title: Nonlinear response of nitric oxide fluxes to fertilizer inputs and the impacts of agricultural intensification on tropospheric ozone pollution in Kenya.
abstract: Crop yields in sub-Saharan Africa remain stagnant at 1 ton ha-1, and 260 million people lack access to adequate food resources. Order-of-magnitude increases in fertilizer use are seen as a critical step in attaining food security. This increase represents an unprecedented input of nitrogen (N) to African ecosystems and will likely be accompanied by increased soil emissions of nitric oxide (NO). NO is a precursor to tropospheric ozone, an air pollutant and greenhouse gas. Emissions of NO from soils occur primarily during denitrification and nitrification, and N input rates are a key determinant of emission rates. We established experimental maize plots in western Kenya to allow us to quantify the response function relating NO flux to N input rate during the main 2011 and 2012 growing seasons. NO emissions followed a sigmoid response to fertilizer inputs and have emission factors under 1% for the roughly two-month measurement period in each year, although linear and step relationships could not be excluded in 2011. At fertilization rates above 100 kg N ha-1, NO emissions increased without a concomitant increase in yields. We used the geos-chem chemical transport model to evaluate local impacts of increased NO emissions on tropospheric ozone concentrations. Mean 4-hour afternoon tropospheric ozone concentrations in Western Kenya increased by up to roughly 2.63 ppbv under fertilization rates of 150 kg N ha-1or higher. Using AOT40, a metric for assessing crop damage from ozone, we find that the increased ozone concentrations result in an increase in AOT40 exposure of approximately 110 ppbh for inputs of 150 kg N ha-1during the March-April-May crop growing season, compared with unfertilized simulations, with negligible impacts on crop productivity. Our results suggest that it may be possible to manage Kenyan agricultural systems for high yields while avoiding substantial impacts on air quality.
authors.affiliation: The Earth Institute of Columbia University, 61 Route 9W, Lamont Hall, PO Box 1000, Palisades, NY, 10964, USA.
authors.forename: Jonathan E
authors.identifier: 0000-0002-7246-642X
authors.initials: JE
authors.lastname: Hickman
authors.affiliation: Department of Geological & Mining Engineering & Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931, USA.|Now at the School of Forestry and Environmental Studies, Yale University, 195 Prospect St., New Haven, CT, 06511, USA.
authors.forename: Yaoxian
authors.initials: Y
authors.lastname: Huang
authors.affiliation: Department of Geological & Mining Engineering & Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931, USA.|Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931, USA.
authors.forename: Shiliang
authors.initials: S
authors.lastname: Wu
authors.affiliation: Millennium Village Project, Milimani Block 10/35, Busia Road, PO Box 2389-40100, Kisumu, Kenya.
authors.forename: Willy
authors.initials: W
authors.lastname: Diru
authors.affiliation: Cary Institute of Ecosystem Studies, 2801 Sharon Turnpike, PO Box AB, Millbrook, NY, 12545, USA.|City University of New York Advanced Science Research Center and Brooklyn College Department of Earth and Environmental Sciences, 85 St. Nicholas Terrace, 5th Floor, New York, NY, 10031, USA.
authors.forename: Peter M
authors.initials: PM
authors.lastname: Groffman
authors.affiliation: University of Maryland, 2108 Plant Sciences Building, College Park, MD, 20742, USA.
authors.forename: Katherine L
authors.initials: KL
authors.lastname: Tully
authors.affiliation: The Earth Institute of Columbia University, 61 Route 9W, Lamont Hall, PO Box 1000, Palisades, NY, 10964, USA.|Now at the University of Florida, P.O. Box 110180, Gainesville, FL, 32611, USA.
authors.forename: Cheryl A
authors.initials: CA
authors.lastname: Palm
mesh_terms: D000383:Agriculture; D000397:Air Pollution; D004784:Environmental Monitoring; D005308:Fertilizers; D007630:Kenya; D009569:Nitric Oxide / Q000737:chemistry*; D010126:Ozone; D012987:Soil
chemical_list: D005308:Fertilizers; D012987:Soil; D009569:Nitric Oxide; D010126:Ozone
keywords: African Green Revolution; fertilizer; greenhouse gas; nitrogen; soils; tropical agriculture