Objective Urban production and construction projects (UPCPs) are an important product of rapid urbanization and a significant source of carbon emissions across their full life cycles. Unlike conventional building projects, UPCPs have a "dual carbon" nature. During land transformation and construction, they generate substantial emissions through intensive material consumption, surface disturbance, and energy use; during operation, however, they can function as long-term carbon sinks through vegetation-based soil and water conservation measures. Accurate assessment of their carbon budget is therefore essential for evaluating ecological performance and supporting China’s "dual carbon" strategy. However, existing studies rarely offer an integrated life-cycle framework that simultaneously incorporates the carbon effects of initial land use adjustment and the long-term sequestration benefits of vegetation. This study addresses that gap by developing a more comprehensive accounting framework for UPCPs.

Methods This study adopted a life cycle assessment framework adjusted for land use adjustment (LCA-LUA) to quantify the carbon budget of UPCPs more accurately. The Xiba flood control embankment municipal supporting facilities and greening engineering project in Hanbin district, Ankang city, Shaanxi province, was selected as a representative case. A carbon accounting boundary was established for five stages: land use adjustment, building material production, on-site construction, operation, and demolition. Within this framework, both carbon sources and carbon sinks were explicitly quantified. Carbon sources included fossil fuel combustion, physicochemical emissions from materials, and transport-related energy consumption, while carbon sinks were primarily derived from vegetation sequestration and conservation-related ecological restoration. Emission factors were obtained from the Chinese Product Carbon Footprint Factor Database and relevant local studies. Vegetation carbon sequestration was estimated using species-specific allometric growth equations for local species, including Betula platyphylla and Prunus lannesiana. The carbon budget was then evaluated over the project’s 50-year life cycle.

Results Over the 50-year period, the project generated total carbon emissions of 521.06 t CO₂ and total carbon sequestration of 557.09 t CO₂, resulting in a net carbon sink of 36.03 t CO₂. These results indicated that a well-managed UPCP could ultimately make a positive contribution to climate mitigation. Distinct stage-specific heterogeneity was observed. Building material production was the dominant emission source, contributing 287.05 t CO₂, accounting for approximately 55% of the total emissions, which highlighted the high embodied carbon of hardscape materials. In contrast, the on-site construction stage accounted for only 1.6% of the total emissions. The operation stage exhibited a distinct pattern of low emissions but high sequestration: vegetation generated a cumulative carbon sink of 545.56 t CO₂ over 50 years, offsetting most of the initial carbon debt. As the plant community matured, the sequestration rate gradually exceeded the emission rate, and the project was estimated to achieve carbon neutrality within the full life cycle (43.3 years). In addition, land use adjustment produced a carbon flux of −0.53 t CO₂ and immediate emissions of 174.05 t CO₂ associated with surface transformation, demonstrating that land conversion constituted a critical early-stage emission event.

Conclusions The LCA-LUA framework proves effective in capturing the full carbon transition of UPCPs from initial ecological disturbance to long-term ecological restoration. Although such projects can eventually achieve carbon neutrality, their payback period remains relatively long. This suggests that future urban planning and project design should move beyond a narrow focus on operational efficiency and instead adopt a coordinated strategy that combines low-carbon embodied materials with high-efficiency carbon-sink plant communities. The study provides a robust methodological basis and empirical evidence for carbon budget accounting, ecological performance evaluation, and low-carbon management of urban municipal projects.