Urban agriculture is increasing in popularity but is an understudied green infrastructure type in terms of nutrients.
Why it matters
Cities are hotspots of nitrogen and phosphorus cycling, and as such play a key role in their management.
These nutrients are essential for plants but can pollute waterways when not properly managed. Cities are trying to improve how they manage nutrients, including through the implementation of blue-green infrastructure programs. However, there is a lack of empirical knowledge on how such programs affect the fate of nutrients and the ecosystem services that depend on nutrient flows.
Urban agriculture is increasing in popularity but is an understudied green infrastructure type in terms of nitrogen and phosphorus.
What we are doing
This project investigates nutrient cycling in food gardens in Linköping Sweden and compares findings with a sister project in Minneapolis USA. We combine discussions with people, field work, and lab work to understand how nutrients are moving around under real-world conditions.
For instance, we survey gardeners to document the diversity of management practices they use and calculate the amount of nutrients applied and harvested in these urban agriculture systems using a garden-gate nutrient budget approach.
We are also measuring nutrient losses as leachate for 3 years with over 30 gardeners in Linköping right now! Every week we go out and collect water which accumulates in a bottle underneath our participants’ gardens. We measure how much water is there and bring samples back to the lab. In the lab we look for how much nitrogen and phosphorus are in the water. Soil is also collected once a year to look at nutrients stored there.
At a later stage, workshops and interviews will be conducted to better understand the benefits (and concerns) stakeholders are interested in related to their gardens and how nutrients fit into this picture.
These results can help cities evaluate how to spend limited resources in adapting green space to meet multiple sustainable development goals, especially related to nutrient sustainability.
Minneapolis sister project
Across the Atlantic ocean, in Minneapolis Saint-Paul, USA, a similar project has been measuring the efficiency of nutrient recycling from compost in urban gardens.
In replicated experimental garden plots, different amounts and different types of compost are applied over five growing seasons. Project results have shown that typical phosphorus (P) inputs in the form of compost far exceed the demand of crops, and that excess P can be lost through leachate.
Targeting compost applications to crop nutrient demand mitigates these losses and increases recycling efficiency, and the application of spent lime (water treatment residual) below gardens can be highly effective at retaining P. They have developed garden mass-balance model based on the data they have collected.
We will be comparing these controlled experiments in the USA to the results from real world gardener plot results In Sweden in the years to come.
The work in the USA is led by Dr. Gaston “Chip” Small and has been funded by a US National Science Foundation CAREER grant focused on the effects of urban agriculture expansion and climate on nutrient cycling and loss in urban ecosystems.
Small, G.E., I. Jimenez, M. Salzl, P. Shrestha. 2020. Urban heat island mitigation due to enhanced evapotranspiration in an urban garden in Saint Paul, Minnesota, USA. WIT Transactions on Ecology and the Environment 243: 39-45.
Shrestha, P., G.E. Small, A. Kay. 2020. Quantifying nutrient recovery efficiency and loss from compost-based urban agriculture. PloS one 15: e0230996
Small, G.E., P. Shrestha, G.S. Metson, K. Polsky, I. Jimenez, A. Kay. 2019. Excess phosphorus from compost applications in urban gardens creates potential pollution hotspots. Environmental Research Communications 1 (9): 091007.
Shrestha, P., M.T. Salzl, I. Jimenez, N. Pradhan, M. Hay, H.R. Wallace, J.N. Abrahamson, G.E. Small. Efficacy of spent lime as a soil amendment for nutrient retention in bioretention green stormwater infrastructure. Water 11:1575