Growing local and circular: The potential of peri-urban agriculture in reducing imports and environmental impacts

Cities are increasingly dependent on food grown far away, shipped across oceans and continents to reach supermarket shelves. This long journey creates a heavy environmental toll, from the fuel burned in transport to the energy used in heated greenhouses. Yet, many cities are surrounded by land that could grow food, known as peri-urban agriculture. The idea of growing food closer to home is not just about reducing travel; it is also about how that food is fed. Traditional farming often relies on synthetic fertilizers made from non-renewable resources, which can pollute water and air. A newer approach suggests using nutrients already present in the city, such as composted food waste or minerals recovered from wastewater, to feed these local crops. This method, called circular nutrient strategy, aims to turn a city's waste into its own fertilizer, closing the loop between consumption and production. The question researchers have long debated is whether this local, circular approach actually offers a better environmental outcome than importing food from distant, conventional farms, or if the challenges of local production outweigh the benefits of shorter supply chains.

A team of researchers set out to answer this question by looking at the Metropolitan Area of Barcelona, a region with a population of 3.4 million people. They wanted to know how much fruit and vegetables the area could grow for itself, and if they could use local waste to feed those crops, would it be better for the planet than buying food from elsewhere? To find out, they did not just guess; they built a detailed model that tracked every step of the process, from the moment a seed is planted or a truck leaves a distant farm, all the way to the wholesale market gate. They compared crops grown locally in open fields against those imported from other parts of Spain and beyond, and they tested different ways of fertilizing the local crops, including using compost from food scraps, minerals recovered from wastewater, and shredded tree branches.

The results revealed a stark reality about the current state of food in the city. Despite having thousands of hectares of agricultural land, the Barcelona area produces only a tiny fraction of the food its residents eat. The researchers calculated that the region grows about 47,000 tonnes of fruit and vegetables each year, but the population consumes roughly 650,000 tonnes. This means the area is only about 7 percent self-sufficient, relying heavily on imports for the vast majority of its fresh produce. The most consumed items, such as potatoes, apples, and watermelons, are almost entirely imported, with local production covering just a sliver of the demand. Even for crops that are grown locally, like tomatoes and onions, the amount produced is far less than what is needed to feed the population.

When the researchers compared the environmental impact of local crops versus imported ones, they found that growing food locally generally caused less harm to the environment, but with important exceptions. For vegetables like tomatoes, peppers, and lettuce, local production was significantly better, often reducing the environmental footprint by up to 94 percent compared to imports. The main reason for this advantage was that the imported vegetables often came from heated greenhouses, which require massive amounts of energy, whereas the local crops were grown in open fields using soil. However, the story was different for fruits. Local fruits sometimes had a higher environmental impact than imported ones, particularly regarding water use and pollution in the oceans, largely because the local farms still relied heavily on synthetic fertilizers and water-intensive practices.

The study then tested whether using circular nutrients could make local farming even better. They simulated scenarios where local farmers replaced synthetic fertilizers with compost from municipal food waste, minerals recovered from wastewater, or shredded pruning residues from city trees. The findings showed that these circular strategies did improve the environmental performance of local crops. Using compost from food waste, for instance, reduced the impact on water pollution and climate change because it avoided the need to treat that waste in landfills and replaced the energy-intensive production of synthetic fertilizers. Using shredded tree branches directly in the soil was particularly effective at reducing carbon emissions, as the branches stored carbon in the ground. However, these benefits came with trade-offs. The process of recovering minerals from wastewater required chemicals that increased the scarcity of mineral resources and the pollution of fresh water. Similarly, shredding large amounts of tree branches required energy and machinery that increased certain types of pollution.

Crucially, the researchers discovered that the overall benefit to the city depended entirely on how much food was actually grown locally. Even when the local circular methods were highly efficient on a per-kilogram basis, the total environmental savings for the city remained small because local production satisfied such a tiny portion of the total demand. For crops where local supply was high, such as peaches, switching to pruning residues as fertilizer could reduce the city's total carbon footprint by up to 37 percent. For other crops like tomatoes, the reduction in marine eutrophication was around 26 percent. But for crops where local supply was negligible, like apples and peppers, the total reduction was generally below 1 percent, no matter how efficient the local farming was. This suggests that simply improving local farming techniques is not enough; the city must also expand the amount of land dedicated to growing food to make a real difference.

The study concludes that while peri-urban agriculture has the potential to be a powerful tool for sustainability, its success depends on a combination of factors. Local crops must be grown in ways that avoid energy-intensive infrastructure, such as heated greenhouses, and they must be paired with fertilization methods that minimize trade-offs. Furthermore, the environmental gains of circular nutrient strategies are only realized if the local production capacity is large enough to displace a significant amount of imports. For the Barcelona area, and likely for many other cities, the path forward involves not just better farming methods, but also a concerted effort to expand the land available for agriculture and to align waste management systems with the needs of local farmers. Without increasing the scale of local production, the environmental benefits of growing food closer to home will remain limited, no matter how circular the system becomes.