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

Abstract

Abstract. In contrast to mineral topsoils, the origin and processes leading to the formation and stabilization of organic matter (OM) in subsoils is 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 movement 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 0.99 ± 0.45 g C m−2 yr−1, and decreased to 0.37 ± 0.10 g C m−2 yr−1 in the upper subsoil (10–50 cm) and 0.01 ± 0.01 g C m−2 yr−1 in the deep subsoil > 100 cm soil depth. 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 %) indicate that recent carbon can be stabilized by interaction with mineral surfaces; although the overall stabilization in the sandy study soils was low. 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 on soil OM formation and subsoil C cycling.

Soil organic carbon stocks in sugar beet rotations differing in residue management and associated rotational crop species

“…This might be explained by the maximum depth of reduced tillage and thus maximum incorporation depth of aboveground plant material at 20 cm soil depth. Possible translocations of organic material from the top‐ to the subsoil were found to be small (Liebmann et al., 2020). Differences in SOC stocks between the crop rotations below the incorporation depth would thus depend on differences in root‐derived C inputs below 20 cm, which were apparently not large enough.…”

Synergy between compost and cover crops in a Mediterranean row crop system leads to increased subsoil carbon storage

“…Carboxylate functional groups' ability to form mineral-associated organic matter through association with charged surfaces or cation bridging (Aquino et al, 2011) would also promote MAOM formation and increase C storage times (Cotrufo et al, 2013;Leinemann et al, 2018), though our results do not provide sufficient support to determine the stability of the increased C stocks at 60-100 cm. As the C cascade is triggered by fresh C inputs which are preferentially sorbed within the top 30 cm (Liebmann et al, 2020), the regular application of soluble C-rich compost and WCC residue combined with increased hydraulic conductivity due to WCC roots can accelerate the process, leading to greater subsoil C transport.…”

Aboveground litter inputs determine carbon storage across soil profiles: a meta-analysis

“…However, aboveground litter inputs can also affect C storage in deeper soil horizons, as dissolved organic carbon (DOC) released by leaching and decomposition processes percolates down the soil profile (Kaiser and Kalbitz 2012;Liebmann et al 2020). Although DOC represents an important pathway for C inputs to the subsoil (Kalbitz and Kaiser 2008;Liebmann et al 2020), litter-derived DOC can also cause priming effects, which stimulate the mineralization of extant C, and could offset increases in soil C concentrations in deeper soil horizons (Chen et al 2019). To date, the effects of aboveground litter inputs on soil C storage through the soil profile remain poorly characterized, especially for subsoil horizons (i.e.…”