Manganese (Mn) may play an outsized role in soil biogeochemical cycles relative to its abundance. The role of Mn-facilitated oxidation of biomacromolecules during litter decomposition is well-established, but the balance between Mn-promoted soil organic carbon (SOC) oxidation and long-term SOC protection in mineral soils is unknown, especially in subsoils. In this study, we used soils collected across the US National Ecological Observatory Network (NEON) to assess the distribution of Mn and relationships between Mn abundance and SOC concentration, potential mineralization, and persistence at a continental spatial scale. Total reducible Mn was not spatially correlated to site moisture (Spearman’s Rho = 0.24), highlighting that Mn abundance may influence SOC cycling independently from other moisture-driven soil chemical properties (e.g., reactive iron and aluminum). However, Mn effects on SOC cycling depended on depth, soil, or site-level properties. In particular, fungal:bacterial biomass ratio, proportion of SOC in the free light fraction, lignin abundance, and/or proportion of undegraded organic matter mediated the effect of Mn on SOC cycling metrics. For example, the effect of Mn on SOC concentration in subsoils shifted from positive (approximately +270 % relative to mean subsoil SOC) to negative (− 125 %) with increasing fungal:bacterial ratio. We propose that convergence of high Mn, lignin-rich substrates, and fungal:bacterial ratio amplifies lignin mineralization in surface soils, but does not result in higher net SOC turnover due to fungal biomass stabilization. In contrast, we suggest that Mn abundance promotes smaller, but more persistent SOC stocks in subsoils by accelerating SOC transformation from particulate to microbial biomass pools.
Too many people focus on surface soils, and too many people ignore Manganese (Mn)! If you don’t know about Mn’s role in ecosystems processes, the introduction is expertly written by Dr. Angela Possinger (also the primary author) and is worth reading over. We show that Mn abundance at the continental scale is highly variable, but that it can be predictive of certain soil carbon metrics (especially lignin abundance). But, the effects of Mn is pretty unique on location, and at each horizon in the soil illustrating the old addage “it’s all about site specificity!”
These year-long incubation data were first explored in (Weiglein 2021). In that paper we focused on differences between A and B horizons to experimental warming and moisture. We explored over 160 different predictors of soil respiration, I recommend reading for a more thorough understanding of how and why surface and subsoils behave differently.