title: Effects of fertilizer on inorganic soil N in East Africa maize systems: vertical distributions and temporal dynamics.
abstract: Fertilizer applications are poised to increase across sub-Saharan Africa (SSA), but the fate of added nitrogen (N) is largely unknown. We measured vertical distributions and temporal variations of soil inorganic N following fertilizer application in two maize (Zea mays L.)-growing regions of contrasting soil type. Fertilizer trials were established on a clayey soil in Yala, Kenya, and on a sandy soil in Tumbi, Tanzania, with application rates of 0-200 kg N/ha/yr. Soil profiles were collected (0-400 cm) annually (for three years in Yala and two years in Tumbi) to examine changes in inorganic N pools. Topsoils (0-15 cm) were collected every 3-6 weeks to determine how precipitation and fertilizer management influenced plant-available soil N. Fertilizer management altered soil inorganic N, and there were large differences between sites that were consistent with differences in soil texture. Initial soil N pools were larger in Yala than Tumbi (240 vs. 79 kg/ha). Inorganic N pools did not change in Yala (277 kg/ha), but increased fourfold after cultivation and fertilization in Tumbi (371 kg/ha). Intra-annual variability in NO-3-N concentrations (3-33 μg/g) in Tumbi topsoils strongly suggested that the sandier soils were prone to high leaching losses. Information on soil inorganic N pools and movement through soil profiles can h vulnerability of SSA croplands to N losses and determine best fertilizer management practices as N application rates increase. A better understanding of the vertical and temporal patterns of soil N pools improves our ability to predict the potential environmental effects of a dramatic increase in fertilizer application rates that will accompany the intensification of African croplands.
authors.affiliation: Department of Plant Science and Landscape Architecture, University of Maryland, College Park, Maryland, 20742, USA. kltully@umd.edu.|Agriculture and Food Security Center, The Earth Institute, Columbia University, New York, New York, 10025, USA. kltully@umd.edu.
authors.forename: Katherine L
authors.initials: KL
authors.lastname: Tully
authors.affiliation: Agriculture and Food Security Center, The Earth Institute, Columbia University, New York, New York, 10025, USA.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Hickman
authors.affiliation: Department of Earth and Environmental Sciences, Columbia University, New York, New York, 10025, USA.
authors.forename: Madeline
authors.initials: M
authors.lastname: McKenna
authors.affiliation: The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts, 02543, USA.|Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island, 02912, USA.
authors.forename: Christopher
authors.initials: C
authors.lastname: Neill
authors.affiliation: Agriculture and Food Security Center, The Earth Institute, Columbia University, New York, New York, 10025, USA.
authors.forename: Cheryl A
authors.initials: CA
authors.lastname: Palm
mesh_terms: D000383:Agriculture; D005308:Fertilizers / Q000032:analysis*; D007630:Kenya; D012987:Soil / Q000737:chemistry*; D013636:Tanzania; D013997:Time Factors; D003313:Zea mays / Q000254:growth & development*
chemical_list: D005308:Fertilizers; D012987:Soil
keywords: 
  Gliricidia sepium
  ; African Green Revolution; fertilizer; maize; nitrogen variability; sub-Saharan Africa