Other leaf chemistry variables, including %N and carbon isotope ratios (δ 13 C), have also shown correlations with pollution, income, and traffic density. For example, foliar %N has been found to be higher in areas of elevated N deposition (Harmens et al., 2011; Hicks et al., 2000) and lower in areas with higher median household income (Cobley et al., 2018). Foliar δ 13 C is often more depleted in areas with higher degrees of urbanization and vehicle emissions (Balasooriya et al., 2009; Cobley & Pataki, 2019; Kiyosu & Kidoguchi, 2000; Lichtfouse et al., 2003; Wang & Pataki, 2012).…”