Nonlinear response of nitric oxide fluxes to fertilizer inputs and the impacts of agricultural intensification on tropospheric ozone pollution in Kenya. (17th March 2017)

• Record Type: Journal Article
• Authors: Hickman, Jonathan E.; Huang, Yaoxian; Wu, Shiliang; Diru, Willy; Groffman, Peter M.; Tully, Katherine L.; Palm, Cheryl A.
• Is Part Of: Global change biology. Volume 23:Number 8(2017)
• Page Start: 3193
• Page End: 3204
• Publication Date: 2017-03-17
• Subjects: African Green Revolution -- fertilizer -- greenhouse gas -- nitrogen -- soils -- tropical agriculture; Climatic changes -- Environmental aspects -- Periodicals; Troposphere -- Environmental aspects -- Periodicals; Biodiversity conservation -- Periodicals; Eutrophication -- Periodicals; 551.5
• DOI: 10.1111/gcb.13644
• Languages: English
• ISSNs: 1354-1013

Abstract

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 thegeos‐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 −1 or 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 −1 during 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. Abstract : Fertilizer use is expected to increase by more than an order of magnitude in sub‐Saharan Africa, to increase food production and reduce food insecurity. We measured emissions of nitric oxide, a precursor to tropospheric ozone pollution, over 2 years from experimental maize plots in western Kenya to understand how agricultural intensification may affect air quality and to quantify the relationship between nitrogen inputs and NO emissions. We found evidence that nitric oxide emissions increase as a sigmoidal function of the rate of nitrogen additions, with the highest emissions occurring above recommended fertilization rates. Using an atmospheric chemical transport model, we find that changes in tropospheric ozone pollution driven by increased fertilizer‐induced nitric oxide emissions are unlikely to negatively impact crop production in western Kenya.