Excess CO₂ in air retards plants growth

an unprecedented study may force certain countries to do some soul searching. Contrary to what a few policy makers advocate, the study reveals that high levels of atmospheric carbon dioxide (co₂) can retard plant growth. The three-year study, conducted at the Stanford University, California, usa, is raising questions about a long-held assumption: by emitting more co₂, our cars and factories create new sources of plant nutrition.

The study is part of the Jasper Ridge Global Change Project - an experiment designed to demonstrate how a typical California grassland ecosystem will respond to future global environmental changes. Scientists assume that in 100 years from now, atmospheric co₂ levels would double up, temperature would rise by 1.1c, precipitation rate would increase by 50 per cent and nitrogen deposition in soil would intensify.

The scientists mapped out 36 circular plots of land, each about six metres in diameter. Four plots were left untouched - they received no additional water, nitrogen, co₂ or heat. Each of the remaining 32 plots were divided into four equal quadrants, separated by underground partitions to prevent roots from one section invading neighbouring tracts.

In these quadrants, the researchers simulated 16 possible combinations of elevated and normal levels of co₂, water, nitrogen and heat. The most surprising find was that elevated co₂ levels stimulated plant growth only when nitrogen, water and temperature were kept at normal levels. "The three-factor combination of increased temperature, precipitation and nitrogen deposition produced the largest stimulation (84 per cent increase), but adding co₂ reduced this to 40 per cent," the report stated. The mean net plant growth for all treatment combinations with elevated co₂ levels was about 4.9 tonnes per 4,840 square yards, compared to roughly 5.5 tonnes per 4,840 square yards for all treatment combinations in which co₂ levels were kept normal.

Why do elevated co₂ levels suppress plant growth? The researchers are struggling to find the exact reasons. One possibility is that excess carbon in the soil allows microbes to out-compete plants for the limited nutrients available in the soil. The researchers hope their study may prompt some researchers and policymakers to take some action to combat global warming.