29 June 2022

Many cities are seeing an increasing interest for homegrown food. People are attracted by locally grown vegetables, the opportunity to get some fresh air, and a community feeling. But researchers want to find out: is there also a downside to urban agriculture in the shape of nutrient loss to waterways?

Two young women collecting samples in a urban garden in front of city houses.PhD student Paulien van de Vlasakker and research assistant Emmy Johansson collect samples in an urban garden in Linköping. Photo credit Anna Nilsen

Does the idea of harvesting ripe tomatoes give you a warm, fuzzy feeling? Do you want to grow different colour carrots, promote biodiversity, or exchange tips with other urban cultivators? If so, you’re in good company.

The increased interest in growing food in urban environments may reflect an increased interest in what we consume. It doesn’t get more local than when you’ve grown something from seed yourself. The pandemic, and war in Europe, may also have got more people interested in growing their own food. Today, many cities allocate parts of public land for urban agriculture. And allotments have existed in Sweden since the end of the 1800s. In recent years, interest in allotments has skyrocketed.Geneviève Metson.Geneviève Metson. Photo credit Magnus Johansson

“Urban agriculture can be a way to relax, and it can provide us with food. But that doesn’t necessarily mean everything happening in a garden is positive. We currently know very little about how nutrients move in these urban agricultural environments”, says Geneviève Metson, associate professor at Linköping University, who leads a research project on urban agriculture.

Plants need nutrients such as phosphorus and nitrogen to grow and provide food. But if these substances end up in our waterways, they act as fertilisers for algae and cyanobacteria. The over-fertilisation of the Baltic Sea is an example of this. There, areas of the sea with little to no oxygen in the water, called dead zones, and algal bloom during the summer, have been caused by nutrient inputs from human activity on land including farming and wastewater. Given urban agriculture is a form of farming, there is a risk it contributes to excess nutrients ending up in the wrong place and worsening water quality. But to get to the root of the problem, researchers need to take a closer look.

“I think that the fact that we work closely together with urban cultivators is what makes our project interesting and unique. We visited shared cultivation spaces, allotments, and some commercial urban agriculture places in Linköping, and asked if people wanted to be a part of the study. They don’t need to change anything. We leave them to their own devices. We just want to take some measurements, ask a few questions, and then share our results with them”, says Geneviève Metson.Two persons talking in a garden shed.PhD student Paulien Van de Vlasakker talks to urban gardener Olle Elfvin. Interviews that she and Associate Professor Karin Tonderski have carried out with cultivators in Linköping show that gardeners sometimes use too much nitrogen and phosphorus, but garden practices vary a lot. Photo credit Anna Nilsen

This approach can give a more realistic picture of what people actually do, which is an important contrast to when researchers do controlled experiments. The cultivators who participate have different methods for taking care of their crops. They are interviewed about their techniques, how much and which nutrients they use, how much they harvest, and their motivation for gardening and the benefits they get from it. The research team has also installed bottles underneath garden plots to collect water from the soil. The team retrieves the water from the bottles every week, and measures the phosphorus and nitrogen levels. They also take soil sample once a year in order to measure nutrient levels.

“It isn’t just about how much we fertilise. Lots of different factors affect these nutrients. For example, which vegetables and crops you grow, how you water them, if you cover the soil during the winter and so on.”

Geneviève Metson has previously worked on Canadian and American studies which have shown that urban cultivators often fertilise with more nutrients than the plants they grow can use.

“On the one hand, urban cultivators often use such fertilisers as compost, manure and diluted human urine. We need to recycle nutrients, so it’s good that they use these sources. But if we add too many nutrients, then there’s a risk that they end up in our waterways. So we suspect that there might be a problem there. But we need to take measurements to know for sure.”Geneviéve Metson collects water from bottles that the researchers have installed underneath garden plots.Geneviéve Metson collects water from bottles that the researchers have installed underneath garden plots. Photo credit Magnus Johansson

This year is the third and last year that the researchers are taking measurements. When the analyses are done, Geneviève Metson hopes that the results will bring them an improved understanding of what actually happens with nutrients in these parts of a city’s green infrastructure. Her and her team also want to get a picture of why people grow food in different ways, and how the environment is affected.

“Urban agriculture lies at the intersection of cities, sustainability and handling of nutrients. I really look forward to seeing how urban agriculture can be a catalyst for more sustainable food production and water stewardship.”

3 tips to urban gardeners

Tips to urban gardeners

It is becoming increasingly popular to grow food in the cities. In collaboration with practitioners, researchers from Linköping University study different aspects of urban agriculture – both benefits and potential problems. Researcher Geneviève Metson shares some tips for gardeners.

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