Terra preta is formed naturally by falling fruits, leaves and branches that get composted on the soil. There's a agriculture method called agroforestry that make use of this concept, it's very promising for organic farms: https://vimeo.com/146953911

I know a couple of people here who have "dry bathrooms" (or banheiro seco) that is just a bucket to collect the poo, when you poo you cover it with dry leaves or grinded wood (they are the source of carbon, and stop the bad smell), when the bucket fills up they take it to an area outside far from the house where it will compost, naturally kill the pathogens (because of the heat it produces) and became terra-preta, much better solution than to send it down stream, easier to decentralize and the end result is the best product you could have.