Biomass over time of C3 grasses and C4 grasses at ambient and elevated CO2. Total biomass (aboveground + 0 to 20 cm belowground) of plots comprising C3 grasses and C4 grasses in ambient CO2 (red) and elevated CO2 (blue) from 1998 to 2017. Data are shown as moving 3-year averages centered over the middle of each 3-year group. Each point represents data pooled across N treatments, and across monoculture and four-species plots (equally weighted), for each functional group (n= 22 plots for each functional group at each CO2 level). Graphic: Reich, et al., 2018 / Science

By Jeff Tollefson
19 April 2018
(Nature) – Grasslands in warm and dry climates could grow faster as carbon dioxide levels rise, according to data from a long-term ecological field experiment in Minnesota. The finding, which runs counter to long-established ideas about how plants will respond to the greenhouse gas, suggests that grasslands could provide a buffer against climate change.The research, published on 19 April 2018 in Science [pdf], delves into a longstanding question about how Earth’s two major groups of plants will respond to the growing level of CO2 in the atmosphere. The biggest group, known as C3 plants, comprise 97 percent of all plant species. These species make energy through photosynthesis, using sunlight to synthesize sugars from CO2 and water. In theory, giving these plants extra CO2 would rev up their energy production.The other group of plants — so-called C4 species — use a two-step process to boost their internal CO2 levels before photosynthesis takes place, making energy production more efficient. For decades, scientists have thought that C4 plants would not benefit from additional CO2 in the atmosphere because they are already turbo-charged. But the Science paper suggests that the opposite might be true.“The main message is don’t count out the C4 grasslands,” says Dana Blumenthal, an ecologist with the US Department of Agriculture in Fort Collins, Colorado. Because C4 plants evolved to live in hot and arid conditions, scientists have long projected that the species will expand their range as the climate warms. Now, it turns out that they also might pull more CO2 out of the atmosphere. […]It’s not yet clear why this has happened, but scientists note that as CO2levels rose, the amount of nitrogen available to the plants also increased. Nitrogen is an essential nutrient that is crucial to photosynthesis. One possibility is that changes in the composition of soil microbes drove the increase in nitrogen. “It’s a huge surprise,” says Peter Reich, an ecologist at the University of Minnesota in Saint Paul who heads the experiment. “I don’t think any scientist in the world would have predicted it.” […]“The lesson is that photosynthesis doesn’t equal growth,” says Richard Norby, an ecologist at the US Department of Energy’s Oak Ridge National Laboratory in Tennessee. He says that if scientists want to understand how an ecosystem’s plants will respond to increased CO2, they need to look at how nutrient cycles change over time. “You can’t get at that with short experiments.” [more]

Grassland plants show surprising appetite for carbon dioxide

ABSTRACT: Theory predicts and evidence shows that plant species that use the C4 photosynthetic pathway (C4 species) are less responsive to elevated carbon dioxide (eCO2) than species that use only the C3 pathway (C3 species). We document a reversal from this expected C3-C4contrast. Over the first 12 years of a 20-year free-air CO2 enrichment experiment with 88 C3 or C4 grassland plots, we found that biomass was markedly enhanced at eCO2 relative to ambient CO2 in C3 but not C4 plots, as expected. During the subsequent 8 years, the pattern reversed: Biomass was markedly enhanced at eCO2 relative to ambient CO2 in C4 but not C3plots. Soil net nitrogen mineralization rates, an index of soil nitrogen supply, exhibited a similar shift: eCO2 first enhanced but later depressed rates in C3 plots, with the opposite true in C4plots, partially explaining the reversal of the eCO2 biomass response. These findings challenge the current C3-C4 eCO2 paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.

Unexpected reversal of C3 versus C4 grass response to elevated CO2 during a 20-year field experiment