Urban agriculture in Braganca, Northeast Portugal: assessing the nutrient dynamic in the soil and plants, and their contamination with trace metals

Urban agriculture is increasing in both developing and developed countries. In spite of its multiple benefits, urban agriculture can cause unwanted aspects such as environmental damage and health risks to consumers. This work was carried out in a social garden project developed by the Polytechnic Institute of Braganca, Portugal. It checked if cropping practices were environmentally sound and monitored the quality of the edible vegetables, in particular their concentration of trace metals. Braganca is a small city of 25,000 inhabitants located in Northeastern Portugal. In this study the plant nutritional status and the inorganic nitrogen (N) levels in the soil as indirect indicators of the risk of N loss to the environment; and the concentrations of trace metals in the soil and plant tissues as a measure of the exposure of gardeners to trace metals were studied. The results showed the existence of low leaf N concentrations, often close to the deficiency range of the respective species, and also low inorganic N levels in the soil during the winter, suggesting reduced risks of nitrate leaching and denitrification. Trace metal concentrations were low in the soils when compared with the limits set by national and international legislation. As a result, the content of trace metals in edible plant parts was also low. The positive results found in this urban agriculture project might be due to its organic farming basis and to the absence of major sources of pollution in the vicinity of the gardens. This information is useful to local gardeners, since they can trust the products they take home, but also to other urban agriculture projects in even more populated cities.