Relevance of aboveground litter for soil organic matter formation – a soil profile perspective

Abstract

Abstract. In contrast to mineral topsoils, in subsoils the origin and processes leading to the formation and stabilization of organic matter (OM) are still not well known. This study addresses the fate of litter-derived carbon (C) in whole soil profiles with regard to the conceptual cascade model, which proposes that OM formation in subsoils is linked to sorption–microbial processing–remobilization cycles during the downward migration of dissolved organic carbon (DOC). Our main objectives were to quantify the contribution of recent litter to subsoil C stocks via DOC translocation and to evaluate the stability of litter-derived OM in different functional OM fractions. A plot-scale stable isotope-labeling experiment was conducted in a temperate beech forest by replacing the natural litter layer with 13C enriched litter on an area of 20 m2 above a Dystric Cambisol. After 22 months of field exposure, the labeled litter was replaced again by natural litter and soil cores were drilled down to 180 cm soil depth. Water extraction and density fractionation were combined with stable isotope measurements in order to link the fluxes of recent litter-derived C to its allocation into different functional OM fractions. A second sampling was conducted 18 months later to further account for the stability of translocated young litter-derived C. Almost no litter-derived particulate OM (POM) entered the subsoil, suggesting root biomass as the major source of subsoil POM. The contribution of aboveground litter to the formation of mineral-associated OM (MAOM) in topsoils (0–10 cm) was 1.88±0.83 g C m−2 and decreased to 0.69±0.19 g C m−2 in the upper subsoil (10–50 cm) and 0.01±0.02 g C m−2 in the deep subsoil >100 cm soil depth during the 22 months. This finding suggests a subordinate importance of recent litter layer inputs via DOC translocation to subsoil C stocks, and implies that most of the OM in the subsoil is of older age. Smaller losses of litter-derived C within MAOM of about 66 % compared to POM (77 %–89 %) over 18 months indicate that recent carbon can be stabilized by interaction with mineral surfaces; although the overall stabilization in the sandy study soils is limited. Our isotope-labeling approach supports the concept of OM undergoing a sequence of cycles of sorption, microbial processing, and desorption while migrating down a soil profile, which needs to be considered in models of soil OM formation and subsoil C cycling.

Contribution of Particulate and Mineral-Associated Organic Matter to Potential Denitrification of Agricultural Soils

“…Thus, POM clearly has a higher potential to fuel denitrification than MOM. This is well in line with previous studies showing that MOM comprises mainly less bioavailable C compounds (e.g., Kalbitz et al, 2005;Liebmann et al, 2020). Correspondingly, we found that only 22-28% of the initial WEOC in soils with MOM addition was emitted as CO 2 within 24 h, while 37-63% of initial WEOC was mineralized in soils with POM addition.…”

“…Thus, MOM represents a continuum of mineral-bound C substances of varying availability and degradability (e.g., Mikutta et al, 2007). Especially in topsoils with high OC inputs and mineral C loadings, OC can repeatedly be displaced and desorbed from mineral surfaces (Leinemann et al, 2016;Liebmann et al, 2020). Consequently, MOM is a potential source of OC fueling denitrification, with its contribution probably depending on the amount and quality of WEOC.…”

Vertical mobility of pyrogenic organic matter in soils: a column experiment

“…Subsoils are unsaturated in OM due to little inputs, such as DOC from upper soil horizons, root-derived and already microbially processed OM, but large mineral surfaces (Kaiser and Kalbitz, 2012;Lützow et al, 2006). Therefore, the probability of mobilized PyOM to interact with the mineral phase is higher in the subsoil, resulting in a higher retention compared to topsoils where OM is already occupying sorption sites on the mineral surfaces.…”

Synergy between compost and cover crops leads to increased subsurface soil carbon storage

“…Compost application is associated both with high levels of mobile DOC (Wright et al, 2008;Zmora-Nahum et al, 2005), as well as a large proportion of aromatic functional groups, attributed to high amounts of lignin and other biomolecules (Leifeld et al, 2002). Research into the transport of organic molecules, such as described by the "cascade theory", has shown that "fresh" carbon inputs are preferentially retained within the top 30 cm (Liebmann et al, 2020) and that aromatic moieties are rapidly removed from the soil solution at the surface (Leinemann et al, 2018). This removal may be a function of the relatively low solubilities of non-polar aromatic functional groups (Maxin and Kogel-Knabner, 1995), as well as their tendency to partition into other non-polar, insoluble organic matter (Pignatello, 1999).…”

“…Carbon inputs to subsurface soils are primarily through dissolved organic carbon (DOC) movement (Kaiser and Kalbitz, 2012) and deeper rooting biomass and exudates (Sokol and Bradford, 2019). This contrasts with the greater variety of inputs to surface soils such as compost (White et al, 2020a), plant aboveground biomass/shoots (Liebmann et al, 2020), atmospheric deposition (Mladenov et al, 2012) and abundant shallower root biomass (Poeplau and Don, 2015).…”