PLANT-RESPONSE TO IRRIGATION WITH WATER ENRICHED WITH CARBON-DIOXIDE

The influence of irrigation with CO2-enriched water on plant development and yield is reviewed. The reason for irrigation with CO2-enriched water was - in most cases - to increase yield. The present evaluation considers results from over a hundred studies performed since the first experiment in 1866. Special emphasis is given to the comparison of 85 experiments made by Mitscherlich in 1910 with 358 irrigation experiments made in the last 80 years. In a statistical analysis of these experiments, the measured plant parameter (often growth and/or gas exchange rates) showed a highly significant mean increase of 2.9 % in plants irrigated with CO2-enriched water as compared with control. Evidence of five mechanisms was found. The subterranean carbon dioxide concentration influences: (a) the rate of nitrification and hence of nitrogen availability; (b) the rate of weathering and pH, and hence the availability of other plant nutrients; (c) the CO2 uptake via roots into the transpiration stream, contributing to the rate of leaf photosynthesis; (d) the hormone levels in the plant; and (e) the rate of pesticide decomposition in soils. After examining the available evidence we found that (a) and (b) in some experiments are important to plant growth, since they change the physiochemical environment of the roots. On the other hand, while (c) could theoretically contribute up to 5% of plant carbon assimilation, it usually contributes less than 1%, while (d) contributes most of the observed effects of CO2-enriched water on plants. In addition, pesticide decomposition in soils can be delayed by supra- or sub-optimal CO2 concentrations.