  Consider a free-space settlement with a closed ecosystem. Controlling the
habitat's carbon dioxide level is a nontrivial problem because the atmospheric
carbon buffer per biosphere area is smaller than on Earth. Here we show that
the problem can be solved by burning agricultural waste. Waste biomass is
stored and dried, and burned whenever plant growth has lowered the atmospheric
carbon dioxide level so that replenishment is needed. The method is robust,
low-tech and scalable. The method also leaves the partial pressure of oxygen
unchanged. In the initial growth phase of the biosphere, one can obtain the
carbon dioxide by burning sugar or carbon, which can be sourced from
carbonaceous asteroid materials. This makes it possible to bootstrap the
biosphere without massive biomass imports from Earth.
