When Christian came to me and asked me if I would like to conduct a field experiment for the Diversify project, I was very enthusiastic about it. I had already worked in the field, but I hadn't ever organised and carried out a whole experiment on my own. This is the story of how it went...

Act one: “seek and you will find”

Since I was writing a research proposal on hydraulic lift and water facilitation, I decided to investigate water use in this field experiment. I was curious how the different crop species use water and how this changed in monoculture or 2- or 3-species mixtures. Therefore, I defined two objectives: (1) to investigate the soil depth the different crop species are taking their water from and (2) to compare how the different crop species in the different cultures using the water during irrigation or rainfall events.

To find out which soil depth the plants are taking their water from, I made use of stable hydrogen isotopes (1H and 2H). The isotope 1H is naturally more dominant compared to the isotope 2H. The proportion of these hydrogen isotopes shows a distinguishable gradient in the soil. One can then compare the proportion between the hydrogen isotopes within the plant and in the specific soil depth and “read out” where the plants are most likely taking their water from.

The second task followed a slightly different approach. Here I used irrigation water which was 2H enriched – meaning the water could be “tracked” since it shows a much higher abundance of 2H than naturally would occur. With this method it is possible to see how much water the different crop species are taking up and compare between the different species.

Act two: “you reap what you sow” or about heatwaves and (almost) records

The crop species that we tested were pea, faba bean, linseed, rapeseed, wheat and barley. In the 2-species mixtures each combination was tested and in the 3-species mixtures only the combinations of the functional groups (functional groups: wheat/barley, pea/faba bean, linseed/rapeseed, e.g. Wheat + Bean + Linseed or Barley + Bean + Linseed or Wheat + Pea + Linseed...). Firstly we had to set up snail-proof fences, to protect our plots, which was rather challenging (Fig. 1 left). We then sowed the seed of our different crop species in early May 2019. The seeds germinated nicely and – despite some heavy rain events and cold weather fronts during May – the plants grew well (Fig. 1 right).

Fig. 1. Germination and growth of the crops. Left: The snail-proof fences which are protecting our plots. Right: Example of a 3-species mixture of rapeseed, pea and barley.

During June the weather changed radically from cold and rainy to hot and dry! The plants liked it and hurried to form flowers and fruits (Fig. 2). Since the plants had to be alive to still be able to take up water, I decided to harvest single plants in each treatment for stable hydrogen isotope analysis as soon as possible. During this time a heatwave had just arrived in Switzerland and weather stations recorded temperatures more than 35°C. Furthermore, it had not rained for 1-2 weeks which was perfect for the application of the 2H enriched irrigation water. Despite the burning sun and the almost unbearable heat we managed the harvest without any problems. It turned out that June 2019 was the second hottest June since records began.

Fig. 2. Flowering and fruit development. Left: Flowering faba bean and linseed. Linseed is simultaneously forming fruits. Right: Flowering and fruit development in pea.

Act three: of mice and birds

Despite some small bug infestations, the plants showed no sign of sickness or nutrient or water limitations. They started to form seeds in July. At that time, I also began to notice holes in the soil within the plots and that the linseed seemed to be disappearing. All this indicated that mice were living in our field. Sadly, after a short time, almost no linseed remained with seed. As the cereals matured, birds also discovered our field. Within weeks, almost all the cereal seed vanished (Fig. 3 left). Our field turned out to be heaven for the birds living in the larger neighbouring field where there weren't any mature cereals.

Next weeds started to accumulate in large numbers (Fig. 3 right) mainly due to the large seed bank present in the soil and the ongoing warm weather conditions during July. This was additionally promoted by the senescence of the crops during August. At some point, we decided that we would not be able to harvest most of the crops apart from the faba bean, which did not lose the seeds to birds or other causes. This is the reality of field work..!

Fig. 3. Issues faced before harvest. Left: Cereal seeds eaten by birds. Right: Weed pressure during senescence of the crops.

Nevertheless, despite mice and birds (and weeds), I learned a lot during this field experiment. Beside the organisation of a field experiment I also learned to measure various traits. And the stable hydrogen isotope data about water source (soil depth) of the different crop species look promising (Fig. 4). This data is not complete yet but already shows the gradient of the proportion of the stable hydrogen isotope in the soil...

Fig. 4. Preliminary soil profile of stable hydrogen isotope abundance (δ2H is calculated by the ratio between 2H and 1H). 2H abundance increases in upper soil layers due to the higher evaporative losses of 1H. 1H is lighter and, therefore, water molecules containing 1H are transformed faster into vapour. This leads to a natural enrichment of 2H isotopes in the upper soil layers. TukeyHSD shows the grouping in soil samples from different depths after ANOVA analysis.

With this I want to thank Christian Schöb for the opportunity to conduct this field experiment and Jianguo Chen and Anna Bugmann for their help in the field. Greetings also from my field neighbours which like to stand in their food containers :-)