Legacy effects of cropping systems on soil carbon stock, millet yield and NPK fertilizer efficiency in Sahel
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Many efforts have been made to set up fertilizer recommendations for millet crop in Sahelian farms. However, most of them resulted from studies carried out in experimental stations, and their relevance to the diversity of cropping systems developed by farmers over a region is often neglected. This study examines the legacy effects of farmers’cropping systems over a Sahelian village on soil attributes, millet yields, and response to inorganic NPK fertilizer. We hypothesized that variations of past organic amendments and the crop rotations may significantly affect these variables. This was tested with a pairwise treatment with and without NPK fertilizer established during two successive years (2016-2017) throughout a network of 19 farmers’ fields representing five histories of cropping systems. We showed that livestock management resulted in a fertility transfer from homesteads-distant fields (Outfields) towards closest fields (Homefields). The outfields were the less amended and had the lowestr soil fertility and millet yield than the homefields. In 2016, millet response to NPK fertilizer was higher in homefield than in outfield and was associated with higher conversion efficiency of N uptake into grain yield. In outfield, fertilizer response was affected likely impaired by low soil P availability and Striga hermontica infestations. In 2017, due to the carryover of the manure applied in 2016 on soil nutrient availability in homefield, millet yield was close to the potential of the variety, and thewas not further stimulated by NPK fertilizer had significant positive effect on straw yield but not on grain yield. Based on these results, practical recommendations were to use a chemical fertilizer supplying more P than the current formula, and increase crop diversification to reduce Striga hermontica infestations in outfields. In homefields, inorganic fertilizer application should be restricted to the year of manuring. The methodological approach developed in this paper could be used more generally to cope with the challenge of setting up differentiated technical recommendations suitable for the diversity of farmers’ fields in a region.