COS 64-3
The effects of African Green Revolution on nitrogen losses from two contrasting soil types in sub-Saharan Africa

Wednesday, August 7, 2013: 8:40 AM
M100HC, Minneapolis Convention Center
Katherine L. Tully, The Earth Institute, Columbia University, NY
Jonathan Hickman, The Earth Institute at Columbia University, NY
Cheryl Palm, Agriculture and Food Security Center, The Earth Institute, Columbia University, Palisades, NY

Nearly 80% of countries in sub-Saharan Africa (SSA) face problems of nitrogen (N) scarcity, which in tandem with poverty causes food insecurity and malnutrition. The Alliance for a Green Revolution in Africa has set a goal of increasing fertilizer use in the region six-fold by 2015. While this will improve crop productivity, if not well managed, fertilizer losses may have important environmental impacts. It is well known that the addition of N to agricultural systems can increase concentrations of mobile nitrate (NO3-) in ground and surface waters. However, it is not clear what the magnitude of impacts will be in SSA given historically low nutrient additions of less than 5 kg N/ha/yr, highly degraded soils due to years of nutrient and soil organic matter depletion, and a wide range of soil types on which increased fertilizer use is occurring.  Experimental maize plots were established in a randomized complete block design in both western Kenya (clayey soil) and central Tanzania (sandy soil).  Plots were amended with 0, 50, 75, and 200 kg N/ha/yr as mineral fertilizer.  Tension lysimeters were installed at three depths in each treatment, and water was collected throughout the maize growing season. Soil water solutions were analyzed for NO3-.   


Soil texture was a major driver of N losses, altering both the pathways and magnitude of losses.  Clayey soils in western Kenya show an enormous potential for loss of NO3- ( 100 mg NO3--N/L) immediately following the onset of rains as they trigger high rates of N mineralization and nitrification in the topsoil (known as the “Birch effect”).  We did not observe this pulse in the sandy soils of central Tanzania. However, NO3- concentrations in leachate were three times lower at 200cm in clayey soils compared to sandy soils as a result of higher anion exchange capacity in clays.  We show that while clayey soils lose NO3- in a large pulse at the onset of rains, sandy soils lose large quantities of NO3- over the course of the maize growing season.