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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.