Published online July 6 , 2006 Carbon Storage by Urban Soils in the United States

We used data available from the literature and measurements from Baltionore, Marylan6 to (i) assas inter-city variebifity of soil oqanic carbon (SOC) pools (1-m depth) of six cities (Atlanta, Balbore, Boston, Cbifago, 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 extrepolrrte 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 approxitnately 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 mP2). On a pervious area basis, the SOC densities increased between 8.32 (Oakland) and 10.82 (Atlanta) kg m-'. In the northeastern United States, Boston and Syracuse had 1.6-fold less SOC postthan in pre-urban development stage. By contrast, cities located in warmer andlor drier climates had slightly higher SOC pools postthan 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 (CA) kg m-', while belowground estimates varied from 4.6 (NV) to 12.7 (NH) kg m-'. 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, espeaally in residential areas where management inputs and the lack of amual 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. I N TERRESTRIAL ECOSYSTEMS and at global scales soil organic carbon (SOC) is primarily a function of the average net primary productivity (or inputs of organic matter) and the rate of organic matter decay (Kirschbaum, 2000). Because rates of organic matter input and decay differentially vary in their sensitivities to temperature and precipitation, a wide variation in SOC exists among life zones (Post et al., 1982). While precipitation and temperature are good predictors of SOC pools at global scales, pools at regional and local scales vary due to soil drainage and the quality of litter entering the soil system (Berg and McClaugherty, 1987; Ciiuteaux et al., 1995). These factors in turn are highly related to topography, soil texture, and plant species composition. In urban landscapes, SOC also may vary due to introducR.V. Pouyat and 1.D. Yesilonis, USDA Forest Service, Northeastern Research Station, c/o Baltimore Ecosystem Study, 5200 Westland Boulevard, Room 134, Gniversity of Maryland, Baltimore County, Baltimore, MD 21227. D.J. Nowak, c/o 5 Moon Library, SUNY-ESF, NY 13210. Received 31 May 2005. *Correspondiog author (rpouyat@Er fed.us). Published in J. Environ. Quai. 35:1566-1575 (2005). Special Submissions doi:10.2134/jeq2005.0215 O ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, W1 53711 USA tions of human disturbances, exotic plants, horticultural management (e.g., fertilization, irrigation, clipping), and urban environmental factors (e.g., urban heat island, elevated atmospheric carbon dioxide). The net result is an "urban soil mosaic'hhere soil conditions, and thus SOC, can vary widely between and within types or patches of soil (Pouyat et al., 2003). Recent research efforts have addressed whether various land-use changes and their associated soil modifications will affect soil C storage at regional and global scales (Houghton et al., 1999; Caspersen et al., 2000). In the case of urban land-use change, very little data are available to assess the spatial variation of SOC pools and whether urban land use leads to a net increase or decrease in these pools (Pouyat et al., 2002). This lack of data has made it problematic to predict or assess the regional effects of land-use change on soil C pools in populated regions of the world (e.g., Ames and Lavkulich, 1999: Tian et al., 1999). In the United States, the conversion of agricultural, grass, and forest land to urban land use is occurring at accelerated rates. Between 1980 and 2000 alone, land devoted to urban uses grew by more than 34% in the United States (USDA Natural Resources Conservation Service, 2001). By contrast, the population grew by only 24% during the same period (United States Department of Commerce, 2001). The resultant urban growth pattern is more dispersed than earlier development patterns and as a result is increasingly affecting the storage of carbon in soils. Urban development can increase or decrease SOC pools depending on the net effect of the previously mentioned factors and the amount of SOC stored in the ecosystem before urban development (Pouyat et al., 2003). In earlier attempts, we calculated urban SOC pools for the conterminous United States (Pouyat et al., 2002, 2003), but did not consider regional differences in native soils (associated with remnants of native ecosystems) and differences in land-use and vegetative cover patterns that occur among cities (Nowak et al., 1996). In this paper we use data that is available from the literature and our own measurements to estimate SOC pools of cities previously assessed for aboveground carbon stocks by trees (Nowak and Crane, 2002), and use the National Land Cover Database (NLCD) to extrapolate total SOC pools by state, region, and the conterminous United States. Specifically, our objectives were to (i) assess inter-city variability of SOC pools (1-m depth) of six cities (Atlanta, Baltimore, Boston, Chicago, Oakland, and Syracuse) where field collected data of tree biomass, land use, and cover were available; (ii) for the same cities calculate the net effect of urban land-use conversion Abbreviations: NLCD, National Land Cover Database; SOC, soil organic carbon. POLTAT ET AL.: CARBON STORAGE BY lJRBAN SOILS IN THE UNITED STATES 1567 on SOC pools; (Ei) use the NLCD to extrapolate total Table 1. Soil organic carbon (SOC) densities for disbhed and SOC pools (1-m depth) for each of the lower 48 states, by made soils in the cities of Baltimore, MD; New York, W Chicago, IL; Hong Kong, Chin% i;d Mosrow, Russia, Except region, and for the United States (lower 48 states); and where inacated, carbon densities were atlculated with data (iv) compare these SOC totals with aboveground carbon collected from soil pit or core chsrscterizations to a depth of storage by urban trees. 1 m (n = number of locations).