Plant‐Soil Relationships Influence Observed Trends Between Manganese and Carbon Across Biomes

Abstract

Manganese (Mn) is a micronutrient essential for plant growth and a redox-sensitive metal whose bioavailability to microorganisms may play an important role in regulating carbon (C) storage in surface soils (Berg et al., 2015;Jones et al., 2020;Kranabetter, 2019;Stendahl et al., 2017). Bioavailable Mn(II) is released into the solution during litter decomposition and through proton-promoted and reductive dissolution of Mn(III/IV)-oxide minerals that are abundant and ubiquitous in soils (Post, 1999). Plant roots take up soluble Mn(II) from the soil rhizosphere since other existing forms of Mn [Mn(III) and Mn(IV)] are unavailable to plants (Alejandro et al., 2020;Pittman, 2005). Although large reservoirs of Mn(II) can slowly accumulate in woody biomass over a long time-scale, most of the Mn(II) taken up by plants is stored in the foliar tissues each year (Herndon et al., 2015), where Mn concentrations can be up to 12,000 μg g −1 dry-mass (Fernando et al., 2009;Foulds, 1993;St. Clair & Lynch, 2005) with negligible reabsorption during senescence (McCain & Markley, 1989). When Mn returns to soil as Mn(II) in litterfall, it either leaches or is oxidized and converted to Mn(III/IV) oxides as Abstract Manganese (Mn) is an essential plant micronutrient that plays a critical role in the litter decomposition by oxidizing and degrading complex organic molecules. Previous studies report a negative correlation between Mn concentrations and carbon (C) storage in organic horizons and suggest that high Mn concentrations in leaf litter reduce soil C storage in forest ecosystems, presumably by stimulating the oxidation of lignin by fungal enzymes. Yet, the relationship between Mn and C in the litter layer and organic soil remains poorly understood and restricted to a few biomes, hampering our ability to improve mechanistic understanding of soil C accumulation. To examine plant-soil interactions that underlie observed relationships between Mn and C across a wide range of biomes, we extracted biogeochemical data reported for plants and soils from the National Ecological Observatory Network (NEON) database. We found that increased C and nitrogen (N) storage in organic horizons were associated with declines in Mn concentrations across diverse ecosystems at the continental scale, and this relationship was associated with the degree of organic matter decomposition (i.e., O i , O e , and O a ). Carbon and N stocks were more strongly correlated with Mn than with climatic variables (i.e., temperature and precipitation). Foliar Mn was strongly correlated with foliar lignin, and both these parameters increased with a decrease in soil pH, indicating links between soil pH, foliar chemistry, and litter decomposability. Our observations suggest that increased Mn bioavailability and accumulation in foliage under moderately acidic soil conditions support fungal decomposition of lignin-rich litter and contributes to lower soil C stocks.Plain Language Summary Soils contain substantial amounts of carbon that can be stored for hundreds to thousands of ye...

Modeling Interactive Effects of Manganese Bioavailability, Nitrogen Deposition, and Warming on Soil Carbon Storage

“…(2023) similarly reported increased CO 2 production and C transfer from POM to MAOM stocks with Mn addition to agricultural soils (Neupane et al., 2023). Overall, total soil C stocks, including the forest floor and mineral soil layers, decreased with higher bioavailable Mn (Figure 4c), similar to patterns observed across multiple biomes (Kranabetter, 2019; Santos & Herndon, 2023; Stendahl et al., 2017).…”

“…High Mn 2+ concentrations in leaf litter and organic horizons are proposed to reduce soil C storage by accelerating decomposition (Kranabetter, 2019; Santos & Herndon, 2023; Stendahl et al., 2017). Soil fungi within Basidiomycetes produce extracellular Mn‐dependent peroxidase (MnP) enzymes that convert Mn 2+ to diffusible Mn 3+ ‐chelates that indiscriminately oxidize phenolic bonds within lignin and polyphenols (Hofrichter, 2002; Keiluweit et al., 2015; Kellner et al., 2014; Morgenstern et al., 2008).…”

Exchangeable manganese regulates carbon storage in the humus layer of the boreal forest

“…Notably, other potential regulators of the C pool, such as soil N concentration ( 6 ) and N deposition ( 9 ), were secondary in their effects on soil C pools and had indirect effects through altering fungal activity. In addition to boreal forests ( 5 , 6 , 8 , 10 ), negative correlations between Mn and humus C pool have been documented in temperate forests ( 8 – 10 ).…”

“…Though climate has always been recognized to play a critical role in regulating the persistence and decomposition of soil organic matter (SOM) in boreal regions ( 4 ), the availability of Mn could overshadow its influence ( 5 , 8 ). Earlier investigations suggest that exchangeable Mn and C storage in organic horizons are negatively correlated ( 8 , 10 ). Keiluweit et al ( 11 ) identified that Mn-redox cycling mediated by litter-decomposing fungi enhanced the oxidative breakdown of lignin and other aromatic litter components, thus implying that Mn bioavailability to the fungal community significantly reduces humus C storage by stimulating the rate and extent of organic matter decomposition.…”

Fine root decomposition in forest ecosystems: an ecological perspective

“…However, significant variation remains unexplained, both worldwide and regionally, as plant tissue breakdown rates are positively associated with MAT and MAP (Santos and Herndon, 2023). Higher precipitation provides moisture, while warmer temperatures increases microbial activity (Wahid et al, 2020).…”