Stream water nutrient and organic carbon exports from tropical headwater catchments at a soil degradation gradient

Abstract: Carbon and nutrient losses were quantified from four small headwater catchments in western Kenya in the year 2008. They include a forested catchment and three catchments under maize continuously cultivated for 5, 10 and 50 years following forest conversion. The C isotopic composition of dissolved organic C (DOC) in stream discharge suggested that soil organic C (SOC) derived from the original forest rather than OC from maize may have contributed to a large extent to watershed OC losses, even 50 years after the forest was removed. Flow-weighted stream water concentrations of DOC and coarse particulate OC, all N species, total P, K and Na significantly (P < 0.05) increased in streams after forest conversion and long-term cultivation. Solute concentrations increased despite the fact that soil contents decreased and total water flow increased indicating mobilization of C and N, P and K from soil with progressing cultivation. In contrast, Ca and Mg concentrations in stream water did not systematically change after deforestation and cultivation, and may be controlled by geochemical weathering rather than by changing water flow paths or topsoil contents. All OC and nutrient exports increased with longer cultivation over decadal time scales (P < 0.05) to the same or greater extent than through deforestation and the first years of cultivation. Fluvial OC and total N losses were 2 and 21 % of total SOC and total N decline, respectively, in the top 0.1 m over 50 years. Fluvial OC losses therefore played a minor role, and SOC losses were mainly a result of microbial mineralization. Resulting total N losses by stream discharge, however, were large with 31 kg ha−1 year−1 after 50 years of continuous cropping in comparison to fertilization of 40 kg N ha−1 year−1. Most (91 %) of the N losses occurred as NO3 . In contrast, P losses by stream discharge were negligible in comparison to plant uptake. Water losses should be managed to reduce soil fertility declines especially through large N export from agricultural headwater catchments. However, stream concentrations of both P (0.01–0.15 mg L−1) and N (0.4–4.8 mg L−1) were moderate or low with respect to possible consequence for human health and not responsible for eutrophication observed in Lake Victoria.