Biogeochemical limitations of carbon stabilization in forest subsoils^#

Abstract

Background Soils are important carbon (C) sinks or sources and thus of utmost importance for global carbon cycling. Particularly, subsoils are considered to have a high potential for additional C storage due to mineral surfaces still available for sorptive stabilization. Aims Little information exists about the extent to which additional litter‐derived C is transferred to and stabilized in subsoils. This study aimed at evaluating the role of litter‐derived dissolved organic matter (DOM) inputs for the formation of stable mineral‐associated C in subsoils. Methods We carried out a multiple‐method approach including field labeling with 13C‐enriched litter, exposure of 13C‐loaded reactive minerals to top‐ and subsoils, and laboratory sorption experiments. Results For temperate forest soils, we found that the laboratory‐based C sink capacity of subsoils is unlikely to be reached under field conditions. Surface C inputs via litter leachates are little conducive to the subsoil C pool. Only 0.5% of litter‐derived C entered the subsoil as DOM within nearly 2 years and most of the recently sorbed C is prone to fast microbial mineralization rather than long‐term mineral retention. Desorption to the soil solution and an adapted microbial community re‐mobilize organic matter in subsoils faster than considered so far. Conclusions We conclude that the factors controlling the current mineral retention and stabilization of C within temperate forest subsoils will likewise limit additional C uptake. Thus, in contrast to their widely debated potential to accrue more C, the role of forest subsoils as future C sink is likely overestimated and needs further reconsideration.

Soil organic carbon stabilization in forest subsoils: Directions for the research community – Comment on “Biogeochemical limitations of carbon stabilization in forest subsoils” by Patrick Liebmann et al., Journal of Plant Nutrition and Soil Science, 185(1), 35–43 (2022)

“…It is unclear how these observations interact with different soil environmental conditions and profile characteristics to govern SOC dynamics over timescales meaningful for SOC stock changes. Liebmann et al (2022) indicated that surface litter derived-C does not strongly contribute to SOC stabilization in a temperate forest subsoil. They also concluded that the role of forest subsoils as a future C sink needs further consideration.…”

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Liebmann et al (2022) unveiled some of the biogeochemical mechanisms that can restrict the stabilization of litter-derived carbon (C) as soil organic carbon (SOC) in subsoils beneath a temperate beech (Fagus sylvatica L.) forest. We applaud the multi-pronged approach the authors invoked to track the potential fate of aboveground C in subsoils.

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“…They suggest that the mineral-controlled potential of temperate forest subsoils to serve as a persistent repository of SOC may be overestimated. Liebmann et al (2022) proposed three reasons why their observed subsoil SOC accrual rates may have been limited: (1) Dissolved organic C inputs into subsoils, which potentially bind to mineral surfaces, may have been too small to induce substantial additional sorption; (2) a further accumulation of C beyond extant SOC stocks would have required substantially enhanced inputs of highly sorptive, plant-derived organic compounds to subsoils over long periods; and…”

“…To help address some of these questions, Liebmann et al. (2022) combined field labelling with 13 C‐enriched litter and exposure of 13 C‐loaded reactive minerals to top‐ and subsoil conditions at a temperate forest site, and laboratory sorption experiments in three temperate forest soils to assess the role of litter‐derived dissolved organic matter (DOM) inputs for the formation of stable mineral‐associated C in subsoils. They suggest that the mineral‐controlled potential of temperate forest subsoils to serve as a persistent repository of SOC may be overestimated.…”

“…Liebmann et al. (2022) proposed three reasons why their observed subsoil SOC accrual rates may have been limited: (1) Dissolved organic C inputs into subsoils, which potentially bind to mineral surfaces, may have been too small to induce substantial additional sorption; (2) a further accumulation of C beyond extant SOC stocks would have required substantially enhanced inputs of highly sorptive, plant‐derived organic compounds to subsoils over long periods; and (3) increased C input into forest subsoils may have promoted the formation of relatively labile mineral‐associated organic C and microbial mobilization of the inherent SOC (i.e., a positive priming effect). We highlight that this study focused on the fate of surface litter derived‐C in a sandy, temperate forest soil where soil C inputs and outputs were supposedly in steady‐state equilibrium after more than a century of no major disturbances or changes in tree species composition.…”