Abstract: To provide support for accurately measuring the carbon sink of garden plants, we investigated the impact of different maintenance methods on the carbon budget of Hydrangea macrophylla ‘Hanatemari’, and explored carbon sequestration and emission reduction potential of three substrate types (S1: garden soil; S2: 50% garden soil + 50% garden waste; S3: 50% garden soil + 40% garden waste + 10% biochar) and two irrigation methods (FI: full irrigation; DI: deficit irrigation) in urban landscaping applications. The results showed that under DI conditions, the S3 substrate significantly promoted leaf dry weight, root dry weight, and total biomass of H. macrophylla (P<0.05), allowing the maintenance of the highest total biomass (68.12 g·plant^-1) and total plant carbon storage (29.38 g·plant-1) with low water consumption. Among all links in the maintenance management of H. macrophylla, carbon emissions followed an order of irrigation > pruning > pest and disease control > fertilization. There were significant differences in carbon budget values under different management methods. Compared to FI, DI increased carbon budget by 92.88%, with the highest value under DI conditions observed in the S3 substrate (-21.18 g·plant^-1). Compared with S1 substrate, the organic carbon storage of S2 and S3 substrates increased by 19.94 times and 15.56 times, respectively. Specifically, compared with traditional burning, the S2 substrate reduced greenhouse gas emissions by 55%, while the S3 substrate reduced CO₂e emissions by 2.35 times and sequestered 1.52 kg CO₂e. Among the examined management practices, the combined use of deficit irrigation and biochar (DI+S3) was the most suitable management combination for improving carbon sink of H. macrophylla and the soil carbon sequestration and emission reduction capacity.

Key words: Hydrangea macrophylla, carbon budget, substrate type, irrigation method, carbon sequestration and emission reduction