Zero-valent iron addition stimulated straw-derived carbon accumulation in submerged soil

Soil organic carbon (SOC) is the key indicator of soil fertility, and its stabilization plays a significant role in terrestrial carbon cycling. The turnover of SOC often interacts with iron due to its active redox characteristics, especially in paddy soils. Zero-valent iron (ZVI) is highly reductive, while how its addition controls SOC turning over in soil received less attention and knowledge gap exists on soil C dynamics. Therefore, we investigated the influence of ZVI addition on soil and straw C dynamics in submerged soil over a period of 30 days in an incubation study. The results showed that ZVI addition enhanced the endogenous Fe(III) reduction, and the exogenous ZVI was oxidized mostly to Fe(II), resulting in substantial Fe(II) accumulation, with the content 33.39-164.9% higher than that in non-ZVI treatments after incubation. At the initial stage of incubation during day 0-10, ZVI addition showed a stimulated effect on OC accumulation as both the cumulative CO2 emission and dissolved organic carbon (DOC) released were reduced in ZVI-amended treatments. This could be attributed to the O2 limitation on OC mineralization caused by the lower oxidation-reduction potential (ORP) by ZVI addition. Besides, endogenous Fe(III) reduction boosted by ZVI addition could release fresh surfaces on Fe mineral for C accumulation by forming organics-Fe mineral complexes. During the latter stage of day 10-30, ZVI addition showed an inhibited effect on OC accumulation as both the cumulative CO2 emission and DOC released were increased in ZVI-amended treatments, and CH4 emission was vigorous after a 10-day lag phase. Labile OC was liberated and vulnerable to decomposition by microorganisms due to the dissolution of OC-Fe(III) oxide complexes as Fe(III) reduction was enhanced by ZVI addition. Furthermore, CH4 emission was promoted by ZVI addition through lowering soil ORP and supplying electrons for methanogenesis. ZVI addition mitigated OC decomposition during the initial stage and stimulated that during the latter stage of incubation. Overall, ZVI addition increased straw C accumulation in the form of insoluble carbon (Caccumulated), but the different responses of C dynamic between days 0-10 and 10-30 led to a statistic insignificant positive effect. However, the association between iron oxides and OC was enhanced in ZVI addition treatments due to the anaerobic corrosion of ZVI, which may increase the stability of OC, indicating a potential to accumulate more straw C in the long term.