Humic and Fulvic Acid Fractions Differentially Regulate Methane-Dependent Arsenate Reduction in Paddy Soils

Area of Science:

• Environmental Science
• Soil Science
• Microbiology

Background:

• Arsenic contamination in paddy soils poses a significant threat to global food security.
• Microbial reduction of arsenate (As(V)) to mobile arsenite (As(III)) is a primary driver of arsenic mobilization.
• Methane (CH4)-dependent As(V) reduction (M-AsR) links methane cycling to arsenic release, but the role of soil organic matter (SOM) is unclear.

Purpose of the Study:

• To investigate how structurally distinct soil organic matter (SOM) fractions regulate the methane (CH4)-dependent arsenate (As(V)) reduction (M-AsR) pathway.
• To elucidate the mechanisms by which humic and fulvic acids influence electron transfer and arsenic mobilization in paddy soils.

Main Methods:

• Experimental analysis of two distinct SOM fractions: aromatic, quinone-rich humic acid and carboxyl-rich fulvic acid.
• Quantification of arsenic species (As(III) and As(V)) and microbial gene abundance (pmoA, mcrA, arrA) under different SOM treatments.
• Thermodynamic assessment of methane-driven metabolism.

Main Results:

• Humic acid enhanced electron transfer, promoting CH4 oxidation coupled to As(V) reduction, leading to a ~1.5-fold increase in As(III) release.
• Fulvic acid promoted acetate accumulation, thermodynamically hindering CH4-driven metabolism and largely blocking M-AsR, with only a 47.9% increase in CH4-driven As(III).
• Humic acid acted as an electron shuttle, while fulvic acid disrupted the coupling via acetate accumulation.

Conclusions:

• Soil organic matter structure significantly controls arsenic mobilization pathways in paddy soils.
• Humic acids facilitate arsenic release by enhancing microbial methane oxidation and arsenate reduction.
• Molecular-level characterization of SOM is crucial for predicting arsenic risks in flooded agricultural systems.