Rising atmospheric carbon dioxide is limiting the amount of nitrogen available to boreal forests, an analysis of stored tree cores in Sweden shows. The findings suggest that as humanity’s carbon footprint continues to rise, trees will have a harder time competing with microbes for nitrogen, slowing forest growth and reducing their importance as carbon sinks.

An important negative feedback in models of anthropogenic climate change is that when carbon dioxide increases in the atmosphere, photosynthetic organisms can absorb carbon dioxide more easily and therefore grow faster. However, other nutrients such as phosphorus and nitrogen are required to do this, and the extent to which these limit faster growth is debated.

Many tree species obtain nitrogen in part through symbiotic interactions between their roots and soil fungi called mycorrhizae, which break down organic matter. As trees are unable to absorb nitrogen from other sources, they become increasingly dependent on this fungal nitrogen source. The fungi cause isotopic fractionation, preferentially retaining the rarer nitrogen-15 isotope and passing nitrogen-14 to the tree. Nitrogen limitation can therefore be detected as a relatively reduced proportion of nitrogen-15 in the wood.

This has been previously observed in several large continental or global studies. However, the causes of this nitrogen limitation are obscured by the fact that multiple human activities, such as fertilizer use and fossil fuel combustion, cause nitrogen pollution. Therefore, it is difficult to distinguish between impacts resulting from recent declines in anthropogenic nitrogen pollution and those resulting from anthropogenic carbon pollution, which is still increasing.

ecosystem ecologist kelly bassettPhD students in Forest Ecology and Management at the Swedish University of Agriculture and their colleagues studied samples of Norway spruce and Scots pine trees collected from across Sweden between 1961 and 2018. “There are millions of samples sitting in archives, just waiting to be discovered,” Bassett says. “The fact that someone had the foresight to keep these samples 60 years ago is truly amazing.” While carbon dioxide in the atmosphere is well mixed and increasing globally, pollution from reactive nitrogen species increased in the more densely populated, highly industrialized areas of southern Sweden and has since declined significantly, but little has changed further north.

Nationally, the strongest influence on nitrogen isotope ratios in both species of wood was atmospheric carbon dioxide levels. These data, along with previous findings that forest growth has apparently leveled off in some parts of Sweden, suggest that nitrogen limitation in boreal forests will become increasingly important as carbon dioxide levels continue to rise, Bassett said. “Plants are in direct competition with organisms in the soil, which are generally much more efficient. …Trees can only use their roots and mycorrhizae to scale up nitrogen extraction, but organisms are present everywhere in the soil, taking up and fixing nitrogen,” she says. Although they see no reason why the results shouldn’t be similar in other forest ecosystems, they say, “One of the things that might come out of publishing this study is that we hope that there are other archives out there.”

ecosystem ecologist andrew elmore He was impressed at the University of California, Merced. “I think they’ve really put an end to the idea that nitrogen deposition can have a similar effect on the nitrogen cycle,” he says. “We’re basically saying that it doesn’t matter in this landscape.” He says an interesting extension of this work would be to study the effects of increased nitrogen fixation on other species in the forest. “Decreasing leaf nitrogen can, for example, reduce the growth rate of caterpillars and reduce the foraging success of birds. So doing this could have a number of effects that affect the things we are interested in.”