Fungi enhance biochar and compost effects on carbon accrual in nutrient-deficient urban greenspace soils

Abstract Urban greenspaces provide critical ecosystem and recreational services but are increasingly threatened by organic matter depletion, fertility decline, and nutrient cycle disruptions under rapid urbanization. Although protective measures like biochar and compost amendments are being widely implemented, their effectiveness across heterogenous urban greenspaces, and the underlying mechanisms governing these responses, remain poorly understood. In a manipulated field experiment across three urban greenspaces with contrasting intrinsic soil nutrient levels, we found that biochar and compost amendments enhanced soil carbon and nitrogen contents, with effects up to 14.4-fold stronger in nutrient-poor greenspace soils compared to nutrient-rich soils. Mechanistically, amendments in nutrient-poor sites elevated fungal diversity and the fungal-to-bacterial richness ratio, driving significant gains in soil carbon and nitrogen. In contrast, nutrient-rich sites exhibited declines in fungal diversity, network connectivity (< − 30%) and stability (− 2% to − 8%), but increased bacterial growth (> 4%) that accelerated carbon consumption, ultimately destabilizing soil carbon pools. These findings demonstrate that baseline soil nutrient status modulates amendment outcomes: in nutrient-poor soils, fungal dominance enhances carbon storage and soil fertility, whereas in nutrient-rich soils, nutrient surplus favors bacterial-driven carbon mineralization. Our study highlights fungi as pivotal drivers of soil restoration in urban greenspaces and underscores the need to prioritize nutrient-poor sites for biochar and compost interventions to maximize ecological benefits. Graphical Abstract