Legacies of Lead in Charm City's Soil: Lessons from the Baltimore Ecosystem Study.

Understanding the spatial distribution of soil lead has been a focus of the Baltimore Ecosystem Study since its inception in 1997. Through multiple research projects that span spatial scales and use different methodologies, three overarching patterns have been identified: (1) soil lead concentrations often exceed state and federal regulatory limits; (2) the variability of soil lead concentrations is high; and (3) despite multiple sources and the highly heterogeneous and patchy nature of soil lead, discernable patterns do exist. Specifically, housing age, the distance to built structures, and the distance to a major roadway are strong predictors of soil lead concentrations. Understanding what drives the spatial distribution of soil lead can inform the transition of underutilized urban space into gardens and other desirable land uses while protecting human health. A framework for management is proposed that considers three factors: (1) the level of contamination; (2) the desired land use; and (3) the community's preference in implementing the desired land use. The goal of the framework is to promote dialogue and resultant policy changes that support consistent and clear regulatory guidelines for soil lead, without which urban communities will continue to be subject to the potential for lead exposure.

A comparison of three empirically based, spatially explicit predictive models of residential soil Pb concentrations in Baltimore, Maryland, USA: understanding the variability within cities.

In many older US cities, lead (Pb) contamination of residential soil is widespread; however, contamination is not uniform. Empirically based, spatially explicit models can assist city agencies in addressing this important public health concern by identifying areas predicted to exceed public health targets for soil Pb contamination. Sampling of 61 residential properties in Baltimore City using field portable X-ray fluorescence revealed that 53 % had soil Pb that exceeded the USEPA reportable limit of 400 ppm. These data were used as the input to three different spatially explicit models: a traditional general linear model (GLM), and two machine learning techniques: classification and regression trees (CART) and Random Forests (RF). The GLM revealed that housing age, distance to road, distance to building, and the interactions between variables explained 38 % of the variation in the data. The CART model confirmed the importance of these variables, with housing age, distance to building, and distance to major road networks determining the terminal nodes of the CART model. Using the same three predictor variables, the RF model explained 42 % of the variation in the data. The overall accuracy, which is a measure of agreement between the model and an independent dataset, was 90 % for the GLM, 83 % for the CART model, and 72 % for the RF model. A range of spatially explicit models that can be adapted to changing soil Pb guidelines allows managers to select the most appropriate model based on public health targets.

The effects of the urban built environment on the spatial distribution of lead in residential soils.

Lead contamination of urban residential soils is a public health concern. Consequently, there is a need to delineate hotspots in the landscape to identify risk and facilitate remediation. Land use is a good predictor of some environmental pollutants. However, in the case of soil lead, research has shown that land use is not a useful proxy. We hypothesize that soil lead is related to both individual landscape features at the parcel scale and the landscape context in which parcels are embedded. We sampled soil lead on 61 residential parcels in Baltimore, Maryland using field-portable x-ray fluorescence. Thirty percent of parcels had average lead concentrations that exceeded the USEPA limit of 400 ppm and 53% had at least one reading that exceeded 400 ppm. Results indicate that soil lead is strongly associated with housing age, distance to roadways, and on a parcel scale, distance to built structures.

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.

Methane uptake in urban forests and lawns.

The largest natural biological sink for the radiatively active trace gas methane (CH4) is bacteria in soils that consume CH4 as an energy and carbon source. This sink has been shown to be sensitive to nitrogen (N) inputs and alterations of soil physical conditions. Given this sensitivity, conversion of native ecosystems to urban, suburban, and exurban managed lawns thus has potential to affect regional CH4 budgets. We measured CH4 fluxes monthly from four urban forest, four rural forest and four urban lawn plots in the Baltimore, MD, metropolitan area from 2001 to 2005. Our objectives were to evaluate the effects of urban atmospheric and land use change on CH4 uptake and the importance of these changes relative to other greenhouse forcings in the urban landscape. Rural forests had a high capacity for CH4 uptake (1.68 mg m(-2) day(-1)). This capacity was reduced in urban forests (0.23 mg m(-2) day(-1)) and almost completely eliminated in lawns. Possible mechanisms for these reductions include increases in atmospheric N deposition and CO2 levels, fertilization of lawns, and alteration of soil physical conditions that influence diffusion. Although conversion of native forests to lawns had dramatic effects on CH4 uptake, these effects do not appear to be significant to statewide greenhouse gas forcing.

Nitrate leaching and nitrous oxide flux in urban forests and grasslands.

Urban landscapes contain a mix of land-use types with different patterns of nitrogen (N) cycling and export. We measured nitrate (NO(3)(-)) leaching and soil:atmosphere nitrous oxide (N(2)O) flux in four urban grassland and eight forested long-term study plots in the Baltimore, Maryland metropolitan area. We evaluated ancillary controls on these fluxes by measuring soil temperature, moisture, and soil:atmosphere fluxes of carbon dioxide on these plots and by sampling a larger group of forest, grass, and agricultural sites once to evaluate soil organic matter, microbial biomass, and potential net N mineralization and nitrification. Annual NO(3)(-) leaching ranged from 0.05 to 4.1 g N m(-2) yr(-1) and was higher in grass than forest plots, except in a very dry year and when a disturbed forest plot was included in the analysis. Nitrous oxide fluxes ranged from 0.05 to >0.3 g N m(-2) yr(-1), with few differences between grass and forest plots and markedly higher fluxes in wet years. Differences in NO(3)(-) leaching and N(2)O flux between forests and grasslands were not as high as expected given the higher frequency of disturbance and fertilization in the grasslands. Carbon dioxide flux, organic matter, and microbial biomass were as high or higher in urban grasslands than in forests, suggesting that active carbon cycling creates sinks for N in vegetation and soil in these ecosystems. Although urban grasslands export more N to the environment than native forests, they have considerable capacity for N retention that should be considered in evaluations of land-use change.

Lead forms in urban turfgrass and forest soils as related to organic matter content and pH.

Soil pH may influence speciation and extractability of Pb, depending on type of vegetation in urban soil environments. We investigated the relationship between soil pH and Pb extractability at forest and turf grass sites in Baltimore, Maryland. Our two hypotheses were: (1) due to lower pH values in forest soils, more Pb will be in exchangeable forms in forested than in turfgrass soils and (2) due to the greater lability of exchangeable Pb in equilibrium with soil solution in forest soils, concentrations of this form will increase with depth more so than in the turfgrass soils, as related to organic matter content and pH. Soil samples were collected from three forested and three turfgrass sites to depths of 20 cm. Lead forms were determined using a sequential extraction technique. Soils under turfgrass and forest vegetation differed in the extractability of soil Pb (P < 0.01) for the Mn(III, IV)- and Fe(III)(hydr) oxide fraction. A greater Pb concentration was bound to this fraction under turfgrass (211 mg kg(-1), 69% of total Pb) than forested soils (67 mg kg(-1), 61% of total Pb), perhaps due to soil pH differences of 5.9 and 5.0, respectively. In the forested soils, as depth increased, the ratio of exchangeable-to-total Pb increased and the ratio of organically bound Pb-to-total Pb decreased. The results suggest changes in pH and organic matter content with depth affect the extractability of Pb, and these soil properties are affected differentially by grass versus tree vegetation in the urban soils investigated.

Carbon storage by urban soils in the United States.

We used data available from the literature and measurements from Baltimore, Maryland, to (i) assess inter-city variability of soil organic carbon (SOC) pools (1-m depth) of six cities (Atlanta, Baltimore, Boston, Chicago, Oakland, and Syracuse); (ii) calculate the net effect of urban land-use conversion on SOC pools for the same cities; (iii) use the National Land Cover Database to extrapolate total SOC pools for each of the lower 48 U.S. states; and (iv) compare these totals with aboveground totals of carbon storage by trees. Residential soils in Baltimore had SOC densities that were approximately 20 to 34% less than Moscow or Chicago. By contrast, park soils in Baltimore had more than double the SOC density of Hong Kong. Of the six cities, Atlanta and Chicago had the highest and lowest SOC densities per total area, respectively (7.83 and 5.49 kg m(-2)). On a pervious area basis, the SOC densities increased between 8.32 (Oakland) and 10.82 (Atlanta) kg m(-2). In the northeastern United States, Boston and Syracuse had 1.6-fold less SOC post- than in pre-urban development stage. By contrast, cities located in warmer and/or drier climates had slightly higher SOC pools post- than in pre-urban development stage (4 and 6% for Oakland and Chicago, respectively). For the state analysis, aboveground estimates of C density varied from a low of 0.3 (WY) to a high of 5.1 (GA) kg m(-2), while belowground estimates varied from 4.6 (NV) to 12.7 (NH) kg m(-2). The ratio of aboveground to belowground estimates of C storage varied widely with an overall ratio of 2.8. Our results suggest that urban soils have the potential to sequester large amounts of SOC, especially in residential areas where management inputs and the lack of annual soil disturbances create conditions for net increases in SOC. In addition, our analysis suggests the importance of regional variations of land-use and land-cover distributions, especially wetlands, in estimating urban SOC pools.

A distinct urban biogeochemistry?

Most of the global human population lives in urban areas where biogeochemical cycles are controlled by complex interactions between society and the environment. Urban ecology is an emerging discipline that seeks to understand these interactions, and one of the grand challenges for urban ecologists is to develop models that encompass the myriad influences of people on biogeochemistry. We suggest here that existing models, developed primarily in unmanaged and agricultural ecosystems, work poorly in urban ecosystems because they do not include human biogeochemical controls such as impervious surface proliferation, engineered aqueous flow paths, landscaping choices, and human demographic trends. Incorporating these human controls into biogeochemical models will advance urban ecology and will require enhanced collaborations with engineers and social scientists.

Controls on mass loss and nitrogen dynamics of oak leaf litter along an urban-rural land-use gradient.

Using reciprocal leaf litter transplants, we investigated the effects of contrasting environments (urban vs. rural) and intraspecific variations in oak leaf litter quality on mass loss rates and nitrogen (N) dynamics along an urban-rural gradient in the New York City metropolitan area. Differences in earthworm abundances and temperature had previously been documented in the stands along this gradient. Red oak leaf litter was collected and returned to its original source stand as native litter to measure decay rates along the gradient. To separate site effects from litter quality effects on decay, reciprocal transplants of litter were also made between stands at the extremes of the environmental gradient (urban and rural stands). Land-use had no effect on mass loss and N dynamics of native litter by the end of the 22-month incubation period. The lack of differences in native litter suggests the factors affecting decay were similar across the stands in this study. However, in the transplant study both environment and litter type strongly affected decay of oak leaf litter. On average urban and rural litter decomposed faster over the incubation period in urban than in rural stands (P=0.016 and P=0.001, respectively, repeated measures ANOVA). Differences in mass loss between urban and rural stands resulted in rural environments having less mass remaining than urban environments at the end of the incubation period (25.6 and 46.2% for urban and rural sites, respectively). Likewise, less N remained in leaf residue in urban sites (71.3%) compared to that in rural sites (115.1%). Litter type also affected mass loss rates during the 22-month incubation period. On average rural litter mass loss rates were faster than urban litter rates in both urban and rural stands (P=0.030 and P=0.026, respectively, repeated measures ANOVA). By the end of the incubation period, rural litter exhibited 43 and 20% greater mass loss and retained 44 and 5% less N than urban litter decomposing in the same urban and rural sites, respectively. These results suggest that different factors were controlling mass loss and N release rates along this urban-rural gradient. In urban stands, exotic earthworms and warmer temperatures may be compensating for what would otherwise be slowly decaying leaf litter because of its lower quality. Likewise, the lower quality litter produced in the urban stands may be decreasing the net release of N from litter despite higher temperatures and earthworm activity. Even though native litter decay rates were similar, the differential importance of the factors affecting decay along this gradient could alter the response of these forests to disturbance and variations in climate.

Soil nitrogen cycle processes in urban riparian zones.

Riparian zones have been found to function as "sinks" for nitrate (NO3-), the most common groundwater pollutant in the U. S., in many areas. The vast majority of riparian research, however, has focused on agricultural watersheds. There has been little analysis of riparian zones in urban watersheds, despite the fact that urban areas are important sources of NO3- to nitrogen (N)-sensitive coastal waters in many locations. In this study, we measured stream incision, water table depths, and pools, production (mineralization, nitrification), and consumption (denitrification) of NO3- in urban soils. Samples were taken from soil profiles (0-100 cm) of three forested urban and suburban zones and one forested reference riparian zone in the Baltimore, Maryland metropolitan area. Our objectives were to determine (1) if stream incision associated with urbanization results in lower riparian water tables, and (2) if pools, production, and consumption of NO3- vary systematically with stream incision and riparian water table levels. Two of the three urban and suburban streams were more incised and all three had lower water tables in their riparian zones than the forested reference stream. Urban and suburban riparian zones had higher NO3- pools and nitrification rates than the forested reference riparian zone, which was likely due to more aerobic soil profiles, lower levels of available soil carbon, and greater N enrichment in the urban and suburban sites. At all sites, denitrification potential decreased markedly with depth in the soil profile. Lower water tables in the urban and suburban riparian zones thus inhibit interaction of groundwater-borne NO3- with near surface soils that have the highest denitrification potential. These results suggest that urban hydrologic factors can increase the production and reduce the consumption of NO3- in riparian zones, reducing their ability to function as sinks for NO3- in the landscape.