Water and agriculture

How does water availability affect the capacity of plants to use nutrients?

Large amounts of nutrients are lost from agricultural fields with negative consequences for the environment. Water is important in agricultural systems not only because is needed for the plants, but also because it carries away nutrients.

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In my work I try to understand how water availability affects the capacity of plants to use the nutrients.

To hear about this research, see a 2-minute video of my research!

The main results from a first publication on nutrient use efficiency shows that with higher temperatures and lower precipitation (as an effect of climate change) there will be increased nutrient retention and therefore decreased nutrient leaching from agricultural fields. The changes in climate that include increased evaporation and decreased precipitation can lead to increase N use efficiencies without decreasing yields.

 

References

 

Scaini et al., 2020. Hydro-climatic controls explain variations in catchment-scale nitrogen use efficiency. ERL

How it all began

Back in 2015 I won the AGU Student Video competition with the video “Drawing a life through a river”, a personal view on living away from the Tagliamento River, the place that I keep going home to.

During my PhD I really wanted to go to a large international conference (AGU, San Francisco) but did not have the necessary funding. However, the winner of a student video competition, sponsored by the conference organizers, was a free ticket to the conference. The story was clear to me: I wanted to show myself without limits, to go beyond the research topic I was working on, to show my love for my roots and passion for water sciences and how those were linked together in a “strengths and weaknesses” kind of way. I convinced my husband and sister to spend their holidays and free time preparing a short movie to submit to the AGU Student video competition. The video was on the Tagliamento river and it had to be done quickly to meet the deadline. By the time we submitted, we were each at the point where we hated my whistling and my voice! We also had a lot of fun. It turned out to be as emotional and philosophical as I am, and a bit messy too. The video made it to the finals, and to our surprise we won! I was able to attend the conference and give two oral presentations that year. I participated to a video-making workshop and had so much positive feedback on the video. I really wish to repeat the struggle!

Find here the link to the award-winning video!

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Tagliamento

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Studying the invisible

“…The invisible is not the opposite of the visible:

the in-visible is the secret counterpart of the visible”

 

Maurice Merleau-Ponty. Visible and invisible, 1964

When it rains, we can follow water drops until they reach different surfaces, and keep checking out water movement on the surface. But what happens to water when it moves through the soil?


Much of our work in hydrology is focused on the movement of rainfall through a watershed – and we have explained the apparently contradictory quick rise of streams after rainfall and the long time (months, years even!) that water resides in catchments using the concepts of velocity and celerity. The difference between velocity and celerity and their relationship is one of the ultimate challenges of modern hydrology. Velocity and celerity are both linked to the concept of “speed”, but the terms are quite specific. In hydrology, we borrow the “hydraulic” definition of celerity, which goes somewhat like this: it is a measure of the impact of a perturbation on a channel. When it rains, in the soil we have a similar effect: water residing in the soil gets “pushed” by the “new” water coming in rain form. The result of this is that we can quantify the “push” with which water infiltrates in the soil during a rainfall event. This is called celerity, and it depends on how much water resides in the soil in the first place, and how much space there is yet to be filled during such “pushing”. Now, the measure of the movement of this new water entering the soil is different. Such new water will have a range of velocities depending not only on the amount of water that already occupies the space, but also how big the pores are, and how easy it is for water to move through them.

Ultimately, we can say that celerity is responsible for increases and decreases in stream discharge (the hydrograph), while velocity controls how old is the water reaching the stream at a given time.

So why do we need to keep these measures separate? Predicting the hydrograph response (celerity), given a certain amount of rainfall, can be done quite easily with modern modelling frameworks. However, equifinality is always around the corner: models can be parameterised in many ways to make a good approximation of the hydrograph, so which parameterisation is actually happening? I think of equifinality as a number of possibilities. From A I can reach B by taking different paths. Now, each path is different as some may be longer than others, some nicer than others, etc., but the point is that there is more than one way to get from one point to the other, they are all possible. In the same way, the water that contributes to the rise of a stream can come from different parts of the area conveyed to that stream.

When we are interested in measuring water in the soil, we have an additional issue to solve: water “disappears” to our eyes when it infiltrates in the soil. The invisibility of water inside the soil makes measuring things more difficult, but also more interesting.

To measure how much water comes from which part of the catchment, we need to design good experiments that allow us to measure velocities as well as possible, to try to understand which is the path that is taken by water. In order to do this, we can artificially apply water, but we need to know how to track the water we are adding. This is usually done by means of substances that can be dissolved in water, in particular salt tracers.

We performed experiments in this way, and as often happens, we found out that all our efforts brought little (that was 2014, my heart-breaking Experiment 1). Experiment 2, in 2015, one year of experiences later, produced a dataset to work with.

My PhD work has a lot to do with the invisibility of water moving through the soil.