The effects of  enrichment can be most simply attained in a greenhouse (see  for its agricultural use). However, for experimentation, the results obtained in a greenhouse would be doubted due to it introducing too many confounding variables. Open-air chambers have been similarly doubted, with some critiques attributing, e.g., a decline in mineral concentrations found in these -enrichment experiments to constraints put on the root system. The current state-of-the art is the FACE methodology, where  is put out directly in the open field. Even then, there are doubts over whether the results of FACE in one part of the world applies to another.