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Abstract A recent publication (Mason et al. in Science 376:261, 2022a) suggested that nitrogen (N) availability has declined as a consequence of multiple ongoing components of anthropogenic global change. This suggestion is controversial, because human alteration of the global N cycle is substantial and has driven much-increased fixation of N globally. We used a simple model that has been validated across a climate gradient in Hawai ‘i to test the possibility of a widespread decline in N availability, the evidence supporting it, and the possible mechanisms underlying it. This analysis showed that a decrease in δ¹⁵N is not sufficient evidence for a decline in N availability, because δ¹⁵N in ecosystems reflects both the isotope ratios in inputs of N to the ecosystem AND fractionation of N isotopes as N cycles, with enrichment of the residual N in the ecosystem caused by greater losses of N by the fractionating pathways that are more important in N-rich sites. However, there is other evidence for declining N availability that is independent of¹⁵N and that suggests a widespread decline in N availability. We evaluated whether and how components of anthropogenic global change could cause declining N availability. Earlier work had demonstrated that both increases in the variability of precipitation due to climate change and ecosystem-level disturbance could drive uncontrollable losses of N that reduce N availability and could cause persistent N limitation at equilibrium. Here we modelled climate-change-driven increases in temperature and increasing atmospheric concentrations of CO₂. We show that increasing atmospheric CO₂concentrations can drive non-equilibrium decreases in N availability and cause the development of N limitation, while the effects of increased temperature appear to be relatively small and short-lived. These environmental changes may cause reductions in N availability over the vast areas of Earth that are not affected by high rates of atmospheric deposition and/or N enrichment associated with urban and agricultural land use. 

Abstract Aquatic ecosystems are subjected to many chemical stressors, including nutrients and emerging contaminants like pharmaceuticals. While pharmaceutical concentrations in streams and rivers are often below the thresholds for acute toxicity, they nonetheless disrupt ecology through changes to organisms' physiology, metabolism, and behavior. However, analyzing samples for the wide range of manufactured pharmaceuticals is often prohibitively expensive for many monitoring efforts. As such, the ability to predict pharmaceutical concentrations over space and time using easier‐to‐monitor water quality parameters would expand our understanding of the scope and consequences of pharmaceutical contamination in aquatic ecosystems. We applied random forest models to data from the Baltimore Ecosystem Study to investigate how well routinely monitored water quality parameters could be used to predict concentrations of nutrients and pharmaceuticals. We found that concentrations of nutrients were accurately predicted by these models, but models for predicting concentrations of pharmaceuticals had high error rates and low predictive ability. Differences in our ability to predict concentrations of nutrients as opposed to pharmaceuticals could be due to differences in their sources, chemistries, or behavior in the environment. More concerted efforts to monitor pharmaceutical concentrations over time in aquatic ecosystems may help to resolve environmental drivers of their concentration and improve our ability to predict them. 

Abstract Hydrologic alterations associated with urbanization can weaken connections between riparian zones, streams, and uplands, leading to negative effects on the ability of riparian zones to intercept pollutants carried by surface water runoff and groundwater flow such as nitrate (NO₃⁻) and phosphate (PO₄³⁻). We analyzed the monthly water table as an indicator of riparian connectivity, along with groundwater NO₃⁻and PO₄³⁻concentrations, at four riparian sites within and near the Gwynns Falls Watershed in Baltimore, MD, from 1998 to 2018. The sites included one forested reference site (Oregon Ridge), two suburban riparian sites (Glyndon and Gwynnbrook), and one urban riparian site (Cahill) with at least two locations and four monitoring wells, located 5 m from the center of the stream, at each site. Results show an increase in connectivity as indicated by shallower water tables at two of the four sites studied: Glyndon and Cahill. This change in connectivity was associated with decreases in NO₃⁻at Glyndon and increases in PO₄³⁻at Glyndon, Gwynnbrook, and Cahill. These changes are consistent with previous studies showing that shallower water table depths increase anaerobic conditions, which increase NO₃⁻consumption by denitrification and decrease PO₄³⁻retention. The absence of change in the forested reference site, where climate would be expected to be the key driver, suggests that other drivers, including best management practices and stream restoration projects, could be affecting riparian water tables at the two suburban sites and the one urban site. Further research into the mechanisms behind these changes and site‐specific dynamics is needed. 

ABSTRACT The timescales over which soil carbon responds to global change are a major uncertainty in the terrestrial carbon cycle. Radiocarbon measurements on archived soil samples are an important tool for addressing this uncertainty. We present time series (1969–2023) of radiocarbon measurements for litter (Oi/Oe and Oa/A) and mineral (0–10 cm) soils from the Hubbard Brook Experimental Forest, a predominantly hardwood forest in the northeastern USA. To estimate soil carbon cycling rates, we built different autonomous linear compartmental models. We found that soil litter carbon cycles on decadal timescales (Oi/Oe: ~7 years), whereas carbon at the organic‐mineral interface (Oa/A), and mineral soil (0–10 cm) carbon cycles on centennial timescales (~104 and 302 years, respectively). At the watershed‐level, the soil system appears to be at steady‐state, with no observed changes in carbon stocks or cycling rates over the study period, despite increases in precipitation, temperature, and soil pH. However, at the site‐level, the Oi/Oe is losing carbon (−15 g C m⁻² year⁻¹since 1998). The observed decline in carbon stocks can be detected when the Oi and Oe layers are modeled separately. This pattern suggests that the rapidly cycling litter layer at the smaller scale is responding to recent environmental changes. Our results highlight the importance of litter carbon as an “early‐warning system” for soil responses to environmental change, as well as the challenges of detecting gradual environmental change across spatial scales in natural forest ecosystems. 

Abstract Microbial biomass is known to decrease with soil drying and to increase after rewetting due to physiological assimilation and substrate limitation under fluctuating moisture conditions, but how the effects of changing moisture conditions vary between dry and wet environments is unclear. Here, we conducted a meta‐analysis to assess the effects of elevated and reduced soil moisture on microbial biomass C (MBC) and microbial biomass N (MBN) across a broad range of forest sites between dry and wet regions. We found that the influence of both elevated and reduced soil moisture on MBC and MBN concentrations in forest soils was greater in dry than in wet regions. The influence of altered soil moisture on MBC and MBN concentrations increased significantly with the manipulation intensity but decreased with the length of experimental period, with a dramatic increase observed under a very short‐term precipitation pulse. Moisture effect did not differ between coarse‐textured and fine‐textured soils. Precipitation intensity, experimental duration, and site standardized precipitation index (dry or wet climate) were more important than edaphic factors (i.e., initial water content, bulk density, and clay content) in determining microbial biomass in response to altered moisture in forest soils. Different responses of microbial biomass in forest soils between dry and wet regions should be incorporated into models to evaluate how changes in the amount, timing, and intensity of precipitation affect soil biogeochemical processes. 

Abstract Despite interest in the contribution of evapotranspiration (ET) of residential turfgrass lawns to household and municipal water budgets across the United States, the spatial and temporal variability of residential lawn ET across large scales is highly uncertain. We measured instantaneous ET (ET_inst) of lawns in 79 residential yards in six metropolitan areas: Baltimore, Boston, Miami, Minneapolis‐St. Paul (mesic climates), Los Angeles and Phoenix (arid climates). Each yard had one of four landscape types and management practices: traditional lawn‐dominated yards with high or low fertilizer input, yards with water‐conserving features, and yards with wildlife‐friendly features. We measured ET_instin situ during the growing season using portable chambers and identified environmental and anthropogenic factors controlling ET in residential lawns. For each household, we used ET_instto estimate daily ET of the lawn (ET_daily) and multiplied ET_dailyby the lawn area to estimate the total volume of water lost through ET of the lawn (ET_vol). ET_dailyvaried from 0.9 ± 0.4 mm d¹in mesic cities to 2.9 ± 0.7 mm d⁻¹in arid cities. Neither ET_instnor ET_dailywas significantly influenced by yard landscape types and ET_instpatterns indicated that lawns may be largely decoupled from regional rain‐driven climate patterns. ET_volranged from ∼0 L d⁻¹to over 2,000 L d⁻¹, proportionally increasing with lawn area. Current irrigation and lawn management practices did not necessarily result in different ET_instor ET_dailyamong traditional, water‐conserving, or wildlife‐friendly yards, but smaller lawn areas in water‐conserving and wildlife‐friendly yards resulted in lower ET_vol. 

Abstract Urbanization has a homogenizing effect on biodiversity and leads to communities with fewer native species and lower conservation value. However, few studies have explored whether or how land management by urban residents can ameliorate the deleterious effects of this homogenization on species composition. We tested the effects of local (land management) and neighborhood‐scale (impervious surface and tree canopy cover) features on breeding bird diversity in six US metropolitan areas that differ in regional species pools and climate. We used a Bayesian multiregion community model to assess differences in species richness, functional guild richness, community turnover, population vulnerability, and public interest in each bird community in six land management types: two natural area park types (separate and adjacent to residential areas), two yard types with conservation features (wildlife‐certified and water conservation) and two lawn‐dominated yard types (high‐ and low‐fertilizer application), and surrounding neighborhood‐scale features. Species richness was higher in yards compared with parks; however, parks supported communities with high conservation scores while yards supported species of high public interest. Bird communities in all land management types were composed of primarily native species. Within yard types, species richness was strongly and positively associated with neighborhood‐scale tree canopy cover and negatively associated with impervious surface. At a continental scale, community turnover between cities was lowest in yards and highest in parks. Within cities, however, turnover was lowest in high‐fertilizer yards and highest in wildlife‐certified yards and parks. Our results demonstrate that, across regions, preserving natural areas, minimizing impervious surfaces and increasing tree canopy are essential strategies to conserve regionally important species. However, yards, especially those managed for wildlife support diverse, heterogeneous bird communities with high public interest and potential to support species of conservation concern. Management approaches that include the preservation of protected parks, encourage wildlife‐friendly yards and acknowledge how public interest in local birds can advance successful conservation in American residential landscapes. 

Abstract In urban areas, anthropogenic drivers of ecosystem structure and function are thought to predominate over larger‐scale biophysical drivers. Residential yards are influenced by individual homeowner preferences and actions, and these factors are hypothesized to converge yard structure across broad scales. We examined soil total C and total δ¹³C, organic C and organic δ¹³C, total N, and δ¹⁵N in residential yards and corresponding reference ecosystems in six cities across the United States that span major climates and ecological biomes (Baltimore, Maryland; Boston, Massachusetts; Los Angeles, California; Miami, Florida; Minneapolis‐St. Paul, Minnesota; and Phoenix, Arizona). Across the cities, we found soil C and N concentrations and soil δ¹⁵N were less variable in residential yards compared to reference sites supporting the hypothesis that soil C, N, and δ¹⁵N converge across these cities. Increases in organic soil C, soil N, and soil δ¹⁵N across urban, suburban, and rural residential yards in several cities supported the hypothesis that soils responded similarly to altered resource inputs across cities, contributing to convergence of soil C and N in yards compared to natural systems. Soil C and N dynamics in residential yards showed evidence of increasing C and N inputs to urban soils or dampened decomposition rates over time that are influenced by climate and/or housing age across the cities. In the warmest cities (Los Angeles, Miami, Phoenix), greater organic soil C and higher soil δ¹³C in yards compared to reference sites reflected the greater proportion of C₄plants in these yards. In the two warm arid cities (Los Angeles, Phoenix), total soil δ¹³C increased and organic soil δ¹³C decreased with increasing home age indicating greater inorganic C in the yards around newer homes. In general, soil organic C and δ¹³C, soil N, and soil δ¹⁵N increased with increasing home age suggesting increased soil C and N cycling rates and associated¹²C and¹⁴N losses over time control yard soil C and N dynamics. This study provides evidence that conversion of native reference ecosystems to residential areas results in convergence of soil C and N at a continental scale. The mechanisms underlying these effects are complex and vary spatially and temporally.