Iron-mediated organic matter decomposition in humid soils can counteract protection

Chunmei Chen,Steven J Hall,Aaron Thompson,Elizabeth Coward

doi:10.1038/s41467-020-16071-5

Chunmei Chen, Steven J Hall + Show 2 more

Open Access

https://doi.org/10.1038/s41467-020-16071-5

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Abstract

Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe also promotes SOM decomposition when oxygen (O2) becomes limited. Here we quantify Fe-mediated OM protection vs. decomposition by adding 13C dissolved organic matter (DOM) and 57FeII to soil slurries incubated under static or fluctuating O2. We find Fe uniformly protects OM only under static oxic conditions, and only when Fe and DOM are added together: de novo reactive FeIII phases suppress DOM and SOM mineralization by 35 and 47%, respectively. Conversely, adding 57FeII alone increases SOM mineralization by 8% following oxidation to 57FeIII. Under O2 limitation, de novo reactive 57FeIII phases are preferentially reduced, increasing anaerobic mineralization of DOM and SOM by 74% and 32‒41%, respectively. Periodic O2 limitation is common in humid soils, so Fe does not intrinsically protect OM; rather reactive Fe phases require their own physiochemical protection to contribute to OM persistence.

Highlights

Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe promotes SOM decomposition when oxygen (O2) becomes limited
One important route of Mineral-associated organic matter (MAOM) formation involves the oxidation of FeII to FeIII at redox interfaces and its rapid hydrolysis to SRO FeIIIoxides, which coprecipitate with DOM20
Consistent with a protective role, under static oxic conditions we found that FeII oxidation in the presence of added 13C-dissolved organic matter (DOM) resulted in SRO Fe–C associations that inhibited the mineralization of 13C-DOM by 35% relative to controls, and suppressed the priming of native SOM mineralization by 47%, which decreased overall CO2 production by 22% (Fig. 1d)

Summary

Introduction

Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe promotes SOM decomposition when oxygen (O2) becomes limited. Consistent with a protective role, under static oxic conditions we found that FeII oxidation in the presence of added 13C-DOM resulted in SRO Fe–C associations that inhibited the mineralization of 13C-DOM by 35% relative to controls, and suppressed the priming of native SOM mineralization by 47%, which decreased overall CO2 production by 22% (Fig. 1d).

Results

Conclusion