Q.Bad Smell In Backyard
I have been noticing an awful odor coming from my backyard, near the fence next to my neighbor and a big tree. I don’t know what’s causing it or how to stop it. It doesn’t seem to smell like a dead body or sewage; I can’t identify the smell but it’s getting worse. I’ve lived her 30 years and never experienced this before.

Certified GKH Gardening Expert
You need to go out there and investigate closely if you haven't already. Examine the tree and its base for any type of fungal growths. Sometimes those can have an odor. If it's not a dead animal or a broken water or sewage pipe, it could be a compost heap if a neighbor has one. Here is more:
https://www.gardeningknowhow.com/ornamental/fungus-lichen/removing-stinkhorn-fungi.htm
https://www.gardeningknowhow.com/composting/basics/managing-compost-odors.htm
https://www.gardeningknowhow.com/composting/basics/compost-smells-bad.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/ammonia-odors-in-garden.htm