Biotic factors dominate carbon fluxes and budget of a drip-irrigated and plastic-mulched cotton system in an arid oasis of Northwest China.

Drip-irrigated and plastic-mulched (DIPM) cotton systems are widely adopted in the arid oases of Xinjiang, where water scarcity poses significant challenges to crop production. Despite the widespread use of these systems, the impact of agricultural management practices, crop development, and environmental factors on carbon fluxes and budget in these arid regions remains insufficiently quantified. This study combined eddy-covariance measurements with micrometeorology, crop growth and agricultural management to assess seasonal and interannual carbon fluxes and carbon budget over ten growing seasons (2009-2013, 2016-2019 and 2022). Net ecosystem exchange (NEE), representing the balance between carbon release (Reco) and carbon uptake (GPP), showed a clear source-to-sink transition, with net uptake peaking during the reproductive period, especially at the flowering stage. Seasonal NEE for the DIPM cotton system ranged from -334.07 to -431.62 g C m-2, with the flowering stage contributing about 61.66% of seasonal GPP and 77.77% of seasonal NEE. Biotic factors, particularly canopy development and biomass accumulation, dominated carbon fluxes during vegetative and early reproductive stages, while temperature, vapor pressure deficit and soil moisture played more influential roles during late reproduction. Carbon budget accounting revealed a trade-off: DIPM increased harvest carbon export but reduced residue return. Despite this, straw return remained the key management practice sustaining a strongly positive net ecosystem carbon budget (NECB) of 332.64 ± 57.52 g C m-2, with straw carbon return of 189.64 ± 46.69 g C m-2 and cotton-seed carbon removal of 214.89 ± 28.59 g C m-2. Overall, biotic factors, particularly crop development during flowering, dominate carbon gains in the DIPM cotton system, with straw return serving as the essential management lever for sustaining the net carbon sink.