Rising CO2 robs crops of protein

New analysis suggests that rising CO2 levels will affect the protein content of major food crops, and indeed this may already be taking place. Experts suggest this change in the composition of the foods we eat could have consequences for human nutrition. Farmers can limit these effects by using more nitrogen-based fertilisers, but these in turn have a high environmental cost. Research investigated the protein content of barley, rice, wheat, soybean and potato crops grown at current CO2 levels and at levels projected for the year 2100.

The findings suggest that increasing atmospheric CO2, at levels likely to be experienced during the 21st century, will reduce the protein content of many staple foods by as much as 14 per cent. This could have a substantial impact on the health of populations in many developing countries, most notably in Bangladesh where over 75 per cent of the diet comes from these crops.

This meta-analysis investigated over 40 studies and reported the effects of high levels of CO2 on five crops (barley, rice, wheat, soybean and potato). All of these crops had lower protein concentrations when grown at the higher CO2 levels projected for the end of the 21st Century. For wheat, barley, rice and potatoes, elevated CO2 cut protein concentration by 10-15 per cent. Soybean showed a smaller reduction of 1.4 per cent. Nitrogen fixing bacteria help ensure that legumes have access to sufficient nitrogen. Nitrogen is the key limiting resource for protein production and this explains why legumes are less strongly affected by elevated CO2 levels than other plant species.

However, CO2 levels were not the only factor that affected the protein content of crops. Both soil nitrogen levels and the ozone concentration in the atmosphere surrounding the crops had an effect on protein levels. In the presence of higher levels of CO2, high ozone concentrations further reduced the protein content of potatoes. In contrast, high ozone combined with high CO2 increased protein levels in soybeans. Increased soil nitrogen levels reduced the negative effect of CO2 on wheat protein levels.

The effects of rising CO2 levels on protein concentrations in our crops this century is likely to be larger for non-legume crops, such as grains and potatoes. Farmers growing wheat and other non-legume crops might also protect their crops against these changes to some extent by increasing the use of nitrogen-based fertilisers. However, nitrogen-based fertilisers have a number of negative ecological impacts that should also be considered.

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