Soil carbon and nitrogen accumulation in residential lawns of the Salt Lake Valley, Utah.

Urban lawn ecosystems are widespread across the United States, with fertilization rates commonly exceeding plant nitrogen (N) uptake rates. While urban soils have been shown to accumulate C and N over time, the long-term balance of N inputs and losses from lawn soils remains largely uncertain. We sampled residential lawn soils aged 7-100 years in the Salt Lake City metropolitan area as a means of inferring changes in total nitrogen (TN) content, organic carbon (OC) content, C:N ratio, and δ15N of bulk soil over time. Core-integrated (0-40 cm) TN and OC stocks increased linearly by 2.39 g N m-2 year-1 and 29.8 g OC m-2 year-1 over the 100-year chronosequence. TN and OC percent were also negatively correlated with elevation. Multiple linear regression models including housing age and elevation as covariates, explained 68 and 62% of variability in TN and OC stocks respectively. δ15N increased with housing age, soil depth, and clay content, suggesting N removal over time, especially in poorly drained soils. We quantified potential hydrologic and gaseous N losses over time by comparing observed N accumulation to different historic fertilization scenarios. Modeling and isotopic results suggest that, while soil N has accumulated over time, the majority of N added to lawns in the Salt Lake Valley over 50 years of fertilization was likely lost from surface soils via denitrification or leaching.

Human-accelerated weathering increases salinization, major ions, and alkalinization in fresh water across land use.

Human-dominated land uses can increase transport of major ions in streams due to the combination of human-accelerated weathering and anthropogenic salts. Calcium, magnesium, sodium, alkalinity, and hardness significantly increased in the drinking water supply for Baltimore, Maryland over almost 50 years (p<0.05) coinciding with regional urbanization. Across a nearby land use gradient at the Baltimore Long-Term Ecological Research (LTER) site, there were significant increases in concentrations of dissolved inorganic carbon (DIC), Ca2+, Mg2+, Na+, and Si and pH with increasing impervious surfaces in 9 streams monitored bi-weekly over a 3-4 year period (p<0.05). Base cations in urban streams were up to 60 times greater than forest and agricultural streams, and elemental ratios suggested road salt and carbonate weathering from impervious surfaces as potential sources. Laboratory weathering experiments with concrete also indicated that impervious surfaces increased pH and DIC with potential to alkalinize urban waters. Ratios of Na+ and Cl- suggested that there was enhanced ion exchange in the watersheds from road salts, which could mobilize other base cations from soils to streams. There were significant relationships between Ca2+, Mg2+, Na+, and K+ concentrations and Cl-, SO42-, NO3- and DIC across land use (p<0.05), which suggested tight coupling of geochemical cycles. Finally, concentrations of Na+, Ca2+, Mg2+, and pH significantly increased with distance downstream (p<0.05) along a stream network draining 170 km2 of the Baltimore LTER site contributing to river alkalinization. Our results suggest that urbanization may dramatically increase major ions, ionic strength, and pH over decades from headwaters to coastal zones, which can impact integrity of aquatic life, infrastructure, drinking water, and coastal ocean alkalinization.

Influence of infrastructure on water quality and greenhouse gas dynamics in urban streams.

Streams and rivers are significant sources of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) globally, and watershed management can alter greenhouse gas (GHG) emissions from streams. We hypothesized that urban infrastructure significantly alters downstream water quality and contributes to variability in GHG saturation and emissions. We measured gas saturation and estimated emission rates in headwaters of two urban stream networks (Red Run and Dead Run) of the Baltimore Ecosystem Study Long-Term Ecological Research project. We identified four combinations of stormwater and sanitary infrastructure present in these watersheds, including: (1) stream burial, (2) inline stormwater wetlands, (3) riparian/floodplain preservation, and (4) septic systems. We selected two first order catchments in each of these categories and measured GHG concentrations, emissions, and dissolved inorganic and organic carbon (DIC and DOC) and nutrient concentrations biweekly for 1 year. From a water quality perspective, the DOC : NO3 - ratio of streamwater was significantly different across infrastructure categories. Multiple linear regressions including DOC : NO3 - and other variables (dissolved oxygen, DO; total dissolved nitrogen, TDN; and temperature) explained much of the statistical variation in nitrous oxide (N2O, r2 = 0.78), carbon dioxide (CO2, r2 = 0.78) and methane (CH4, r 2 = 0.50) saturation in stream water. We measured N2O saturation ratios, which were among the highest reported in the literature for streams, ranging from 1.1 to 47 across all sites and dates. N2O saturation ratios were highest in streams draining watersheds with septic systems and strongly correlated with TDN. The CO2 saturation ratio was highly correlated with the N2O saturation ratio across all sites and dates, and the CO2 saturation ratio ranged from 1.1 to 73. CH4 was always supersaturated, with saturation ratios ranging from 3.0 to 2157. Longitudinal surveys extending form headwaters to third-order outlets of Red Run and Dead Run took place in spring and fall. Linear regressions of these data yielded significant negative relationships between each gas with increasing watershed size as well as consistent relationships between solutes (TDN or DOC, and DOC : TDN ratio) and gas saturation. Despite a decline in gas saturation between the headwaters and stream outlet, streams remained saturated with GHGs throughout the drainage network, suggesting that urban streams are continuous sources of CO2, CH4, and N2O. Our results suggest that infrastructure decisions can have significant effects on downstream water quality and greenhouse gases, and watershed management strategies may need to consider coupled impacts on urban water and air quality.