Dry run for cropping systems

The heatwave of 2018 was yet another warning to both farmers and the public on the effects of climate change: in the future, Switzerland will see less and less rain during the summer months. Given the prospect of longer and more frequent periods of drought, Nina Buchmann, Professor of Grassland Sciences, is concerned about the future of agriculture. She argues that drought and resulting crop failures pose an increasing threat to food production and asserts, “our agriculture is inadequately prepared”. For her, the answer lies in a greater use of cropping systems that also deliver good, stable yields under dry conditions. At present, however, it is by no means clear which of the cropping systems common to Switzerland is the most resistant to drought.

Together with her team and colleagues from Agroscope, the Swiss centre of excellence for agricultural ­research, Buchmann is now aiming to find out. The research team is conducting a field experiment to investigate how well the common cropping systems in Switzerland react to sustained drought. Launched in 2017 and scheduled to run for three years, the experiment seeks to compare drought responses under conventional and organic cultivation with and without soil tillage – or, in the case of organic cultivation, with reduced tillage to keep weeds in check. The crops chosen for the experiment are maize, a mixed crop of peas and barley for forage, and winter wheat.

The project is funded through the World Food System Center’s Mercator Research Program and the ETH Zurich Foundation, and is based on FAST, a long-term trial conducted by Agroscope, which for the past ten years has been studying the agronomic performance of these cropping systems.

Field experiments for agriculture

Out in the field, the ETH researchers use simple rainout shelters to keep the rain off and simulate drought conditions. The experiment itself is more complex. It comprises 4 different cropping systems applied to 32 plots, 16 of which are covered by a rainout shelter, while the rest are open to the sky. Each of the 32 plots is equipped with a device known as a PhenoCam, which captures images of plant growth hourly. Sensors in the soil and above ground measure a range of ecological and physiological variables. Four doctoral students are responsible for gathering the field data.

The experiment rests upon the hypothesis that organic production would be more resistant to summer drought than conventional agriculture. Buchmann justifies this reasoning as follows: “We make this assumption because yields from organic production are known to be lower, so water consumption should be lower as well. You would also expect to find a greater presence of plant symbionts in the soil, such as nitrogen-fixing bacteria and mycorrhizae, both of which can favour greater resistance to drought and stress.”

Various factors can influence the ability to withstand dry conditions. Researchers are therefore seeking to capture the performance of the crops, and also of the entire ecosystem and its services. Ecosystem services here include plant growth, the amount and quality of the crop yields, and the ability to withstand fungal infection and her­b­ivory by insects. Fundamental soil characteristics such as soil fertility and the ability to decompose organic ma­terial are measured – as well as the presence of plant symbionts, the form in which different nutrients are avail­able to plants, and how much nitrogen is being leached out of the soil. “We try to measure a lot of services, but using simple, established methods, so that we get the full picture and comparable results,” explains Yujie Liu, who is working on a doctorate in Buchmann’s group.

Embolism of plant vascular systems

The project also addresses a second assumption that cropping systems that involve less ploughing are better suited to withstanding summer drought. This is because less ploughing leads to a more stable soil structure, which should improve the availability of water and nutrients for the crops.

To test this hypothesis, Buchmann’s team uses stable isotopes of hydrogen and oxygen to determine how much water is present at which soil depth and from which depth plants take up water. In addition, the researchers want to find out exactly how much the crops suffer as a result of the drought, and when this becomes crit­ical for the plants. On a regular basis, Qing Sun, another doctoral student in Buchmann’s group, heads out before sunrise to measure a specific physiological variable that tells her how much stress the plants experience as a result of the drought.

Plants have a vascular system to transport water through the stem. Evaporation from the leaves generates a suction that draws water from the tip of the root to the top of the plant. This creates a negative pressure in the vascular system, which can be measured. If the soil is dry, this pressure can increase to such a degree that the continu­ous water column in the vascular system breaks, leading to the formation of small air bubbles, which block the vessels. Depending on the location of this “embolism”, one leaf may wilt or the whole plant may die. According to Sun, the team is mapping the fraction of vascular embolism over time. “Such measurements are very rare,” she says. “They’ll give us a better understanding of how plants react to drought stress under different cropping systems.”

Waiting for the full picture

The experiment will run until the end of the coming year. Preliminary ana­lyses suggest that cropping systems may have less impact on ecosystem services than originally assumed. Although the mixed crop of peas and barley appears to be more resistant to dry conditions when cultivated organically, it may well be that maize and winter wheat react differently. “We still don’t have the full picture,” Buchmann explains. For the time being, only the plants know whether there is one cropping system that is more resistant to drought than others – or whether each particular crop favours a specific cropping system.