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| 04/23/15

Plants won’t soak up as much carbon as we’d hoped


Climate models take into account large-scale uptake by plants of the carbon dioxide (CO2) that we pump into the atmosphere when we burn fossil fuels. Indeed, plants are one of our most powerful weapons in the battle against climate change. But fresh research suggests that due to insufficient availability of nutrients from natural sources, plants are not going to grow as much as expected in a higher-CO2 atmosphere, meaning a lot more work will be needed to cut emissions. Plants need nitrogen to produce chlorophyll—the pigment that allows plants to absorb energy from light—and phosphorus for the development of new tissue. If there is a lot more CO2 available, plants will grow faster, but only if there is sufficient nitrogen and phosphorus at hand too.

Despite this, only two future warming models in the latest Intergovernmental Panel on Climate Change report incorporated possible limitations on the amount of nitrogen that will be available to plants, and none considered limits on phosphorus. But when the researchers behind the paper out this week in Nature Geoscience did so, they found plants won’t grow any where near as much as had been assumed—25 percent less growth overall in fact. That translates as much less ‘breathing in’ of CO2 from fossil fuels. Worse still, their calculations suggest that by 2100, the land will switch from being a net absorber of CO2 to being a net emitter. This is because in a warming world, soil microbes, which help fix nutrients in most plants, will increase how fast they exhale CO2. This has “profound policy and social implications”, write the researchers, as our models will have to be adjusted accordingly, and we either have to significantly up our emissions-reduction ambition, or start to think about artificially fertilizing plants—feeding them more nitrogen and phosphorus than they get naturally.

Here in British Columbia, researchers have looked at aspects of this latter option. Temperate forests and taiga—or boreal forests—are usually nitrogen limited because they are tend to be far away from nitrogen-rich air pollution, and soils are naturally nitrogen deficient as well. As a result, nitrogen fertilization is a common forest management practice in the timber industry. An investigation from 2010 in the journal Global Change Biology looked into how successful such practices are in boosting carbon sequestration for coastal Douglas fir forest in BC. They found that carbon uptake increases significantly, with potential consequences for climate mitigation efforts, although other forest types show less success and at very high levels of nitrogen input, the process actually reduces carbon uptake. Relatedly, some artificially added nitrogen can be converted to nitrous oxide by bacteria in the soils. Nitrous oxide is a powerful greenhouse gas in its own right, some 300 times more powerful than CO2 , and any even inadvertent production of the gas from forest-fertilzation schemes would be climatically problematic.

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