Fig. 1: Main urban agriculture nutrient flows and leachate measurements.

a Conception of nutrient mass balance in gardens. Imports (left-hand side) of nitrogen (N) and phosphorus (P) are often organic in nature, and quite diverse. Once applied to soils these inputs, in addition to on-site nutrient recycling through composting practices, are taken up by crops. Intentional exports of N and P (right-hand side) leave the system through intentional harvest, and unintentional export (bottom) can occur via leaching. Losses (unintentional exports) are also possible via runoff and erosion (N and P), and atmospheric pathways (for N, e.g., denitrification) but are not quantified in our work. The system boundaries are the garden plot (middle of the image) including crops and soil (30 cm down), and assume a one growing-season time step. If imports are larger than intentional exports there is accumulation of N and P in the system and/or an increased risk of unintentional losses. If intentional imports and exports match, they are balanced. If imports are smaller than intentional exports, there is mining of N and P soil pool and/or potentially lower yields/exports over time. Hays include straw and other types of mulch or organic soil coverings. b Sampling scheme common across the three studies. This figure was created in https://BioRender.com.