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Forecasting climate change is like assembling a jigsaw puzzle when you don’t know its overall pattern. Sometimes you find a piece you didn’t even know was missing. A discovery that arid deserts may be soaking up a lot of carbon dioxide is a case in point. And other times you see a puzzle piece in a helpful new perspective. That has just happened in a study of how nitrogen fixation affects the CO2-absorbing capacity of forests.

The latter takes some explaining. It involves subtleties of plant and soil chemistry that scientists are just beginning to appreciate. Plants need nitrogen to grow. That nitrogen has to be in a chemical form, such as ammonia, that plants can process. Some plants have made a deal with different kinds of bacteria that can take nitrogen out of the air and turn it into ammonia. The bacteria get a home on the plant roots. In exchange, the plants get the nitrogen fertilizer. This relationship is called nitrogen fixation. Legumes do it. So do some trees.

Nitrogen-fixing plants have an advantage over other plants in nitrogen-poor soils. The problem is that nitrogen fixation is a “gas guzzler” – plants that use it burn a lot of energy. That’s a fair trade-off in poor soil, but it’s a disadvantage in soils already nitrogen rich. In lakes and oceans, nitrogen-fixers are less abundant in nitrogen-poor soils than in rich circumstances. But in forests, it’s the opposite. Nitrogen-poor temperate forests are short on nitrogen-fixing trees. Nitrogen-rich tropical forests have them in abundance.

Benjamin Houlton at the University of California at Davis says that, finally, “we asked why.” Last week, he and several colleagues published their answer in the online edition of Nature. It turns out that forest trees are acting like commuters at the gas pump. They are weighing the high energy cost of nitrogen fixation against its benefits.

It takes more energy to fix nitrogen in cool temperate forests than in warmer soils. The gain in fertility isn’t worth the higher energy cost. Tropical forests have plenty of available nitrogen but are short on phosphorus, which plants also need. “The extra nitrogen added to the soil by nitrogen-fixers helps mobilize the phosphorus, making it easier for roots to absorb,” Dr. Houlton explains. Here the benefit is worth the energy cost.

Factoring this new understanding of nitrogen fixation into global estimates of forest productivity will sharpen estimates of how much CO2 forests can remove from the atmosphere.

Climate scientists are also taking a new look at the CO2-absorbing capacity of deserts. A review of this research by the journal Science two weeks ago reported the counterintuitive discovery that desert soils soak up CO2 at night. On a global basis, the absorption may amount to half the CO2 from burning fossil fuels. This is only a hopeful suggestion at this stage of the research. Scientists don’t know how the absorption works, where the absorbed carbon is stored, or whether it is eventually released back into the atmosphere with no net gain in CO2 sequestration.

One conclusion is clear. Scientists cannot completely assemble the CO2 part of the climate-forecast puzzle until they figure out all the pieces that relate to the interaction of plant and soil chemistry.

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