Accumulation of Carbon and Nitrogen in Residential Soils with Different Land-Use Histories

Urban areas are growing in size and importance; however, we are only beginning to understand how the process of urbanization influences ecosystem dynamics. In particular, there have been few assessments of how the land-use history and age of residential soils influence carbon (C) and nitrogen (N) pools and fluxes, especially at depth. In this study, we used 1-m soil cores to evaluate soil profile characteristics and C and N pools in 32 residential home lawns that differed by previous land use and age, but had similar soil types. These were compared to soils from eight forested reference sites. Residential soils had significantly higher C and N densities than nearby forested soils of similar types (6.95 vs. 5.44 kg C/m² and 552 vs. 403 g N/m², P < 0.05). Results from our chronosequence suggest that soils at residential sites that were previously in agriculture have the potential to accumulate C (0.082 kg C/m²/y) and N (8.3 g N/m²/y) rapidly after residential development. Rates of N accumulation at these sites were similar in magnitude to estimated fertilizer N inputs, confirming a high capacity for N retention. Residential sites that were forested prior to development had higher C and N densities than present-day forests, but our chronosequence did not reveal a significant pattern of increasing C and N density over time in previously forested sites. These data suggest that soils in residential areas on former agricultural land have a significant capacity to sequester C and N. Given the large area of these soils, they are undoubtedly significant in regional C and N balances.     

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.     

Metal concentrations in urban riparian sediments along an urbanization gradient

Urbanization impacts fluvial systems via a combination of changes in sediment chemistry and basin hydrology. While chemical changes in urban soils have been well characterized, similar surveys of riparian sediments in urbanized areas are rare. Metal concentrations were measured in sediments collected from riparian areas across the urbanization gradient in Baltimore, MD. Average metal concentrations are similar to those observed in other regional studies. Two important spatial patterns are evident in the data. First, calcium concentrations double across the urbanization gradient, regardless of changes in underlying geochemistry at the boundary between the Eastern US Piedmont and Coastal Plain physiographic provinces. Alkali-earth metal ratios indicate that the additional Ca is very pure and possibly arises from cement common to urban systems. Second, hot spots of trace metals typically associated with urban systems (e.g., Cu, Zn, and Pb) occur in areas that have been artificially filled to create additional real estate in high land value areas. Together, these data indicate that riparian sediments exhibit unexpected patterns of metal contamination. If these sediments are remobilized, during events such as droughts or floods, this contamination may perpetuate legacy impacts to ecosystem health from a history of fluvial contamination.     

Moving Towards a New Urban Systems Science

Ecosystems - Research on urban ecosystems rapidly expanded in the 1990s and is now a central topic in ecosystem science. In this paper, we argue that there are two critical challenges for ecosystem...