Steve M. Raciti

NITROGEN RETENTION IN URBAN LAWNS AND FORESTS

Lawns are a dominant cover type in urban ecosystems, and there is concern about their impacts on water quality. However, recent watershed-level studies suggest that these pervious areas might be net sinks, rather than sources, for nitrogen (N) in the urban environment. A 15N pulse-labeling experiment was performed on lawn and forest plots in the Baltimore (Maryland, U.S.A.) metropolitan area to test the hypothesis that lawns are a net sink for atmospheric-N deposition and to compare and contrast mechanisms of N retention in these vegetation types. A pulse of 15N-NO3-, simulating a precipitation event, was followed through mineral soils, roots, Oi-layer/thatch, aboveground biomass, microbial biomass, inorganic N, and evolved N2 gas over a one-year period. The 15N label was undetectable in gaseous samples, but enrichment of other pools was high. Gross rates of production and consumption of NO3- and NH4+ were measured to assess differences in internal N cycling under lawns and forests. Rates of N retention were similar during the first five days of the experiment, with lawns showing higher N retention than forests after 10, 70, and 365 days. Lawns had larger pools of available NO3- and NH4+; however, gross rates of mineralization and nitrification were also higher, leading to no net differences in NO3- and NH4+ turnover times between the two systems. Levels of 15N remained steady in forest mineral soils from day 70 to 365 (at 23% of applied 15N), but continued to accumulate in lawn mineral soils over this same time period, increasing from 20% to 33% of applied 15N. The dominant sink for N in lawn plots changed over time. Immobilization in mineral soils dominated immediately (one day) after tracer application (42% of recovered 15N); plant biomass dominated the short term (10 days; 51%); thatch and mineral-soil pools together dominated the medium term (70 days; 28% and 36%, respectively); and the mineral-soil pool alone dominated long-term retention (one year; 70% of recovered 15N). These findings illustrate the mechanisms whereby urban and suburban lawns under low to moderate management intensities are an important sink for atmospheric-N deposition.

The engaged university: providing a platform for research that transforms society

Despite a growing recognition that the solutions to current environmental problems will be developed through collaborations between scientists and stakeholders, substantial challenges stifle such cooperation and slow the transfer of knowledge. Challenges occur at several levels, including individual, disciplinary, and institutional. All of these have implications for scholars working at academic and research institutions. Fortunately, creative ideas and tested models exist that provide opportunities for conversation and serious consideration about how such institutions can facilitate the dialogue between scientists and society.

Methane emissions from natural gas infrastructure and use in the urban region of Boston, Massachusetts

Significance Most recent analyses of the environmental impact of natural gas have focused on production, with very sparse information on emissions from distribution and end use. This study quantifies the full seasonal cycle of methane emissions and the fractional contribution of natural gas for the urbanized region centered on Boston. Emissions from natural gas are found to be two to three times larger than predicted by existing inventory methodologies and industry reports. Our findings suggest that natural-gas–consuming regions may be larger sources of methane to the atmosphere than is currently estimated and represent areas of significant resource loss. Methane emissions from natural gas delivery and end use must be quantified to evaluate the environmental impacts of natural gas and to develop and assess the efficacy of emission reduction strategies. We report natural gas emission rates for 1 y in the urban region of Boston, using a comprehensive atmospheric measurement and modeling framework. Continuous methane observations from four stations are combined with a high-resolution transport model to quantify the regional average emission flux, 18.5 ± 3.7 (95% confidence interval) g CH4⋅m−2⋅y−1. Simultaneous observations of atmospheric ethane, compared with the ethane-to-methane ratio in the pipeline gas delivered to the region, demonstrate that natural gas accounted for ∼60–100% of methane emissions, depending on season. Using government statistics and geospatial data on natural gas use, we find the average fractional loss rate to the atmosphere from all downstream components of the natural gas system, including transmission, distribution, and end use, was 2.7 ± 0.6% in the Boston urban region, with little seasonal variability. This fraction is notably higher than the 1.1% implied by the most closely comparable emission inventory.

Mapping carbon storage in urban trees with multi-source remote sensing data: relationships between biomass, land use, and demographics in Boston neighborhoods.

High resolution maps of urban vegetation and biomass are powerful tools for policy-makers and community groups seeking to reduce rates of urban runoff, moderate urban heat island effects, and mitigate the effects of greenhouse gas emissions. We developed a very high resolution map of urban tree biomass, assessed the scale sensitivities in biomass estimation, compared our results with lower resolution estimates, and explored the demographic relationships in biomass distribution across the City of Boston. We integrated remote sensing data (including LiDAR-based tree height estimates) and field-based observations to map canopy cover and aboveground tree carbon storage at ~1m spatial scale. Mean tree canopy cover was estimated to be 25.5±1.5% and carbon storage was 355Gg (28.8MgCha(-1)) for the City of Boston. Tree biomass was highest in forest patches (110.7MgCha(-1)), but residential (32.8MgCha(-1)) and developed open (23.5MgCha(-1)) land uses also contained relatively high carbon stocks. In contrast with previous studies, we did not find significant correlations between tree biomass and the demographic characteristics of Boston neighborhoods, including income, education, race, or population density. The proportion of households that rent was negatively correlated with urban tree biomass (R(2)=0.26, p=0.04) and correlated with Priority Planting Index values (R(2)=0.55, p=0.001), potentially reflecting differences in land management among rented and owner-occupied residential properties. We compared our very high resolution biomass map to lower resolution biomass products from other sources and found that those products consistently underestimated biomass within urban areas. This underestimation became more severe as spatial resolution decreased. This research demonstrates that 1) urban areas contain considerable tree carbon stocks; 2) canopy cover and biomass may not be related to the demographic characteristics of Boston neighborhoods; and 3) that recent advances in high resolution remote sensing have the potential to improve the characterization and management of urban vegetation.

Denitrification in suburban lawn soils.

There is great uncertainty about the fate of nitrogen (N) added to urban and suburban lawns. We used direct flux and in situ chamber methods to measure N and NO fluxes from lawns instrumented with soil O sensors. We hypothesized that soil O, moisture, and available NO were the most important controls on denitrification and that N and NO fluxes would be high following fertilizer addition and precipitation events. While our results support these hypotheses, the thresholds of soil O, moisture, and NO availability required to see significant N fluxes were greater than expected. Denitrification rates were high in saturated, fertilized soils, but low under all other conditions. Annual denitrification was calculated to be 14.0 ± 3.6 kg N ha yr, with 5% of the growing season accounting for >80% of the annual activity. Denitrification is thus an important means of removing reactive N in residential landscapes, but varies markedly in space, time, and with factors that affect soil saturation (texture, structure, compaction) and NO availability (fertilization). Rates of in situ NO flux were low; however, when recently fertilized soils saturated with water were incubated in the laboratory, we saw extraordinarily high rates of NO production for the first few hours of incubation, followed by rapid NO consumption later in the experiment. These findings indicate a lag time between accelerated NO production and counterbalancing increases in NO consumption; thus, we cannot yet conclude that lawns are an insignificant source of NO in our study area.

Depleted soil carbon and nitrogen pools beneath impervious surfaces.

Urban soils and vegetation contain large pools of carbon (C) and nitrogen (N) and may sequester these elements at considerable rates; however, there have been no systematic studies of the composition of soils beneath the impervious surfaces that dominate urban areas. This has made it impossible to reliably estimate the net impact of urbanization on terrestrial C and N pools. In this study, we compared open area and impervious-covered soils in New York City and found that the C and N content of the soil (0-15 cm) under impervious surfaces was 66% and 95% lower, respectively. Analysis of extracellular enzyme activities in the soils suggests that recalcitrant compounds dominate the organic matter pool under impervious surfaces. If the differences between impervious-covered and open area soils represent a loss of C and N from urban ecosystems, the magnitude of these losses could offset sequestration in other parts of the urban landscape.

Modeling and validation of on-road CO2 emissions inventories at the urban regional scale

On-road emissions are a major contributor to rising concentrations of atmospheric greenhouse gases. In this study, we applied a downscaling methodology based on commonly available spatial parameters to model on-road CO(2) emissions at the 1 × 1 km scale for the Boston, MA region and tested our approach with surface-level CO(2) observations. Using two previously constructed emissions inventories with differing spatial patterns and underlying data sources, we developed regression models based on impervious surface area and volume-weighted road density that could be scaled to any resolution. We found that the models accurately reflected the inventories at their original scales (R(2) = 0.63 for both models) and exhibited a strong relationship with observed CO(2) mixing ratios when downscaled across the region. Moreover, the improved spatial agreement of the models over the original inventories confirmed that either product represents a viable basis for downscaling in other metropolitan regions, even with limited data.

Nitrate production and availability in residential soils.

The rapid increase in residential land area in the United States has raised concern about water pollution associated with nitrogen fertilizers. Nitrate (NO3-) is the form of reactive N that is most susceptible to leaching and runoff; thus, a more thorough understanding of nitrification and NO3(-) availability is needed if we are to accurately predict the consequences of residential expansion for water quality. In particular, there have been few assessments of how the land use history, housing density, and age of residential soils influence NO3(-) pools and fluxes, especially at depth. In this study, we used 1 m deep soil cores to evaluate potential net nitrification and mineralization, microbial respiration and biomass, and soil NO3(-) and NH4+ pools in 32 residential home lawns that differed by previous land use and age, but had similar soil types. These were compared to eight forested reference sites with similar soils. Our results suggest that a change to residential land use has increased pools and production of reactive N, which has clear implications for water quality in the region. However, the results contradict the common assumption that NO3(-) production and availability is dramatically higher in residential soils than in forests in general. While net nitrification (128.6 +/- 15.5 mg m(-2) d(-1) vs. 4.7 +/- 2.3 mg m(-2) d(-1); mean +/- SE) and exchangeable NO3(-) (3.8 +/- 0.5 g/m2 vs. 0.7 +/- 0.3 g/m2) were significantly higher in residential soils than in forest soils in this study, these measures of NO3(-) production and availability were still notably low, comparable to deciduous forest stands in other studies. A second unexpected result was that current homeowner management practices were not predictive of NO3(-) availability or production. This may reflect the transient availability of inorganic N after fertilizer application. Higher housing density and a history of agricultural land use were predictors of greater NO3(-) availability in residential soils. If these factors are good predictors across a wider range of sites, they may be useful indicators of NO3(-) availability and leaching and runoff potential at the landscape scale.

Field and remotely sensed measures of soil and vegetation carbon and nitrogen across an urbanization gradient in the Boston metropolitan area

Understanding the impact of urbanization on terrestrial biogeochemistry is critical for addressing society’s grand challenge of global environmental change. We used field observations and remotely sensed data to quantify the effects of urbanization on vegetation and soils across a 100-km urbanization gradient extending from Boston to Harvard Forest and Worcester, MA. At the field-plot scale, the normalized difference vegetation index (NDVI) was positively correlated with aboveground biomass (AGB) and foliar nitrogen (N) content and negatively correlated with impervious surface fraction. Unlike previous studies, we found no significant relationship between NDVI or impervious surface area (ISA) fraction and foliar N concentration. Patterns in foliar N appeared to be driven more strongly by changes in species composition rather than phenotypic plasticity across the urbanization gradient. For forest and non-residential development, soil nitrogen content increased with urban intensity. In contrast, residential land had consistently high soil N content across the gradient of urbanization. When field observations were scaled-up to the Boston Metropolitan Statistical Area (MSA), we found that soil and vegetation N content were negatively correlated with ISA fraction, an indicator of urban intensity. Our results demonstrated the importance of accounting for the influence of impervious surfaces when scaling field data across urban ecosystems. The combination of field data with remote sensing holds promise for disentangling the complex interactions that drive biogeochemical cycling in urbanizing landscapes. Empirical data that accurately characterize variations in urban biogeochemistry are critical to gain a mechanistic understanding of urban ecosystem function and to guide policy makers and planners in developing ecologically sensitive development strategies.

Tree Productivity Enhanced with Conversion from Forest to Urban Land Covers.

Urban areas are expanding, changing the structure and productivity of landscapes. While some urban areas have been shown to hold substantial biomass, the productivity of these systems is largely unknown. We assessed how conversion from forest to urban land uses affected both biomass structure and productivity across eastern Massachusetts. We found that urban land uses held less than half the biomass of adjacent forest expanses with a plot level mean biomass density of 33.5 ± 8.0 Mg C ha-1. As the intensity of urban development increased, the canopy cover, stem density, and biomass decreased. Analysis of Quercus rubra tree cores showed that tree-level basal area increment nearly doubled following development, increasing from 17.1 ± 3.0 to 35.8 ± 4.7 cm2 yr-1. Scaling the observed stem densities and growth rates within developed areas suggests an aboveground biomass growth rate of 1.8 ± 0.4 Mg C ha-1 yr-1, a growth rate comparable to nearby, intact forests. The contrasting high growth rates and lower biomass pools within urban areas suggest a highly dynamic ecosystem with rapid turnover. As global urban extent continues to grow, cities consider climate mitigation options, and as the verification of net greenhouse gas emissions emerges as critical for policy, quantifying the role of urban vegetation in regional-to-global carbon budgets will become ever more important.