I. Land Use Change and Lateral Carbon Fluxes
Urban areas alter local, regional, and global biogeochemical cycles due to their inherent heterotrophy; in other words, by importing the food, building supplies, and water required to sustain their populations, cities act to concentrate the effects of altered nitrogen and carbon cycles. Approximately 50% of the world’s population and 79% of the U.S. population lives in urban areas, with more and more people moving to cities each year (US Census, 2000; UNDP, 2006). By 2030, 60% of the world’s population is expected to dwell in urban centers with the fastest growing regions occurring in the developing world (UNDP, 2006). Despite occupying less than 2% of the Earth’s surface, urban centers have large ecological footprints (Grimm et al. 2008), producing 78% of the global greenhouse gases, inextricably altering biogeochemical cycles.Given that the U.S. landscape is rapidly urbanizing and greater than 50% of residents live within 50 miles of the coast, estuarine, and coastal ecosystems are heavily impacted. Thus, by characterizing nitrogen and carbon biogeochemistry, our research contributes to the understanding of the mechanisms linking these critical elemental cycles and the impacts of the evolution and development of the landscape on ecosystems.
Recent work has shown that cities are not only sources of CO2 to the atmosphere but also a large source of dissolved inorganic carbon (DIC = CO2 + HCO3 + CO3) to rivers (Barnes & Raymond, 2009). These observations suggest we need to change the way we think about human alteration of the carbon cycle – not only are we changing the amount of CO2 in the atmosphere, we are changing the lateral flux of carbon from land to the ocean.
Land Use Change = Soil Disturbance
Over 40% of the terrestrial landscape is used to grow food (crop and rangeland) and another 5% represents the densest settlements, i.e. urban areas. Using a global dataset of 14C-DOC values from rivers spanning equatorial (Amazon, Congo) to Arctic (Lena, Yukon), we found that human induced land use change (both urban and croplands) results in the export of millennial aged DOC (see Butman et al. 2015 Nature Geoscience).
We continued to investigate the sources of aged organic carbon by using proxies for hydrology (isotopes, water yield) as well as the stoichiometric relationship between the DOC and the sum of base cations to examine if shifting flow paths – both seasonally and spatially – can explain the export of aged DOC from large watersheds (Barnes et al. 2018 ES&T).
- Barnes, R.T. and P.A. Raymond. 2009. The contribution of urban and agricultural activities to inorganic carbon fluxes in Southern New England. Chemical Geology, 266: 327-336.
- Butman D., H.F. Wilson, R.T. Barnes, M. A. Xenopoulos, Raymond, P.A. 2015. Disturbance mobilizes aged carbon to rivers, Nature Geoscience,8: 112-116, DOI:10.1038/ngeo2322
- Barnes, R.T., D.E. Butman, H. Wilson, & P.A. Raymond. 2018. Riverine export of aged carbon driven by flow path depth and residence time, Environmental Science & Technology, doi: 10.1021/acs.est.7b04717
II. Controls on Watershed N Export
Using a comparative watershed approach my dissertation work used the dual isotopic composition of nitrate (δ15N and δ18O) from 20 streams over 2 years to examine controls on nitrate export. Agricultural (row crops and pasture) and urban (50-88% impervious) streams retained a strong seasonal signal and model results suggest they were retaining between 40 and 65% of all nitrate inputs on an annual basis (Barnes et al. 2010). The average amount of nitrate retained or removed within the watershed was greatest in watersheds with less development – e.g. catchments dominated by pasture retained more than catchments with row crops and/or active cattle grazing.
The stoichiometric relationship between available organic matter and inorganic nitrogen parallel shifts in redox conditions and microbial community (Taylor & Townsend 2009; Helton et al. 2015). Thus these are also dominant controls on ecosystem nitrogen and carbon cycling – especially at terrestrial-aquatic interfaces. To tease apart biological vs. hydrological controls on the transformation of N and C within ecosystems I often use modeling and laboratory experiments in addition to sampling (water, soil) and monitoring (water level, redox, oxygen).
- Barnes, R.T. and P.A. Raymond. 2010. Land use controls on the delivery, processing, and removal of nitrogen from small watersheds: Insights from the dual isotopic composition of stream nitrate. Ecological Applications, 20 (7): 1961-1978.
- Barnes, R.T., R.L. Smith, & G.R. Aiken. 2012. Linkages between denitrification and organic matter quality, Boulder Creek Watershed, CO. Journal of Geophysical Research- Biogeosciences,DOI:10.1029/2011JG001749
- Barnes, R.T., M.W. Williams, J.N. Parman, K. Hill, & N. Caine. 2014. Thawing Glacial and Permafrost Features Contribute to Nitrogen Export from Green Lakes Valley, Colorado Front Range, USA, Biogeochemistry, doi:10.1007/s10533-013-9886-5