Biogeochemical soil processes may hold key to global warming

Experimental CO2 enrichment of trees reduces their phosphorus uptake from soil

As humanity continues to burn fossil fuels, the delicate balance of carbon in ecosystems is changing. That's true of trees, which are expected to grow faster as a result of increased carbon dioxide (CO2) concentrations in our atmosphere and thereby contribute to reducing the rise of atmospheric CO2. But not all trees are responding in this way. In particular, eucalypts - Australia's iconic forest trees - haven't benefited from the increase in CO2. A new study, recently published in Nature, suggests that the low response of these trees is due to a below-ground struggle for access to phosphorus, a mineral nutrient in soils that is essential for trees to grow. According to the study, in some parts of the world, increased CO2 triggers soil microorganisms to hold on to their phosphorus, making it less available for trees. This reduces the increase of forest growth otherwise expected from increasing CO2, and thus the tree-based contribution to mitigate global warming

This text is based on a press release provided to us by Western Sydney University.

Our study used data from a Western Sydney University experiment known as "Eucalyptus Free Air CO2 Enrichment", or EucFACE, located in a century-old woodland. CO2 is released into the woodland through a computer‐controlled system. Scientists then monitor the effects on trees, soils and the broader ecosystem. Over six years, CO2 was raised to the levels expected around the year 2050, according to the current business-as-usual emissions trajectory.

Our previous studies already found that woodland trees did not show any extra growth at high CO2 levels. In the recent study, we analysed data collected at EucFACE, which sampled phosphorus in all parts of the ecosystem, tracing its journey from the soil to the trees to test whether the low availability of soil phosphorus was the cause. Phosphorus is crucial to the process of photosynthesis, which fixes atmospheric CO2 and makes trees grow. Phosphorus in soil is provided by microorganisms that break down dead and decaying matter, thus changing phosphorus into a form that plants can take up with their roots. Using these new data data, we found that under high-CO2 conditions soil microbes retain more of the phosphorus they produce in order to support their own metabolism. This change in soil functioning left less phosphorus available for trees to take up.

Less phosphorus uptake under high CO2 reduces forest growth response to increasing atmospheric CO2 levels and thus its carbon sink potential

Our study is the first to show how the phosphorus cycle at ecosystem level is affected by high CO2, and in particular to highlight the role of soil microbes. Most Australian soils are naturally low in phosphorus, because they are derived from ancient, nutrient-depleted rocks. The same is true for soils in tropical and subtropical regions. That makes the phosphorus service provided by microbes even more important.

Current climate projections assume that increasing atmospheric CO2 will lead to more forest growth globally. Forests are a vital carbon "sink" since they draw down carbon from the atmosphere, thus mitigating the effects of climate change. However, these results add to a growing body of literature suggesting that, extra forest growth is at least limited and future warming could be higher than current projections made with state-of-the-art Earth system models.

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