Cover cropping induces the formation of Mn(II) hotspots within close proximity of particulate organic matter

Manganese plays a crucial role in the cycling of soil organic matter. Under aerated conditions, Mn (IV) is the most common oxidation state of Mn(IV). Mn(IV) can be reduced to Mn(III) and then to Mn(II) by processes such as waterlogging which constrains O2 diffusion into the soil and prompts microbes to use oxidised metals, such as Mn(IV), as terminal electron acceptors. The oxidation of these reduced Mn species leads to the formation of Mn oxides, which occlude and adsorb organic matter, thereby protecting soil organic matter against microbial degradation. Cover crops, known to enhance soil C content and modify soil pore structure, may affect Mn oxidation and reduction, potentially contributing to soil C gain. The objective of this study was to examine the effects of cover cropping on the oxidation state and microscale distribution of Mn in relation to pores and particulate organic matter. Intact cores (5 cm diameter, 2.5 cm height) were collected from 7.5 to 10 cm depth in soils of two experimental sites. The first was an Alfisol from Kellogg’s Biological Station Long-Term Ecological Research Experiment in Central Michigan. The second was an Ultisol from the USDA Farming Systems Project Long-Term Agricultural Research Experiment in Beltsville, Maryland. All treatments underwent annual tillage under a corn-soybean-wheat rotation. The cover crop treatments included a leguminous winter cover crop. The Alfisol and Ultisol soils had sandy-loam and silty-clay textures, respectively. To determine the Mn oxidation state and distribution in relation to the soil pores, X-ray µCT, X-ray absorption near-edge structure (XANES) spectroscopy, and X-ray Fluorescence (XRF) imaging were used. We identified that under conventional agriculture, Mn(II) and Mn(III) hotspots were located 150 µm from the soil pores, whereas Mn(IV) was found to be directly adjacent to soil pores. In both soils, cover cropping increased the area of Mn(II) by 1.15 times, with these hotspots surrounding particulate organic matter. While this was also true for Mn(III) in the Maryland soil, with Mn(III) being located at close distances to particulate organic matter under cover cropping, we did not observe a similar trend in the Michigan soil which may be due to contrasting soil textures. These findings suggest that additional organic matter inputs through cover cropping may induce the formation of reduced Mn. Re-oxidation of these lower oxidation state hotspots to Mn(IV) may play a pivotal role in enhancing soil C persistence.