Dissolved organic matter molecular diversity and transformation potential are associated with microbial carbon use efficiency in paddy soils.

Microbial carbon (C) use efficiency (CUE) is a key regulator of soil C sequestration, reflecting how assimilated C is partitioned between biomass production and respiration. In paddy soils, how dissolved organic matter (DOM) molecular characteristics and microbial specialization jointly relate to CUE across straw amendments and moisture regimes remains unclear. We incubated paddy soil for 180 days under flooded and unflooded conditions, adding equal C (1.32 g C kg-1 soil) as fresh straw, straw-derived biochar, or decomposed straw (DS). CUE was quantified by H218O labeling; DOM composition and its transformation potential were characterized using FT-ICR MS and paired mass distance (PMD) analysis, respectively. Microbial specialization was quantified using the H2' index derived from microbial-DOM bipartite networks. All three amendments significantly increased CUE under both moisture regimes, with DS producing the largest and most consistent increase. Relative to the control, DS increased microbial C growth by 204% and 172% under unflooded and flooded conditions, respectively, while maintaining the highest CUE. DS increased DOM molecular diversity and showed the highest top 30 PMDs. Partial least squares path modeling showed that higher CUE was associated with greater nutrient availability and lower microbial specialization under unflooded conditions, whereas metabolic constraints showed the strongest negative association with CUE under flooded conditions. Overall, CUE responses varied with substrate quality, DOM molecular properties, microbial specialization, and moisture rather than C input quantity alone. These findings clarify how straw and water management may influence microbial C allocation and potential C retention in paddy soils.