Soil Carbon Sequestration: Role of Fe Oxides and Polyphenol Oxidase Across Temperature and Cultivation Systems

The "enzyme latch" and "Fe gate" mechanisms are crucial factors influencing soil carbon sequestration capacity, playing a key role in understanding the dynamic changes in soil organic carbon (SOC). However, there is a lack of research regarding polyphenol oxidase (PPO) activity and the concentration of iron oxides in paddy soils under varying incubating temperatures and cultivation practices. This study was conducted over three years in a double-cropping rice area in southern China, incorporating systematic soil sampling to measure PPO activity, Fe oxide concentration, and basic physicochemical properties. The results showed that temperature did not significantly affect either PPO activity or the concentration of Fe oxides. Additionally, compared to conventional management (CK), organic management led to a decrease in Fe oxides (Fe bound to organic matter, reactive Fe, and total free Fe) by 19.1%, 16.2%, and 13.7%, respectively (p < 0.05). At the same time, PPO activity did not show any significant changes. Our results indicated that short-term (5 weeks) incubation temperature did not affect PPO activity or Fe oxides, while organic farming decreased Fe oxides without influencing PPO activity. PPO activity increased with the length of the incubation period.