Predicting nitrogen leaching losses from intensifying agriculture in sub-Saharan Africa: the role of soils and fertilizer type

Abstract:

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. We present data on N leaching losses in the vadose zone 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. We combine measuring soil pore water concentrations from tension lysimeters with a variably saturated hydrologic flow model to estimate N leaching losses under different fertilizer scenarios in two soil types. Vertical N fluxes are given at the soil surface, within, and below the root zone. We find N losses from high-clay soils to be nearly two orders of magnitude lower than low-clay soils likely due to lower fluid flux and higher anion exchange in the high-clay soils. Organic material additions (from leguminous tree prunings) reduced N leaching losses substantially in low-clay soils likely by changing the timing of N-release and by increasing soil organic matter (and thus improving water retention and the formation of soil aggregates). Predicting the fate of added N in SSA is very important as N application is poised to increase in this vulnerable and under-studied region.