Conversion from double-rice to maize-rice increases iron-bound organic carbon by "iron gate" and "enzyme latch" mechanisms.

[Display omitted] • Double rice conversion to maize-rice (MR) increased Fe-SOC content. • Ferrous iron oxidation and low phenol oxidase activity stabilized Fe-SOC under MR. • Iron gate theory dominated in maize season whereas enzyme latch in rice in MR. • Tradeoffs between enzyme latch and iron gate preserve Fe-SOC in MR system. To assess the impact of the incorporation of maize into double-rice (RR) on soil organic carbon (SOC) sequestration, a two-year experiment was conducted and changes in iron-bound SOC (Fe-SOC), as a proxy for stable carbon pool, were measured. The introduction of a maize-rice (MR) rotation enhanced Fe-SOC by 12–17 % in the maize season, and it stayed 13–36 % higher during rice cultivation compared to RR. Following the "iron gate" theory, Fe oxidation in the maize season was the leading mechanism for improving Fe-SOC content. However, "enzyme latch", i.e. the suppression of Fe-SOC loss by decreased phenol oxidase activity, was a dominant factor controlling the increase of Fe-SOC in the rice season of MR. Thus, the introduction of maize into the paddy field increased Fe-SOC, which was caused by combination of "iron gate" mechanism in maize season and "enzyme latch" in rice season. [ABSTRACT FROM AUTHOR]