A groundbreaking 20 year study into the impact of rising carbon dioxide on important grazing grasses could change way we think about pasture management.
Until now, the accepted theory said the C3 type of plants grow more under elevated levels of atmospheric CO2 while C4 plants do not respond with elevated growth.
But new research shows that after 10 years under eleveated CO2, the roles are reversed and C4 grasses are “increasingly enhanced” while the growth rate of C3 plants drops off.
The impact of elevated CO2 on plant growth is a critical question.
According to NASA atmospheric CO2 is at its highest level for the past 400,000 years and emissions are forecast to rise over the next century.
C3 and C4 grass species are both present in most pastures across the continent. C3 are more prevalent in cooler grazing reginos, while C4 grasses dominate the mix in tropical and subtropical climates.
In western NSW for example the pasture composition is split evenly between C3 and C4 and even in Tasmania, there are significant C4 grazing plants such as kangaroo grass.
But before we delve into what this breakthrough means for pastures we’ll need to take a quick look at a few fundamentals of plant biology.
It’s a bit technical but stick with it, we may be witnessing the dawn of a new era for pasture management.
The breakthrough came from one of the few long-term projects investigating plant response to increased levels of atmospheric CO2, which was a 20 year experiment by the University of Minnesota.
The fickle nature of publicly-funded science means most research is conducted under three year project cycles.
Carbon fixation is the way plants extract carbon from the CO2 in the atmosphere and convert it into sugars and other organic matter. The most recently evolved mechanism is C4 photosynthesis, which is more energy efficient than the ‘ancestral’ C3 photosynthesis.
“The ancestral method of photosynthesis (C3) combines CO2 with a five-carbon molecule to produce two identical three-carbon molecules,” the article said.
“However, the enzyme that catalyzes this reaction also combines the same five carbon compound with dioxygen, thus reducing the carbon assimilation rate and, hence, growth.
“Hatch and Slack (who first described the C4 mechanism) discovered that some plants can avoid this by first combining CO2 from the atmosphere with a three-carbon molecule, producing a four-carbon molecule as the first stable product in photosynthesis.”
University of Tasmania plant ecologist, Associate Professor Mark Hovenden, provided a technical investigation of the ramifications from the University of Minnesota’s research in a recent article which was co-authored by New Zealand’s AgResearch scientist Paul Newton.
Prof Hovendeon’s team focus on manipulative field experiments or “work in the paddock” as he puts it, providing background information and explanations of why the C3 growth declines.
Prof Hovenden said improved pastures, which are typically dominated by a C3 plant in perennial ryegrass, would be particularly impacted by the reduced long-term growth rate under rising CO2.
Nitrogen Fixation is one of the major underlying reasons C3 grass growth slows under elevated CO2.
For ten years or so C3 grasses increase growth under rising atmospheric carbon.
But the process of returning Nitrogen, another building block of plant growth, to the soil breaks down over time.
“I found in my experiments C3 plants end up with slower rate of decomposition of organic matter that goes back into the soil,” Prof Hovenden said.
“As CO2 goes up, less of those nutrients are released in a form that’s available to plants. That’s definitely nitrogen but we think it’s potentially phosphorus.”
Conversely, C4 plants actually improved their return of nutrients to the soil over time.
Producers relying on improved pastures, like dairy farmers, could need to increase their fertiliser application to maintain pasture growth.
Prof Hovenden also noted that differing temperature responses could see a shift in pasture production to later in the season, with C4 growing more in summer and C3 doing better in spring.
Another significant factor was C4 grasses tend to be less nutritious “which has implications for anything that lives off them”, including native and agricultural grazing species, Prof Hovenden said.
Big picture is complex
Current research is looking at how elevated CO2 interacts with changes in temperature and whether, as predicted under climate change, an increase of two degrees or so changes plant responses, Prof Hovenden said.
Crops such as rice and wheat are C3 plants, but rising CO2 may not impact them as profoundly it does grasses. Crops are managed more intensely.
There is greater capacity for plant breeding research and fertiliser inputs to improve growth.
Climate Council member and Australian National University professor Will Steffen said atmospheric CO2 levels presently sit on 405 parts per million and are expected to rise by about 2.5ppm a year in the near term, regardless of emissions reduction action.
Emissions are on trajectory to hit 600ppm CO2 or higher by 2100 unless there is a shift in industrial practices.
“If we really get our act together we would see CO2 concentrations of no more than 450ppm by the middle of the century,” Prof Steffen said, noting that investments in carbon capture and emissions reduction could potentially cause CO2 levels to fall by the end of the century.
Prof Steffen said there was still “great uncertainty” over the global impact of the latest research on all C3 plants, include woody varieties, which comprise more than 85 per cent of all plant species.
Currently plants absorb about one-quarter of human CO2 emissions, and another quarter is absorbed by the ocean and its plants.
But this scenario may shift, as climate change alters the biosphere, with shifting rainfall and fire patterns.
“We need to look at CO2 impacts on woody C3 plants and how heat and rain impact plant growth. It’s a very complex system,” Prof Steffen said.
The story Breakthrough overturns theory of plant growth under rising CO2 first appeared on Farm Online.