Soil Microbial Populations Shift as Processes Protecting Organic Matter Change During Podzolization

Marie-Liesse Vermeire,Sebastian Doetterl,Jean-Thomas Cornélis,Eric Van Ranst,Bruno Delvaux,Steeve Bonneville

doi:10.3389/fenvs.2018.00070

Abstract

In the upper part of the solum of mineral soils, soil organic and mineral constituents co-evolve through pedogenesis, that in turn impacts the transformation and stabilization of soil organic matter (SOM). Here, we assess the reciprocal interactions between soil minerals, SOM and the broad composition of microbial populations in a 530-year chronosequence of podzolic soils. Five pedons, derived from beach sand, are studied. From young to old soils, net acidification parallels mineral dissolution and the formation of eluvial and illuvial horizons. Organo-mineral associations (OMA) accumulate in the illuvial B horizon of the older soils (330-530 yrs). Apart from contributing to SOM stabilization and protection, organo-mineral compounds progressively fill up interparticle voids. The subsequent loss of porosity leads to horizon induration, decrease of hydraulic conductivity, which promote redoximorphic processes. While recalcitrant SOM is preserved in the topsoil of the old soils, the largest quantity of protected SOM occurs in the indurated, temporalily waterlogged B horizons, through both the OMA accumulation and inhibition of microbial decomposition. SOM protection is thus both time- and horizon-specific. The microbiota also evolve along the chronosequence. Fungi dominate in all horizons of the younger soils and in the topsoil of the older soils, while bacteria prevail in the cemented B horizons of older soils. This shift in microbial community composition is due to the interdependent co-evolution of SOM and minerals during pedogenesis. Our results call for considering the microenvironment and parameters inherent to decomposer microorganisms to understand SOM protection processes in soils.

Highlights

The fate of soil organic matter (SOM) involves the decomposition of macromolecules into small oxidized and reactive molecules that can interact with the pedogenic products of mineral weathering, and influence their formation (Cotrufo et al, 2013; Basile-Doelsch et al, 2015; Lehmann and Kleber, 2015)
In the surface horizons (P1-BC1, P2-Bw and E in P3, P4, P5), Total Reserve in Bases (TRB) decreases along the sequence from 439 (P1) to 430 (P2), 311 (P3), 236 (P4), and 169 (P5)
Forest development along the chronosequence induces an increasing SOM input from the litter, favored by humid climatic conditions, which promote leaching of solutes produced by mineral weathering

Summary

Introduction

The fate of SOM involves the decomposition of macromolecules into small oxidized and reactive molecules that can interact with the pedogenic products of mineral weathering, and influence their formation (Cotrufo et al, 2013; Basile-Doelsch et al, 2015; Lehmann and Kleber, 2015). During the initial stages of SOM decomposition, some organic compounds are selectively preserved because of intrinsic molecular-level properties that limit their biodegradation (“recalcitrant” compounds). This represents a short-to-medium term C stabilization process— i.e., years or decades—(Kögel-Knabner et al, 2008a; Marschner et al, 2008; Schmidt et al, 2011). In later stages of SOM decomposition, the microbial biomass and byproducts bind to reactive, mineral clay-sized surfaces (Kögel-Knabner et al, 2008b; Kleber et al, 2015). The resulting OMA and soil microaggregates are responsible for the long-term persistence of SOM—i.e., decades to millennia—(Sollins et al, 1996; Baldock and Skjemstad, 2000; von Lützow et al, 2006; Kögel-Knabner et al, 2008b; Kleber et al, 2015)

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