Land Use Affects Manganese Bioavailability in Soils

Agricultural management practices alter manganese cycling in agroecosystems compared to unmanaged forest soils.

February 25, 2025

Image is described in caption.

Conceptual diagram of manganese cycling between plants and soils and interactions with organic carbon in temperate forests and agroecosystems, emphasizing differences in plant availability and organo-mineral associations.

[Reprinted with permission from Montgomery, A., et al. 2025. “Linking Manganese Fractions and Organic Carbon in Soils of Contrasting Land Use Systems,” ACS Agricultural Science and Technology 5(3), 316–20. DOI:10.1021/acsagscitech.4c00546. © 2025 American Chemical Society.]

The Science

Manganese (Mn) is an essential micronutrient stored in soils in forms with varying availability to plants. Fertilizer containing Mn is often added to agricultural soils to overcome nutrient limitation, but chemical reactions within the soil can convert added Mn to forms inaccessible to plants. Furthermore, Mn oxide minerals that form within soils can bind organic matter and protect it from degradation, influencing soil health. To test the effects of land use on Mn cycling and Mn-carbon interactions, researchers conducted Mn speciation and carbon analyses on similar soils collected from managed agricultural systems and adjacent unmanaged forest land.

The Impact

This study provides information on how management practices affect Mn storage in agricultural soils relative to unmanaged forest soils. Findings indicate while management practices can increase total soil Mn content, crop harvest and soil liming limit plant-available Mn by removing Mn in aboveground biomass and promoting formation of Mn oxides. Conversion of Mn fertilizer to nonplant-available forms should be considered when planning treatments.

Summary

Mn influences carbon cycling by binding or oxidizing soil organic matter. Mn fractions in soil and their plant availability depend largely on soil pH, which is commonly altered by agricultural practices. Mn fractions in soils range from readily available (e.g., bioavailable Mn and exchangeable Mn) to stabilized (e.g., Mn-oxide minerals and Mn contained in silicates). The distribution of soil Mn with depth was assessed in contrasting land-use systems (organic agriculture, conventional agriculture, and unmanaged forest) using a sequential extraction method that targets Mn fractions ranging in bioavailability. Both agricultural sites had similar amounts of total Mn but lower plant-available Mn (1 to 7% of total Mn) than the unmanaged forested site (15% of total Mn). Lower Mn availability in the agricultural sites likely derived from both crop harvest that removed Mn from the system each year and liming that increased soil pH and favored conversion of bioavailable Mn to Mn oxides. Mn and organic carbon were generally positively correlated, but this relationship depended on soil depth, Mn fractions, and concentrations, while land management change had little influence. Mn oxides and soil carbon were strongly correlated in subsoils of agricultural systems, indicating formation of Mn oxides in these less acidic soils promoted carbon storage through organo-mineral associations.

References

Montgomery, A., et al. "Linking Manganese Fractions and Organic Carbon in Soils of Contrasting Land Use Systems." ACS Agricultural Science and Technology 5 (3), 316–20  (2025). https://doi.org/10.1021/acsagscitech.4c00546.