Nitrogen is a funny thing, and not just because it's a main ingredient in laughing gas. As the gas N2, nitrogen—which is critical to life, forming protein building blocks—makes up four-fifths of the molecules in the air around us. But N2 is useless to most crops, which have no way of drinking from the nitrogen consommé. The plants rely on soil microbes to blend the nitrogen into larger molecules, like nitrate, that they can suck up through roots and turn into plant meat.
Plants also need carbon, which they suck right out of the air. The burning of fossil fuels and felling of forests has pushed carbon dioxide levels in the atmosphere up by more than a third so far. All that extra carbon dioxide is warming the planet, and it's also helping to green it by stimulating plant growth. Which sounds like a blessing for farmers—a rare blip of positivity amid terrifying climate upheaval.
But climate change is truly a depraved beast.
"Because nitrate is an important nitrogen source in most agricultural soils, elevated CO2 will generally have a negative effect on food security."
Even as crops grow more quickly, at least initially, following a CO2 bump, they tend to become less nutritious.
"Plants reliant on nitrate as a nitrogen source become protein deprived," says the University of California-Davis' Arnold Bloom. "Because nitrate is an important nitrogen source in most agricultural soils, elevated CO2 will generally have a negative effect on food security."
Bloom led a team of scientists to a discovery that helps explain this unfortunate phenomenon, which is known as CO2 acclimation. The breakthrough could help agricultural scientists breed new crop varieties to feed the world's population—even as carbon levels rise.
To figure out how carbon dioxide might affect crops of the future, Bloom exhumed an experiment from the past. The team studied wheat crops that were grown with varying nitrate and carbon dioxide levels in Arizona test fields in the 1990s. New research techniques measured the concentrations of various forms of nitrogen in the long-frozen samples. The scientists found similar overall nitrogen levels in plants grown in high and low CO2 levels. But the high-CO2 plants contained a higher ratio of nitrate to other forms of nitrogen.
That suggests that these CO2-saturated crops were perfectly capable of drawing nitrate up from the ground through their roots, but that they weren't so good at actually incorporating the nitrogen into their cells to form useful proteins.
"NO3− assimilation was slower under elevated CO2," the team writes in a paper published this week in Nature Climate Change. "Breeding crops for enhanced root NO3− and NH4+ assimilation has the potential to compensate ... as atmospheric CO2 rises, but this approach is yet untapped."
With atmospheric carbon dioxide levels expected to reach those that were simulated in the Arizona field trials in the coming decades, now might be a good time to start tapping.