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

Manganese (Mn) is a redox‐active micronutrient that has been shown to accelerate plant litter decomposition; however, the effect of Mn‐promoted decomposition on soil C storage is unclear. We present a novel biogeochemical model simulating how Mn bioavailability influences soil organic C (SOC) stocks in a soil profile (<50 cm) within a temperate forest. In our model, foliar Mn increased in response to increasing soluble Mn released through Mn‐oxide (birnessite) dissolution in mineral soil layers. The ensuing Mn enrichment in leaf litter redistributed Mn to the surface forest floor layer, promoted enzymatic oxidation of lignin, and decreased SOC stocks. Total SOC loss was partially mitigated by accumulation of lignin‐oxidation products as mineral‐associated organic C. We also explored how Mn‐driven changes to C storage interacted with effects of N deposition and warming. Nitrogen enrichment inhibited Mn‐dependent lignin degradation, increasing SOC stocks and weakening their dependence on Mn bioavailability. Warming stimulated decomposition and reduced C stocks but was less effective at low Mn bioavailability. Our model results suggest that SOC stocks are sensitive to Mn bioavailability because increased plant uptake redistributes Mn to surface soils where it can enhance litter decomposition. Based on our simulations, we predict that Mn becomes limiting to litter decomposition where Mn is poorly soluble. Depletion of bioavailable Mn or other cofactors that are critical to decomposition could limit the response of organic C stocks to warming over time, but quantitative projections of the role of Mn bioavailability in regulating decomposition requires additional measurements to constrain model uncertainties. Plain Language Summary: Carbon that is removed from the atmosphere by plants and stored in soils has the potential to partially offset greenhouse gas emissions and mitigate climate change. However, predictions of soil carbon storage are challenged by limited understanding of complex interactions between biological and geochemical processes that influence how quickly organic matter decomposes in the soil. We developed a novel model that simulates how cycling of the micronutrient manganese between soils and plants impacts carbon storage in a soil profile. Although it has been demonstrated that manganese is involved in breakdown of lignin, an important component of plant litter, its effects on soil carbon storage are unknown. We also explore how effects of manganese interact with two other major global perturbations: warming and nitrogen deposition to soils. From our model results, we determine that high levels of manganese uptake by plants ultimately generate manganese‐rich surface soils that promote litter decomposition and decrease soil carbon storage. Nitrogen deposition increased carbon storage by inhibiting effects of manganese on decomposition. Decomposition increased with warming but was inhibited by low manganese bioavailability. Our model provides a novel framework for simulating how soil properties that control micronutrient availability to plants can impact soil carbon dynamics. Key Points: We developed a biogeochemical model simulating how manganese bioavailability impacts soil carbon storage in temperate forestsManganese redistribution to surface soils by plants enhanced decomposition and decreased carbon storage in the soil profileOur model predicts that low manganese bioavailability may generate nutrient limitation that decreases warming effects on decomposition [ABSTRACT FROM AUTHOR]