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Growing maize in sub-Saharan Africa

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Fertilizer use is rapidly increasing in sub-Saharan Africa (SSA). However, we currently have little understanding of the consequences of increased nitrogen (N) fertilizer use on surface and groundwater resource quality in these tropical croplands. This is because there are few field studies that examine N dynamics in SSA, and extrapolation is difficult because soil biogeochemistry and land management differ from the regions where most of our understanding of N losses from agriculture has been developed. I will present data on N balances from a high-clay soil in western Kenya and a low-clay soil in mid-western Tanzania. Experimental fields were established in both sites with fertilizer rates ranging from 0 to 200 kg N ha-1 yr-1. In the high-clay fields (Kenya) N removed in the maize harvest comprised the largest N export, with little N lost via leaching or through soil emissions. In the low-clay soils fields (Tanzania), N leaching losses comprised the largest N export, with moderate exports in the maize harvest, and low soil N emissions. Nitrogen is used farm more more efficiently in maize cultivated in the high-clay soils compared to the low-clay soils, which has huge implications for both environmental and food security outcomes in these two regions.

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Growing maize in sub-Saharan Africa

  1. 1. Growing maize efficiently in sub- Saharan Africa: the interactions of soils and fertilizer Kate Tully Minisymposium in Plant Biology
  2. 2. How do we maximize productivity and minimize nutrient loss? 2
  3. 3. Tropical Agriculture • Majority of population growth set to occur • Intensifying and expanding • Rain-fed • Small-holder systems • Operating at the “margin” • Effect of management? 3
  4. 4. I. AFRICAN GREEN REVOLUTION Yield stagnation in Africa 4 Hazell and Wood 2008- PNAS
  5. 5. Initial goal of African Green Revolution 5 <7 kg/ha 50 kg/ha
  6. 6. 6 Nutrient dynamics in sub-Saharan Africa – terra incognita
  7. 7. Yala, Kenya 7 Tumbi, Tanzania 85% sand 928 mm 1 rainy season 35% clay 1816 mm 2 rainy seasons
  8. 8. Sand vs. Clay 8 Sand Particles Clay Particles
  9. 9. How do we maximize productivity and minimize nutrient loss... ...if we increase fertilization rates? ...in different soil types? 9
  10. 10. 6 fertilizer treatments • 0, 50, 75, 100, 150, and 200 kg N/ha applied by hand • Replicated in 4 blocks Fertilizer application split between two events • 1/3 at planting (DAP) • 2/3 at topdressing (urea) Experimental Design
  11. 11. Organic treatment 11 N-fixer: Gliricidia sepium 75 kg N/ha/yr
  12. 12. Nitrogen losses 12
  13. 13. Calibrate solute transport model 13
  14. 14. Daily downward nitrate flux - Kenya 14
  15. 15. Nitrogen losses - leaching 15 200 cm 960 mm rain Apr-June 500 mm rain Apr-June
  16. 16. What about yields? 16
  17. 17. What about yields? 17 Average maize yields in Africa are 1 ton per hectare 16 bushels per acre Average maize yields in USA & MD are 11 tons per hectare 170 bushels per acre
  18. 18. Kenyan maize yields 18 960 mm 520 mm 300 mm
  19. 19. Tanzanian maize yields 19
  20. 20. How do we maximize productivity and minimize nutrient loss? 20
  21. 21. Nutrient balances can help us manage maize more efficiently 21
  22. 22. Kenya N balance 2012 22 Treatment (kg N/ha) N leaching (kg/ha/seas on) N2O+NO (kg/ha/seas on) Maize N (kg/ha) N Balance (kg/ha) 0 11.0 0.3 75 -86 50 2.2 0.3 102 -54 75 7.1 0.4 95 -28 200 9.4 0.9 122 +68 Over 90 percent of the N is exported in the harvest
  23. 23. Kenya N balance 2012 23 Treatment (kg N/ha) N leaching (kg/ha/seas on) N2O+NO (kg/ha/seas on) Maize N (kg/ha) N Balance (kg/ha) 0 11.0 0.3 75 -86 50 2.2 0.3 102 -54 75 7.1 0.4 95 -28 200 9.4 0.9 122 +68 75 kg N maximized yields and minimized losses
  24. 24. Tanzania N balance 2013 24 Treatment (kg N ha-1) N leaching (kg/ha/seas on) N2O+NO (kg/ha/seas on) Maize N (kg/ha) N Balance (kg/ha) 0 57.3 2.7 15 -73 50 74.2 4.7 44 -70 75-org 13.0 3.8 36 +25 75-min 80.0 4.5 47 -54 200 119.7 5.1 48 +31 Over 70 percent of the N is lost the plots receiving mineral fertilizer
  25. 25. Tanzania N balance 2013 25 Treatment (kg N ha-1) N leaching (kg/ha/seas on) N2O+NO (kg/ha/seas on) Maize N (kg/ha) N Balance (kg/ha) 0 57.3 2.7 15 -73 50 74.2 4.7 44 -70 75-org 13.0 3.8 36 +25 75-min 80.0 4.5 47 -54 200 119.7 5.1 48 +31 Only 30 percent of N is lost in the organic plots Organic maximized yields while minimizing loss.
  26. 26. Soil organic matter(s)! 26 FAO 1999
  27. 27. Nitrate-N loading in sandy soils 27
  28. 28. Yala, Kenya 28 Tumbi, Tanzania Remember, different soil types
  29. 29. 29 Remember, different soil types… means different solutions.
  30. 30. How do we maximize productivity and minimize nutrient loss? 30 • Kenya • 75 kg N with low N losses and high yields • Long term? • Tanzania • High N losses at low fertilizer levels • Add organics to reduce N losses
  31. 31. Acknowledgements The Earth Institute at Columbia University National Science Foundation Gates Foundation The Marine Biological Laboratory 31 Kenya Wilson Ondiala Samuel Jato Stephen Ogendo Herine Okoth Jessica Masira Tanzania Kassim Masibuka Ismail Rashid Peter Matata Gilbert Temba Gerson Nyadzi Laboratory Madeline McKenna Aja Ould Rich McHorney
  32. 32. 32 Questions? Maswali?
  33. 33. Where does the N go? 33 KE - 0N Yr1 KE - 0N Yr2 KE - 200N Yr2 KE - 0N Yr3 KE - 200N Yr3 TZ - 0N Yr0 TZ - 0N Yr2 TZ - 75N (org) Yr2 TZ - 75N (min) Yr2 TZ - 200N Yr2
  34. 34. Field method validation 34 n = 160 Tully and Weil (in press) - Comm Soil Sci Plant Anal
  35. 35. Nitrogen losses - leaching 35 15 cm 200 cm 120 cm
  36. 36. 36 “Birch Effect” Nitrate at 15 cm in Kenyan Clay
  37. 37. 37 Nitrate at 50 cm in Tanzanian Sand
  38. 38. Tanzania: NO3-N exceeds MCL at 200 cm 38 Nitrate at 200 cm in Tanzanian Sand
  39. 39. Nitrate at 200 cm in Kenyan Clay 39 Nitrate at 200 cm in Kenyan Clay
  40. 40. What about productivity? 40
  41. 41. Maize yields plateau 41
  42. 42. Nitrogen in gas 42 Static chambers N2O Chemiluminescent analyzer NO “Nox Box”
  43. 43. Kenya Fertilizer-N2O responses differ in sands and clays Threshold behavior? Hickman unpublished data 5 4 3 2 1 0 MeanN2Oflux(gN/ha/day) Fertilizer N (kg/ha) 0 50 100 150 Tanzania2012 N2Oflux(gN/ha/day) 100 75 50 25 0 Fertilizer N (kg/ha) 0 50 100 150 200200 2012 43 Hickman et al. (in review) Ecol Letters Post-topdressing

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