Leaching losses from Kenyan maize cropland receiving different rates of nitrogen fertilizer

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.