How relevant is recalcitrance for the stabilization of organic matter in soils?

Abstract

Traditionally, the selective preservation of certain recalcitrant organic compounds and the formation of recalcitrant humic substances have been regarded as an important mechanism for soil organic matter (SOM) stabilization. Based on a critical overview of available methods and on results from a cooperative research program, this paper evaluates how relevant recalcitrance is for the long-term stabilization of SOM or its fractions. Methodologically, recalcitrance is difficult to assess, since the persistence of certain SOM fractions or specific compounds may also be caused by other stabilization mechanisms, such as physical protection or chemical interactions with mineral surfaces. If only free particulate SOM obtained from density fractionation is considered, it rarely reaches ages exceeding 50 y. Older light particles have often been identified as charred plant residues or as fossil C. The degradability of the readily bioavailable dissolved or water-extractable OM fraction is often negatively correlated with its content in aromatic compounds, which therefore has been associated with recalcitrance. But in subsoils, dissolved organic matter aromaticity and biodegradability both are very low, indicating that other factors or compounds limit its degradation. Among the investigated specific compounds, lignin, lipids, and their derivatives have mean turnover times faster or similar as that of bulk SOM. Only a small fraction of the lignin inputs seems to persist in soils and is mainly found in the fine textural size fraction (<20 lm), indicating physico-chemical stabilization. Compound-specific analysis of 13 C : 12 C ratios of SOM pyrolysis products in soils with C3-C4 crop changes revealed no compounds with mean residence times of > 40-50 y, unless fossil C was present in substantial amounts, as at a site exposed to lignite inputs in the past. Here, turnover of pyrolysis products seemed to be much longer, even for those attributed to carbohydrates or proteins. Apparently, fossil C from lignite coal is also utilized by soil organisms, which is further evidenced by low 14 C concentrations in microbial phospholipid fatty acids from this site. Also, black C from charred plant materials was susceptible to microbial degradation in a short-term (60 d) and a long-term (2 y) incubation

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

“…This result was of high importance, supporting the largely anecdotal evidence that shrub dominated the areas across the basin study area that is used to be dominated by grasses [Buffington and Herbel, 1965]. At the Los Piños sites, due to the similarities between grama grass and juniper n-alkane signatures, there was a degree of difficulty in determining between sources; based upon n-alkane signature, a problem was previously experienced when using n-alkane signatures alone [Marschner et al, 2008;Gocke et al, 2011]. However, the use of n-alkane signatures from piñon and combined with the δ 13 C values of both piñon and juniper demonstrated that the same trends transcend to the mountainous, grassland to piñon-juniper transition [Wilcox et al, 2003b].…”

Persistence of soil organic matter as an ecosystem property

“…New observations show these to be only marginally important for organic matter cycling 12,13 . Furthermore, loose use of the term 'recalcitrance' has significantly confused the discussion in the past.…”

“…However, using compound-specific isotopic analysis, molecules predicted to persist in soils (such as lignins or plant lipids) have been shown to turn over more rapidly than the bulk of the organic matter ( Fig. 1) 12,[15][16][17] . Furthermore, other potentially labile compounds, such as sugars, can persist not for weeks but for decades.…”

“…Even components that appear chemically labile, including proteins and saccharides of plant and microbial origin ('Different biological sources'), instead seem to turn over (on average) at rates similar to those of bulk SOM, that is, on the order of years or even decades. Thus, over time, the importance of initial quality fades and the initially fast-cycling compounds are just as likely to persist as the slow 12,15 . This figure compiles data from surface horizons of 20 long-term field experiments (up to 23 years) in temperate climate, using 13 C labelling to trace the residence time of bulk SOM and of individual molecular compounds.…”

Mechanisms of organic matter stabilization and destabilization in soils and sediments: conference introduction

“…In a talk considering the importance of recalcitrance as a stabilization mechanism for soil organic matter, Bernt Marschner reported that there was little evidence for it, except in the case of black carbon and fossil carbon (see : Marschner et al 2008). The recalcitrance of black carbon was further discussed by both Michael Schmidt (see: Preston and Schmidt 2006) and Johannes Lehmann (see: Nguyen et al 2008), with the latter providing evidence that even here interactions with minerals may play an important role in retarding degradation.…”