I'm Quitting Heroing manga volume 6 features story by Quantum and art by Nori Kazato and Hana Amano. 

The Hero Faces a New Formidable Foe—Environmental Degradation! The Demon Queen and her trusty aide, former hero Leo, have returned to the demon realm…only to find a barren, polluted wasteland on the verge of destruction. The crops aren't growing, the water is contaminated, and the realm's demonic citizens are fed up. To prevent things from getting even worse, Leo must roll up his sleeves and start working to solve this environmental crisis! And as he toils away in the fields, the story of another Demon Heart sibling comes to light…