Co-application of compost and biochar promotes soil carbon sequestration: Evidence from eco-enzymatic stoichiometry

Adding organic materials to soil can effectively mitigate the weak, or even non-existent carbon (C) sequestration in the soil of urban green spaces. However, the potential of this strategy to enhance C sequestration and the underlying mechanisms are still unclear. In this study, effects of compost and biochar application on microbial metabolism and C sequestration in the soils of green spaces were investigated by analyzing the ecological enzyme stoichiometry, based on a potting experiment. The results showed that the application of compost and biochar intensified the microbial C limitation by increasing the stoichiometric imbalance of dissolved resources and microbial communities in soil, which led to a decrease in the microbial C use efficiency (CUE). The highest CUE of 0.531 was observed, when soil was amended with 7.5 % compost and 4 % biochar (BCC4 treatment). However, no significant linear relationship was noticed between CUE and net organic-C sequestration in the green space soils. Combined application of compost and biochar resulted in increased organic C sequestration in the green space soils, mainly by promoting plant C inputs. BCC4 treatment led to the highest soil organic C sequestration of 2.42 kg m −2 , which was significantly higher than that in the control group. This study demonstrated that the synergistic application of compost with optimal biochar dosage can effectively enhance C sequestration capacity in urban green space soils. This effect is primarily driven by increased plant-derived C inputs, rather than an improvement in microbial CUE, highlighting the pivotal role of plant contributions in soil C dynamics. • Compost and biochar induced stoichiometric imbalance in the soils of green spaces. • Stoichiometric imbalance reduced microbial CUE by exacerbating C limitation. • Microbial CUE did not show a linear relationship with SOC sequestration. • Organic materials enhanced SOC sequestration mainly by increasing plant C inputs. • 7.5 % compost + 4 % biochar (v/v) significantly enhanced SOC sequestration.