University of St Thomas (MN) St. Paul, MN, United States
Background/Question/Methods Expansion of urban agriculture in recent years has increased the importance of understanding its potential role as green-blue infrastructure. However, there is little information available on possible externalities on the surrounding environment in terms of nutrient losses. In this study, we quantified nutrient losses for nitrogen (N) and phosphorus (P) by measuring weekly nutrient leachate from May to November in 2020 and 2021 by using lysimeters at 47 measuring points divided among 19 ground- and soil based urban gardens in Linköping, Sweden. In addition, we collected annual soil samples and gathered data about nutrient management practices by interviews with gardeners and calculated nutrient balances (inputs – estimated harvest outputs).
Nutrient inputs applied by gardeners often exceeded average estimated crop uptake, resulting in high nutrient surpluses with mean values of 28,7 g N m-2 and 6,4 g P m-2 per growing season. The results show that current nutrient management in urban agriculture can result in high nutrient leachate losses, with values up to 56,90 g N m-2y-1 and 9,62 g P m-2y-1. Even for gardens with low nutrient inputs, leachate losses can still be high, influenced by other factors such as precipitation and soil structure. Soil nutrient values were high in most gardens. We found a high variability of nutrient inputs and losses between gardens, but also between years. A decrease in inputs for most gardens may have contributed to lower average cumulative half-year leachate losses of 3,52 g N m-2 and 0,34 g P m-2 in 2021 versus 4,81 g N m-2 and 0,47 g P m-2 in 2020. Our ongoing study demonstrates that over-application of nutrients in urban gardens, often derived from organic materials, may be leading to high nutrient leachate losses. As nutrient leachate seems to be affected by garden nutrient management and soil structure, there are concrete opportunities for improving nutrient cycling in urban agriculture.