Effect of decomposition products produced in the presence or absence of epigeic earthworms and minerals on soil carbon stabilization
Introduction
It is generally accepted that microbial activity is a major control of soil organic matter (SOM) formation (Schimel and Scheffer, 2012). In recent years, microbial decomposition products have been identified as the main contributors to SOM (Miltner et al., 2012; Kallenbach et al., 2016), by favoring aggregation and also through their association with clay-sized minerals (Cotrufo et al., 2013). They are a source of stabilized SOM, and may be composed of degraded plant litter as well as microbial-derived material (Rumpel et al., 2015; Barré et al., 2018). In this study, we addressed the fate of end products from decomposition processes, which we hypothesized to contain both, microbial residues and partly decomposed plant material.
In soils, earthworms are considered as key engineers that may affect decomposition processes by mixing and simultaneous ingesting fresh organic matter (OM) and minerals. Through the digestion and transformation of these materials in their gut, earthworms may not only enhance carbon mineralization (Lubbers et al., 2015), but also stabilize SOM through the formation of new aggregates (Bossuyt et al., 2005; Shipitalo and Protz, 1989; Schrader and Zhang, 1997). These new aggregates might enhance the physical protection of carbon from microbial mineralization for years to decades (Brown et al., 2000). Recently, experimental evidence showed that earthworms may also enhance the transformation of fresh OM into microbial necromass (Angst et al., 2019) and affect OM transformations in the presence of minerals through their impact on microbial biomass and microbial habitat (Barthod et al., 2020). Despite their potential importance for the stabilization of SOM, the impact and fate of the various types of decomposition products resulting from OM degradation in presence and absence of earthworms is not well understood. Such knowledge may advance our process understanding of SOM turnover.
In this study we focused on decomposition products generated in a decomposition system with high OM concentrations in the presence and absence of clay-sized minerals and epigeic earthworm species. Such conditions may occur at soil surfaces, where aboveground litter inputs are abundant (e.g. agricultural no-till systems, grassland and forest).
We investigated the effect of adding different types of decomposition products, to a silty agricultural soil, on carbon mineralization, microbial biomass and soil organic carbon (SOC) allocation to physicochemical fractions. To this end we carried out a laboratory incubation with materials produced during a 6 months decomposition experiment transforming an OM mixture in the presence or absence of epigeic earthworms and clay-sized minerals. We monitored CO2 emissions after soil addition of decomposition products, studied their effect on microbial biomass and determined SOC contribution to different SOM pools after the end of the experiment.
The general paradigm is that epigeic earthworm species ingest exclusively OM and do not contribute to organo-mineral complex formation. However, recent studies indicated that these species may be able to form such interactions (Barthod et al., 2020), and stabilize OM through the formation of aggregates and/or organo-mineral complexes containing microbial-derived material (Le Mer et al., 2020).
We thus hypothesized that epigeic earthworms are able to form organo-mineral complexes and that the addition of decomposition products, containing minerals, formed in the presence of earthworms decreases SOC mineralization.
The aim of the study was to evaluate:
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how contrasting decomposition products produced in the presence or absence of minerals and earthworms affect microbial activity and carbon use efficiency after soil application.
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how decomposition products affect soil physicochemical parameters, SOC pool allocation and soil nitrogen
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if decomposition products of epigeic earthworms contribute to SOC stabilization
Section snippets
Decomposition products
Decomposition products were obtained during a 6 months decomposition experiment carried out in a model system inspired from composting or vermicomposting systems (Barthod et al., 2016). To this end, fresh organic materials composed of lettuce, apples, residual maize, spent coffee grounds and pieces of cardboard (2 × 3cm) were decomposed in the presence or absence of epigeic earthworms (Eisenia andrei and foetida) and different types and quantities of minerals during 196 days. We used products
Physico-chemical soil parameters after incubation
After 79 days of incubation, the carbon content of the un-amended control soil remained unchanged, whereas the addition of organic materials to soil increased their total SOC and total N contents, with total SOC contents ranging between 11 and 16 mg g−1 and a N content ranging between 1.5 and 2.0 mg g-1 (Table 3). SOC contents and C/N ratios of amended soils showed significant differences when the decomposition products were produced in the presence of a kaolinit/goethite mixture (KG). For the
Discussion
Effect of decomposition products on SOC mineralization, CUE and microbial biomass carbon.
The nature of decomposition products and in particular their production in the presence or absence of earthworms affected SOC mineralization. Generally, the addition of No EW decomposition products induced increased SOC mineralization. This is in agreement with studies on compost and vermicompost addition to a similar soil type (Ribeiro et al., 2010) and may be explained by priming effects induced by the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The study was financed by CNRS under the framework EC2CO program (LOMBRICOM project) and ADEME under the framework of the DOSTE program (VERMISOL project). The PhD fellowship of J.B. was provided by Sorbonne University. This study contributes to the U2 worm project financed by ANR (ANR-20-CE01-0015-01).
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2022, GeodermaCitation Excerpt :For instance, the fresh casts of L. castaneus emitted on average significantly more CO2 compared to the ones of endogeic species showing lower OC contents (Table 1). These relationships were consistent with previous studies that demonstrated a stimulating effect of OM incorporation into earthworm casts on OC mineralisation (Barthod et al., 2020, 2021; Brown et al., 2000; Chotte et al., 1998; Hamer et al., 2004; Kuzyakov and Kuzyakov, 2010). On the other hand, the casts’ microstructural features (Table 3) seemed to drive an important part of the OC mineralization dynamics during cast ageing (Fig. 3), in particular (connected) porosity, pores (Fig. 1) and (connected) POM contents (Fig. 2).