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

Background/Question/Methods

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^-.   

Results/Conclusions

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.