Effect of long-term straw return on organic matter transformation by hydroxyl radical during paddy soil oxygenation

The generation of hydroxyl radicals (center dot OH) mediated by Fe(II) has drawn increasing amounts of attention due to its significant role in the transformation of soil organic carbon (SOM) during redox fluctuations in paddy soil. This study further examined the effect of long-term (since 1984) straw return on Fe(II) on driving center dot OH formation and SOM transformation upon paddy soils oxygenation. We found that long-term straw return could inhibit the center dot OH and CO2 production in abiotic processes. The center dot OH production decreased from 190.4 to 133.4 mu M as the straw return amount increased from 0 to 19.16 t center dot hm  2 center dot year  1. Moreover, 14.7-38.6 % of the CO2 production was associated with center dot OH during oxygenation. By combining radical quenching and OM extraction, we determined that straw return could increase O2 reduction to H2O2 through a two-electron transfer pathway. Based on chemical extraction, Mo center dot ssbauer spectra and multilinear regression model, we demonstrated that straw return resulted in more active Fe(II) species that controls the center dot OH production. The results of 3DEEMs and FT-ICR MS showed that oxidative transformation can be accomplished by the introduction of center dot OH to aromatic structures or olefins through ring opening and hydroxylation mechanisms. And OM at high straw return concentrations was likely oxidized by center dot OH into bioavailable aliphatic compounds. Therefore, this study highlights the underappreciating effects of long-term straw return on Fe(II) oxidation in driving center dot OH generation and SOM transformation in paddy soil redox fluctuation events.