Applying recovered nutrients and pruning waste to enrich and regenerate agricultural soils in urban agriculture: Effects on emissions and yield of spinach and sweet potato

Peri-urban agricultural soils have degraded from intensive tillage and chemical fertilizer use, leading to reduced soil organic carbon (SOC), loss of microbial diversity, and declining crop yields. Nutrient recovery from urban waste can substitute chemical fertilizers, but nutrient cycling in depleted soils can be ineffective. A promising strategy is the incorporation of unstable, carbon-rich biomass waste, which can regenerate soil while closing carbon and nutrient loops. We tested this by applying 150 t ha−1 of ramial chipped wood (RCW) and evaluated yield, soil health, and emissions of CO2, CH4, N2O, and NH3 for spinach and sweet potato cultivation. The outcomes of RCW were compared with mineral fertilization, compost, and struvite combined with organic nitrogen. Spinach grown with RCW showed the lowest productivity amongst treatments (1.16 – 0.32 kg m−2), whereas sweet potato reached high yields (1.72 – 1.99 kg m−2), reflecting its nitrogen-fixation capacity and better adaptation to RCW–amended soils. High RCW doses showed higher CO2 emissions than other treatments initially and significantly enhanced SOC and soil N stocks (+96.3% and +82%, respectively, for sweet potato). We found that N2O emissions are reduced significantly from 0.36 kg ha−1 emitted with mineral fertilizer treatment to 0.07 kg ha−1 with RCW for spinach cultivation, which we attribute to the ability of the unstable carbon available in RCW to bind more easily to the nitrogen. Overall, applying RCW favours sweet potato production while reducing N2O emissions and improving soil fertility, highlighting agronomic and environmental trade-offs for sustainable urban food systems.