I burnt my plants - hotbed trouble

I had successfully used a decomposing wood chip pile to keep my seedlings warm. But I tab out of space and put tomatoes, peppers and eggplant on top of a new pile just heating up with wood chip and asparagus. It had a slightly foul smell. ..
The next morning the plants were brown and wilted. I guess it was the gases. Ah well, lesson learned. Next time I'll make sure I cap the heap with soil and it doesn't go anaerobic on me before I use it as a hotbed.
Below pics of the wood chip hotbed.