Soil texture mediates nitrogen losses from intensifying agriculture in Sub-Saharan Africa
The intensification of agriculture in the temperate zone has fed a growing world population but has caused widespread pollution of the atmosphere and waterways as a result of nitrogen fertilizer losses. As intensive agriculture spreads across the tropics, the interaction between tropical soils, climate, and management may mediate the environmental consequences of intensification in unforeseen ways. This is particularly true in Sub-Saharan Africa, were fertilizer inputs are increasing after millennia of subsistence agriculture on a complex mosaic of tropical soils. We measured the effect of fertilization rate on soil nitrate accumulation under maize crops in Kisumu, Kenya. We hypothesized that anion exchange capacity associated with clays in these soils prevents nitrate leaching despite high fertilizer application. To test this hypothesis, we surveyed 14 maize farms across a gradient of surface soil textures (57% to 86% clay) that varied in fertilizer management (6 high fertilizer, 5 low fertilizer, 2 improved fallow). We extracted 2 cores (0-400 cm) and 2 cores (0-100 cm) from each farm field and performed potassium chloride extractions on soils to determine inorganic nitrogen along the soil profile.
We grouped farms according to surface soil clay content as being either “High”, “Medium” or “Low” (>84%, 72-84%, <72% clay, respectively), and depths were grouped as “Upper”, “Middle”, and “Lower” (0-50, 50-200, and 200-400 cm, respectively). Results of two-way analysis of variance on log transformed soil nitrate concentrations found clay content and depth to be significant sources of variation. Nitrate concentrations of all three clay content groups differed significantly (P<0.0001) with means for High, Medium and Low clay content groups of 3.85 ± 1.21, 2.02 ± 1.13, and 0.68 ± 1.21 µg/g, respectively. The Upper depths of the soil profile were found to have significantly lower (P=0.0005) nitrate concentrations than that of the Middle and Lower depths, with means of 0.98 ± 1.19 µg/g, 2.70 ± 1.19, and 2.16 ± 1.17 µg/g, respectively. We found no significant differences in nitrate concentrations between fertilizer management treatments. These results demonstrate the potential for soil properties in Sub-Saharan Africa to alleviate nitrate contamination to waterways, and highlight that the environmental consequences of intensive agriculture need to be considered in the context of the soils on which that agriculture takes place.