The Australian Grain Free Air CO2 Enrichment (AGFACE) program was established in 2007 to investigate how field grown crops would respond to future elevated carbon dioxide (CO2) levels. However, as knowledge about how plant growth was effected by elevated CO2 came to light, it became evident that little was known about soil-crop interactions and nutrient dynamics under elevated CO2. To fill this knowledge gap, SoilFACE, a sub-project of the AGFACE program, was established in the Wimmera four years ago.
Background
By 2050 CO2 levels are almost certain to rise by at least 50 per cent and this will have both positive and negative effects on grain production. Increases in atmospheric CO2 have been shown to enhance photosynthetic gain and net primary productivity (growth), and to also increase water use efficiency. However, it is uncertain whether this extends to grain crops in semi-arid agricultural systems such as ours which are often N and (almost always) water limited.
SoilFACE researchers are investigating the effect elevated CO2 levels have on the interaction between the crop growth and the soil properties. Specifically, SoilFACE examines a pulse (field pea) -wheat rotation grown under elevated CO2 in three contrasting soil types – Mallee sands, Wimmera cracking clays, and heavy clay from high rainfall zone near Hamilton in the Western District.
The project is looking to identify any direct effects CO2 has on the crop rhizosphere, any agronomic carry-over effects on soil nitrogen from the pulse crop to wheat and the potential long-term effects of soil nitrogen depletion, phosphorous and carbon.
Method
The SoilFACE research facility near Horsham consists of eight four metre ‘bunkers’ which each house up to 50 large intact soil cores (encased in PVC) that were extracted from Walpeup in the Mallee, Horsham in the Wimmera and Hamilton in the Western District. Wheat (Yitpi) or a pulse (field pea) is planted in each core on an annual rotation basis. The SoilFACE cores are not irrigated and only phosphorus fertiliser is added.
Each soil core is 100cm deep, 30cm in diameter and weigh between 140 and 160 kilograms (depending on soil type) ; the bunkers have been designed so the top of each core is at ground level. Four of the eight bunkers are fed carbon dioxide at 550ppm; the predicted level of atmospheric CO2 in 2050. The crops in the other four bunkers are grown under the current CO2 concentration (390ppm).
Plant sampling is undertaken at flowering and grain maturity. Soil water and nitrogen are measured at the beginning and end of the season and non-destructive measurements taken between these dates allow scientists to quantify growth and N fixation by the legume. Measurements look at the growth and grain yield of the crops, changes in soil water and mineral nitrogen status and nitrogen fixation by the peas.
Results and interpretation
Wheat and pea yields grown under raised CO2 conditions at Horsham have consistently yielded, on average, 25 per cent higher than their counterparts grown in ambient CO2 conditions.
Early SoilFACE results suggest that along with increasing crop growth, elevated CO2 stimulates grain yields and nitrogen content. So in effect, the legumes will supply a greater quantity of nitrogen to the following cereal crops. However, if plants grown in elevated CO2 are bigger and produce more biomass, their nutrient and water need will also be greater, and quite possibly, beyond what the preceding pea crop can supply.
SoilFACE project leader Dr Roger Armstrong says the total amount of nitrogen fixed by pulses is greater under elevated CO2 because the crop produces more dry matter.
“This is a consequence of the CO2 ‘fertilisation effect’ rather than through achieving a greater rate of N fixation per se,” he said. “CO2 is essential for photosynthesis but it also increases water-use efficiency in plants. So while increases in CO2 can positively effect on plant growth and yields, when temperatures increase, rainfall efficiency decreases as rates of transpiration also increase in order for the crop to maintain internal temperatures. If predictions of a hotter and dryer future climate are correct the benefits from elevated CO2 may be offset by the crops increased nutrient demand.”
The research suggests fertiliser demand will increase in the future with heavier crops requiring between 25 and 60 per cent more nitrogen. Additionally, the phosphorus (P) requirement of selected crop and pasture species is likely to increase under elevated CO2, which again, will have consequences for farmers endeavouring to meet their crops nutritional needs.
More encouraging are recent findings which have revealed that the ability for plants to access ‘non-available’ pools of soil P is enhanced under elevated CO2. “It’s quite possible increased root growth will go some way towards compensating for an increased P requirement,” Dr Armstrong said.
Where to from here?
Maintaining productivity in the future will require a good understanding about how elevated CO2 will affect below ground processes so that appropriate management adaptations can be made (to agronomic practices as well as breeding better adapted varieties).
Knowledge of nitrogen mineralisation rates, as well as the amount of N and carbon that is exudated from roots in the plant rhizosphere, will help deliver better understandings about the capacity of the soil to supply nitrogen to the crop and the amount of carbon that the crop can sequester in the soil.
The research suggests that in the future crop yields may increase, but so too will their water and nutritional needs. However, these results are being achieved under current temperatures.
“Whether future yields increase will depend largely on rainfall and temperature changes, and if farmers can, or are willing to, supply their crops increased nutritional demands,” Dr Armstrong said. “What the research so far tells us is that in regions where biomass is predicted to increase, more nitrogen fertiliser many be required to take advantage of increased yield potential. Legumes in rotation might be able to supply at least some of this.”
The AGFACE program is a collaborative venture between the Victorian Department of Primary Industries and the University of Melbourne, with crucial additional funding from the Grains Research Development Corporation and the Australian Commonwealth Department of Agriculture, Fisheries and Forestry. For more information visit the Primary Industries Climate Challenges Centre website at: www.piccc.org.au/AGFACE.
