The composting units they designed to demonstrate this low-tech approach to enriching greenhouse air with CO2 were made of wood and had a height of 1.2 meter and diameter of 0.6 meter. The unit's bottom was constructed of 6-cm-wide wood bars with a 2-cm-wide interval between them that ensured sufficient aeration. The composting material consisted of 25 kg of crop residue (rice straw, in their case) and 8 kg of moist manure, to which mixture 90 liters of water were added initially, after which the mixture was kept at a moisture content of approximately 70%. Last of all, the CRAM mixture was inoculated with three different species of fungi (Panusconclmtw zj3, Trichoderma viride zj2, and Aspergillas niger zj1), in order to increase CO2 production via accelerated fermentation. Then, "to prevent NH3 emission from the compost into the greenhouse," as they describe it, "a sponge layer permeated with 2 M H2SO4 was placed on top of the composting mixture to trap volatizing NH3."

The six scientists tested their technique within three pairs of greenhouses placed over a uniform soil surface, wherein they planted celery (Apium graveolens L.), leaf lettuce (Lactuca virosa L.), stem lettuce (Lactuca saiva L.), oily sowthistle (Sonchus oleraceus L.), and Chinese cabbage (Brassica chinensis L.). One of each of these greenhouse pairs acted as a control, while the other housed one of their low-tech CO2 generators, which they emptied every 21 days onto an adjacent field to provide it with an organic fertilizer over the course of their 63-day vegetable crop growing season.

As best we can determine from Jin et al.'s graphs of hourly CO2 measurements made on specific days, as well as every-day or every-other-day measurements of CO2 made at one specific time of day, mean daylight CO2 concentrations in the CO2-enriched greenhouses were just a little over twice as great as those measured in the control greenhouses. As for the consequences of this slightly more than doubling of the air's CO2 content, they report that "the average percentage of yield increases of all three sites were 270%, 257%, 87%, 140% and 227% for celery, leaf lettuce, stem lettuce, oily sowthistle, and Chinese cabbage, respectively." In addition, they say that the extra CO2 from the composting units increased the concentration of vitamin C in all five species: "by 13%, 39%, 25%, 72% and 37% for celery, leaf lettuce, stem lettuce, oily sowthistle, and Chinese cabbage, respectively."

In concluding their paper, the five Chinese researchers and their Australian collaborator say their "CRAM procedure is an efficient way to elevate CO2 concentrations," and that it "represents a simple, cheap and adoptable management tool for use in greenhouse vegetable production," while its "use of crop residues and animal manures decreases the possible environmental problems caused by burning and [other less-than-desirable] practices of disposal of these agricultural by-products."

Considering these environmental benefits, the hugely increased crop yields, and the increased nutritive value of the edible produce they harvested, Jin et al.'s low-tech approach to enriching greenhouse air with CO2 would appear to be a winning proposition, especially where it may not feasible to employ more sophisticated techniques.

Sherwood, Keith and Craig Idso

Reference
Jin, C., Du, S., Wang, Y., Condon, J., Lin, X. and Zhang, Y. 2009. Carbon dioxide enrichment by composting in greenhouses and its effect on vegetable production. Journal of Plant Nutrition and Soil Science 172: 418-424.