COS 40-6 - Winter matters: nitrogen and phosphorus leaching from urban agriculture in Southern Sweden

Background/Question/Methods

Cities are usually viewed a point-sources of nitrogen (N) and phosphorus (P) pollution to waterways, but as they rely more on decentralized (and multi-functional) ecological infrastructure it becomes increasingly important to understand nutrient dynamics in these ‘diffuse’ spaces to protect water quality. Urban agriculture is one type of urban ecological infrastructure which has the potential to promote nutrient recycling but also nutrient pollution. Perhaps because it is often practiced at a smaller scale, few studies quantify losses of N and P to water from urban agriculture despite the fact that nutrient inputs can be very high and are often unregulated. In temperate climates, if nutrient losses are measured in urban agriculture, it is during the summer months when people are active in their garden. However, losses of nutrients from agricultural land can occur year-round.



Here we investigate the seasonality of N and P leaching from allotment garden plots in Linköping Sweden. We installed 8 lysimeters collecting water 30 cm below the surface in 4 gardens and collected leachate weekly for one year (May 2020-2021). Samples were analysed for total N, total P, nitrate, and phosphate.

Results/Conclusions

Annual cumulative losses of total N (3.9-19 g m^-2) and P (94-243 mg m^-2) were high compared to leachate studies in conventional rural agriculture and variability among plots was large. We found that, for all but one plot, the majority of losses happened in the non-growing season (22 to 97% of annual losses). P losses generally followed the proportion of leachate water collected by season and matched the differences in water volumes collected among lysimeters. One hypothesis is that differences in soil compaction and macropores in the clay soils above the lysimeters help explain this pattern. Snowmelt, precipitation patterns, and irrigation also seem to play a role. Still, N losses did not follow water volume patterns as closely as with P. Gardeners used organic nutrient sources as fertilizers, including some leaving crop residues in place or covering the soil in the autumn with grass clippings and hay. The high mineralization potential of this material may explain the high nitrate losses observed. Our results indicate that monitoring year-round is important to better understand nutrient losses from urban agriculture in temperate settings. Additional guidance on garden management outside of the growing season, including soil covering and fertilization practices, would also be beneficial.