Abstract
Meeting food security requirements in sub-Saharan Africa (SSA) will require increasing fertilizer use to improve crop yields, however excess fertilization can cause environmental and public health problems in surface and groundwater. Determining the threshold of reasonable fertilizer application in SSA requires an understanding of flow dynamics and nutrient transport in under-studied, tropical soils experiencing seasonal rainfall. We estimated leaching flux in Yala, Kenya on a maize field that received from 0 to 200 kg ha−1 of nitrogen (N) fertilizer. Soil pore water concentration measurements during two growing seasons were coupled with results from a numerical fluid flow model to calculate the daily flux of nitrate-nitrogen (NO3−-N). Modeled NO3−-N losses to below 200 cm for 1 year ranged from 40 kg N ha−1 year−1 in the 75 kg N ha−1 year−1 treatment to 81 kg N ha−1 year−1 in the 200 kg N ha−1 treatment. The highest soil pore water NO3−-N concentrations and NO3−-N leaching fluxes occurred on the highest N application plots, however there was a poor correlation between N application rate and NO3−-N leaching for the remaining N application rates. The drought in the second study year resulted in higher pore water NO3−-N concentrations, while NO3−-N leaching was disproportionately smaller than the decrease in precipitation. The lack of a strong correlation between NO3−-N leaching and N application rate, and a large decrease in flux between 120 and 200 cm suggest processes that influence NO3−-N retention in soils below 200 cm will ultimately control NO3−-N leaching at the watershed scale.
Highlights
Hunger and malnutrition persist in many developing countries despite technological advances in agricultural food production and distribution during the last 50 years
Pore water NO3--N concentrations rose quickly at 120 cm following the start of the rains, while concentrations measured at 200 cm tended to rise slowly throughout the growing season
NO3--N concentrations at both depths generally rose through September and October during the beginning of the short rains, likely due to downward transport of remaining applied N fertilizer in the soil, and mineralization of organic matter in the soil followed by nitrification of the applied ammonium
Summary
Hunger and malnutrition persist in many developing countries despite technological advances in agricultural food production and distribution during the last 50 years. Agricultural productivity in sub-Saharan African (SSA) has lagged behind the rest of the world (Hazell and Wood 2008; Monfreda et al 2008) and provides motivation for the African Green Revolution (AGR), a movement that aims to increase food production by combining science, technology, and policy (Annan 2004). While there is substantial evidence that fertilizer application will increase crop yield (Sanchez et al 2007; Denning et al 2009; Nziguheba et al 2010; Snapp et al 2010; Sanchez 2015; Mafongoya and Jiri 2016), excess fertilizer application does not improve yield and can have environmental and public health consequences (Goulding 2000; Ju et al 2009).
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