Constant Water-Content Analogues Reveal Wetting–Drying-Regime Effect on Rewetting CO2 Pulses in Six Composite Agricultural Soils from Madhya Pradesh

Mean soil water status can mask the effect of wetting–drying regime on post-rewetting carbon release. This study tested whether repeated drying–rewetting altered CO2 pulses under closely similar time-averaged gravimetric water content across six contrasting composite agricultural soils from Madhya Pradesh, India. Soils were incubated for six 144 h cycles under continuously moist, moderate/severe drying–rewetting (DRW-M/DRW-S), and constant water-content analogue treatments (CA-M/CA-S) designed to approximate the time-averaged gravimetric water content of the oscillatory regimes. The matched contrasts were close in achieved mean gravimetric water content (DRW-M versus CA-M, −0.56 percentage points; DRW-S versus CA-S, 0.15 percentage points), but produced different respiration responses. DRW-S exceeded CA-S in every soil for peak flux and cumulative CO2 release over 0–24, 0–48 and 0–144 h, with mean increases of 12.07 mg C kg–1 h–1 and 186.75, 315.90 and 456.88 mg C kg–1, respectively. The moderate contrast was positive for peak flux and first-day CO2 release, but weakened over longer windows. Early-pulse fractions suggested that severe oscillation shifted a larger share of full-cycle CO2 into the first day. Cycle 6 microbial biomass carbon (MBC) summaries suggested that oscillatory and constant dry treatments differed in endpoint MBC, whereas aggregate metrics were mixed. The wetting–drying regime therefore increased and shifted rewetting CO2 release toward the early post-rewetting period beyond what time-averaged gravimetric water content alone predicted.