Effectiveness of direct application of top dressing with spent coffee grounds for soil improvement and weed control in wheat-soybean double cropping system

With increasing annual global coffee consumption, the amount of spent coffee grounds (SCGs) increases. Direct application of SCGs in large agricultural fields can potentially improve soil and control weeds in an environmentally safe manner. However, except for composting, the use of SCGs during cropping has not been implemented thus far because of inhibitory effects on crop growth observed in pot-based studies. We evaluated the effect of a top dressing approach to direct SCG application on crop biomass, soil improvement, and weed control, through double cropping field experiments using wheat and soybean. The field experiments were conducted over six successive cropping seasons in an upland field converted from paddy, and crop and weed biomass as well as soil total carbon and nitrogen were investigated. To avoid growth inhibition, the SCGs were surface broadcasted after crop germination to mimic living mulch. The soil total carbon and nitrogen contents increased significantly under an SCG concentration of 5 kg m(-2) or more, approximately 20 months after the first application, whereas SCG application did not significantly affect crop yield except for the first cropping of wheat. In addition, the 10 kg m(-2) SCG application reduced the weed biomass by 50% or more during cropping, except for the wheat cropping in the second year. We concluded that top dressing with SCGs after crop germination is an efficient method for sustainable agricultural production, although further detailed studies of SCGs' effect on the crop growth and soil characteristics are required.