I think it's funny that people find this contradicting or ironic. Nature is absolutely everywhere and everything. Not just the green stuff. But I guess nature has many definitions, and most people think of green grass when they hear the word.
I believe this is what people believe in when they think of nature- The phenomena of the physical world collectively, including plants, animals, the landscape, and other features and products of the earth, as opposed to humans or human creations not just green grass. So this wonderful drawing obviously contradicts nature.
Wow, the lines are fantastic. You know, I think you should try and paint this with watercolors with a contrasting color scheme, or if you don't feel to confident, let it on the hands of a great painter?