Fertilizer microdosing enhances maize yields but may exacerbate nutrient mining in maize cropping systems in northern Benin

Fertilizer microdosing is a promising technology to complement traditional fertility management strategies, yet little is known regarding its performance in maize systems in western Africa. This study assessed to what extent the application of fertilizer microdosing to maize in northern Benin may contribute towards improved yields, greater nutrient use efficiency and reduced nutrient mining, in combination or not with various manure management practices. In a 2-year on-station experiment at Ina (northern Benin), four fertilizer options were tested [no fertilizer control, microdosing options 1 (MD1, 23.8 kg N ha−1, 4.1 kg P ha−1 and 7.8 kg K ha−1) and 2 (MD2, 33.1 kg N ha−1, 8.2 kg P ha−1 and 15.6 kg K ha−1), broadcast fertilizer at recommended rate (RR, 76 kg N ha−1, 13.1 kg P ha−1, and 24.9 kg K ha−1)] within five manure strata [manure applied through corralling in the same year (Cor-0) as well as one (Cor-1) and two years (Cor-2) before the experiment, transported manure (TM, 3 t ha−1), and no manure (NM)]. On average across all manure strata and years, fertilizer application significantly increased grain yields by 64% for MD1, 81% for MD2 and 93% for RR compared to the unfertilized control. Yields in MD2 were never different from those in RR. Across the manure strata, there was a general tendency for FUEs to decrease from MD1 (8.0-19.0 kg grain kg−1 fertilizer) to RR (4.6-8.0 kg grain kg−1 fertilizer). Maize response to fertilizer microdosing was best in the absence of organic amendments and tended to decrease with increasing fertility. Indeed, the greatest grain yield increases were observed for the NM (+1611 kg ha−1) and Cor-2 (+1468 kg ha−1) strata, followed by TM (+1258 kg ha−1), Cor-1 (+1183 kg ha−1) and Cor-0 (+1126 kg ha−1). Consequently, FUE was also best in NM plots and lowest in recently corralled plots (Cor-0). Fertilizer-induced yield increases resulted principally from larger numbers of grains per cob (+52% on average) and larger 1000-grain weights (+13%). For the NM and TM strata, the partial nutrient balances for the two cropping seasons ranged between −44 and +21, −24 and −9 and −78 and −45 kg ha−1 year−1 respectively for N, P and K depending on the mineral fertilization treatment. The balances ranged between −17 and +54, −77 and −50 and −345 and −228 kg ha−1 over a three-year corralling cycle, respectively for N, P and K. Except for N applied at the recommended rate, partial nutrient balances were equally or more negative on fertilized plots than on the unfertilized controls, indicating that the tested fertilization options may exacerbate nutrient mining. This was particularly the case for P and K and suggests that microdosing should probably not be used for extended periods. Nevertheless, fertilizer microdosing appears better adapted to the realities of smallholder farmers than the RR while still ensuring very significant yield increases. There is a need to evaluate these nutrient management options in farmer's fields, taking into account climatic, soil and management conditions to better assess and understand the maize response and the magnitude of nutrient mining and to evaluate its economic benefits and risk.