Co-composting to close the cycle of resources during rose cultivation in Kenya: An agronomic and pesticide residue assessment

Abstract

Recycling green waste through composting is a promising practice for the transition towards a bio-based circular economy in the floricultural sector of Africa, especially for Kenya where cut flower export accounts for nearly 14 % of its total export value in 2017. Rose waste is a large waste stream, but its intrinsic properties make it challenging to recycle. Composting on commercial scale was studied on a rose farm near Lake Naivasha, (Kenya). Three mixtures were examined: (1) rose waste (RW) only, (2) 80 % RW + 20 % tomato waste and (3) 90 % RW + 10 % mature rose compost. Trapezoidal piles of approximately 4000 kg green waste were composted following the turned windrow approach, samples were taken at six occasions. The nine-month composting study, including pesticide fate assessment, showed consistent performance across tested mixtures. All mixtures resulted in mature and stable compost with C/N ratios below 10 and a high fertilizing potential, meeting international sanitation requirements. Final average volume reduction was 82 %, total N values ranged between 8.1 and 8.9 mg g⁻¹ compost and pH values were alkaline (8.0–8.3). Out of the approximately 50 pesticides commonly used in rose cultivation only 8–12 pesticides could be detected in the matured composts with the highest contribution of flubendiamide and fluopyram. Scenario analysis showed the feasibility of closing the resource cycle in the African floricultural sector via continuous crop rotation over eight years with an amendment rate of 11.5 kg per m². Overall, this study provided straightforward implementable practices for rose waste management, which facilitates the re-use of valuable green waste in Africa and thereby contributes to the transition towards a global circular economy.