Plant-fungal partnership key for plant responses to increased atmospheric CO2

Date: 1 July 2016

Introduction: New research shows that the stimulatory effect of atmospheric CO2 on plants depends on both soil nitrogen and on the fungi that can help the plants to acquire the nitrogen.

It is common knowledge in horticulture that pumping extra carbon dioxide into a greenhouse stimulates plant growth, but there has been great debate about whether people pumping carbon dioxide into the atmosphere through burning fossil fuels also stimulates plant growth around the planet. Some hope that such a “CO2 fertilization effect” will increase the productivity of farms and forests. Indeed, recent research suggests that the earth has become greener over the past several decades, primarily as a result of increases in atmospheric CO2 concentrations. However, as most scientists are quick to point out, the greening of the earth is not occurring everywhere, and critical questions about the persistence of CO2 fertilization effects remain unanswered.

Experiments demonstrate that some plants sustain positive growth responses to elevated CO2, but many others do not. Such divergent responses fuel debate about the role of plants in slowing climate change as atmospheric CO2 concentrations rise. In a paper published this week in Science (HYPERLINK), Sara Vicca (Global Change Ecology Excellence Centre, Research Group Plant Ecology) and colleagues from Imperial College and two collaborating institutions have begun to answer this decades-old question. The team synthesized 83 experimental studies from across the globe that increased CO2 concentrations to about 650 ppm (currently we are at 400 ppm). They show that plants do grow more with extra CO2 if there is plenty of available soil nitrogen. But they also showed that some plants can respond even when native soil N availability is low. However, the plants then need the right microbial partners to help them access the nitrogen.

Mycorrhizal symbiosis

These microbial partners are the mycorrhizal fungi. The fungi provide their host plant with nutrients and water and receive carbohydrates in return: a symbiosis. Mycorrhizal fungi are more effective in taking up nutrients than plant roots, owing to their ability to explore a greater volume of soil and to produce enzymes that release nutrients from soil.

But not all mycorrhizae are the same. Arbuscular mycorrhizal fungi specialize in taking up phosphorus (P) from the soil, but are limited in their capacity to take up nitrogen (N). Ectomycorrhizal fungi, in contrast, are limited in their ability to take up P from the soil, but are especially effective in taking up N. Most herbaceous species show a partnership with arbuscular mycorrhizal fungi, while the majority of needle-leaved trees are associated with ectomycorrhizal fungi. Among deciduous trees, some, like maple and cherry, live in symbiosis with arbuscular mycorrhizal fungi, while others, like beech and oak, live with ectomycorrhizal fungi.

“This species-specificity of the symbiosis makes it easy to determine whether an ecosystem is dominated by ecto- or by arbuscular mycorrhizal fungi” says lead author César Terrer-Moreno. “By categorizing ecosystems based on their type of dominant mycorrhizal fungi, we were able to determine that mycorrhizal type influences the magnitude of the CO2 fertilization effect”.

Nitrogen availability

In most temperate and boreal ecosystems, nitrogen (N) is the primary element limiting plant growth. Without enough N, it is thought that plants will be unable to respond to rising CO2. Sara Vicca: “Still, some N-limited ecosystems have shown a significantly positive growth response to elevated CO2, which has puzzled scientists for many years. Our study demonstrates that the CO2 fertilization effect increased plant biomass by about 30% in ecosystems dominated by ectomycorrhizal fungi, but had little to no effect in ecosystems dominated by arbuscular mycorrhizal fungi, where soil N availability was low.” At high N availability, both groups responded positively to elevated CO2. César: “Plants need N to respond to high CO2, whether they find it readily available in the soil, or whether their mycorrhizal partners can help them get it."

The terrestrial carbon sink

Land ecosystems currently absorb about 30% of human CO2 emissions, without which climate change would be happening even faster than it is now. The future of this terrestrial carbon sink will depend on how plants and soil respond to the interplay between different global change factors. This includes elevated CO2 concentrations and enhanced nitrogen inputs (through atmospheric deposition, another result of burning fossil fuels), but also for example higher temperatures and more frequent and severe extreme events like droughts, floods and hurricanes. If we are to correctly quantify and project how land ecosystems feed back to the climate system, and thus determine reliable global warming thresholds (avoiding ‘dangerous’ climate change), taking mycorrhizal fungi into account turns out essential. Fortunately, it is reasonably well known which plants live in symbiosis with arbuscular mycorrhizal fungi, and which associate with ectomycorrhizal fungi, so adding mycorrhizal type to earth system models is a tractable challenge.