Nutrient balances in two agricultural watersheds in southern Malawi

Malawi soils are highly depleted resulting in severe food shortages. The study investigates nutrient fluxes into and from small agricultural watersheds in Southern Malawi. Nutrient balances for N, P and K were calculated in two watersheds (Matipa and Kawanula). Data were obtained on the use of mineral fertilizers, animal manure and crop residues using household interviews. Biological N fixation was estimated from measured N uptake, while stream flow losses were calculated from sediment and soluble nutrient concentrations measured in streams. Wet atmospheric deposition and gaseous losses were estimated based on data from the study. Estimated annual nutrient balances were 45 N, 16 P and -1 K kg/ha/year for Matipa and 25 N, 5 P and -5 K kg/ha/year for Kawanula. The positive nutrient balances for N and P were attributed to nutrient importation through mineral fertilizers outstripping nutrient export. The comparatively low N losses through stream flow for Matipa and Kawanula (10-15 kg/ha/year) based on discharge, supported the hypothesis that a significant portion of eroded and leached nutrients are redistributed in lower slopes rather than being entirely exported from the watershed. Using an alternative farm balance approach, erosion estimates of 35 kg N/ha/year for each watershed and leaching losses of 35-42 kg N/ha/year and negative N balances of -32 and -46 kg N/ha/year for Matipa and Kawanula. The watershed approach may provide opportunities for farmers to judiciously manipulate nutrient flows to achieve a positive nutrient balance.

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