Abiotic and biotic regulation on carbon mineralization and stabilization in paddy soils along iron oxide gradients

Iron (Fe) oxides regulate soil organic carbon (C) content via balancing C processes of stabilization and mineralization. However, abiotic and biotic mechanisms are involved in stabilization (e.g., by adsorption and/or coprecipitation) and decomposition (e.g., by shifting the microbial community) of paddy soil rich in iron oxides remains poorly understood. We examined the mineralization and stabilization of maize-straw-derived C (delta 13C = 5000 parts per thousand), soil priming effects (PE), and soil microbial community structure in four paddy soils, along with Fe oxide concentrations gradient ranging from 13.7 to 55.8 g kg-1 soil (Fe-13, Fe-25, Fe-42, and Fe-55). The paddy soil with the highest Fe content (Fe-55) stabilized 20.5 mg 13C kg 1 soil of the maize-straw-derived C, being significantly greater (P < 0.05) than Fe-13 (5 mg 13C kg 1 soil). The high C:Fe molar ratio of Fe-55 suggests the main pathway of stabilizing the maize-straw-derived C via co-precipitation as Fe-OM. Larger stabilization in Fe55 led to less CO2 emission from maize and SOM, e.g., Fe-55 had 12-16% lower straw mineralization and 8-11% lower PE than Fe-13 during the first 7 days of incubation. Random forest analysis further revealed that Proteobacteria and Actinobacteria (the keystone species, i.e., Gaiella) gave the largest contribution to maize-straw mineralization and PE, while microbial diversity and some microorganisms featured with filamentous hyphae contributed to C stabilization. This study confirmed that the concentration of Fe oxide in paddy soils plays a central role in C sequestration via biotic and abiotic processes, including i) modulation of microbial community diversity and composition, especially the abundance of fungi and Actinobacteria, and ii) physicochemical stabilization of maize-straw-derived C through the formation of Fe-OM complexes via co-precipitation.

Automated summary

This study found that the concentration of iron oxides in paddy soils plays a crucial role in regulating soil organic carbon content through balancing carbon stabilization and mineralization processes. Soils with higher iron content were able to stabilize carbon more effectively, leading to reduced carbon dioxide emissions and influencing soil microbial community structure.