Review
A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics

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Abstract

Since the 1900s, the link between soil biotic activity, soil organic matter (SOM) decomposition and stabilization, and soil aggregate dynamics has been recognized and intensively been studied. By 1950, many studies had, mostly qualitatively, investigated the influence of the five major factors (i.e. soil fauna, microorganisms, roots, inorganics and physical processes) on this link. After 1950, four theoretical mile-stones related to this subject were realized. The first one was when Emerson [Nature 183 (1959) 538] proposed a model of a soil crumb consisting of domains of oriented clay and quartz particles. Next, Edwards and Bremner [J. Soil Sci. 18 (1967) 64] formulated a theory in which the solid-phase reaction between clay minerals, polyvalent cations and SOM is the main process leading to microaggregate formation. Based on this concept, Tisdall and Oades [J. Soil Sci. 62 (1982) 141] coined the aggregate hierarchy concept describing a spatial scale dependence of mechanisms involved in micro- and macroaggregate formation. Oades [Plant Soil 76 (1984) 319] suggested a small, but very important, modification to the aggregate hierarchy concept by theorizing the formation of microaggregates within macroaggregates. Recent research on aggregate formation and SOM stabilization extensively corroborate this modification and use it as the base for furthering the understanding of SOM dynamics. The major outcomes of adopting this modification are: (1) microaggregates, rather than macroaggregates protect SOM in the long term; and (2) macroaggregate turnover is a crucial process influencing the stabilization of SOM. Reviewing the progress made over the last 50 years in this area of research reveals that still very few studies are quantitative and/or consider interactive effects between the five factors. The quantification of these relationships is clearly needed to improve our ability to predict changes in soil ecosystems due to management and global change. This quantification can greatly benefit from viewing aggregates as dynamic rather than static entities and relating aggregate measurements with 2D and 3D quantitative spatial information.

Introduction

The interest in long-term sustainability and reduction of environmental costs of agricultural ecosystems have emerged and only recently augmented. To achieve this interest, soil organic matter (SOM) dynamics and nutrient cycling need to be better understood and subsequently managed. In trying to further our understanding of these important dynamic soil properties, recent research focuses often on the role played by the soil matrix, the soil biota and their multiple interactions. It is this multitude of interactions that makes it a very complex research subject to be elucidated.

Studies tackling this complexity often use aggregate measurements as surrogates of the, in itself, complex soil matrix. Aggregates not only physically protect soil organic matter (e.g. Tisdall and Oades, 1982), but also influence microbial community structure (e.g. Hattori, 1988), limit oxygen diffusion (e.g. Sexstone et al., 1985), regulate water flow (e.g. Prove et al., 1990), determine nutrient adsorption and desorption (e.g. Linquist et al., 1997, Wang et al., 2001), and reduce run-off and erosion (e.g. Barthes and Roose, 2002). All of these processes have profound effects on SOM dynamics and nutrient cycling.

The objective of the first section of this review is to give a historical account of the development of theories related to aggregate–SOM interactions. In the second section influences and interactions between the major factors controlling aggregation are synthesized with a special reference to microaggregation and macroaggregate turnover.

Section snippets

Before 1950

All major factors playing a role in aggregate formation and stabilization were already identified in the early 1900s. By then, it was already clear that the following factors influenced soil aggregation: (1) soil fauna; (2) soil microorganisms; (3) roots; (4) inorganic binding agents; and (5) environmental variables (Fig. 1). The most extensively studied group of soil fauna in relation to aggregation is the earthworms. Recently, termites have received a well-deserved greater attention for their

Major factors controlling macroaggregate turnover and microaggregation

In this section, we review and synthesize the research progress since 1950 related to the five major factors (see Fig. 1) influencing aggregation. We focus on the controls of formation and stabilization of microaggregates and the formation and breakdown of macroaggregates. We opted for these two focuses because microaggregates are the structural units within the soil where SOM is predominantly stabilized in the long term and the balance between formation and breakdown of macroaggregates

Conclusions

Major progress has been made in the understanding of the link between aggregates, soil biota and soil organic matter dynamics. This is evident from the continuous evolving of the conceptual models depicting our theoretical understanding of this link. The most important concepts constituting our current understanding are: (1) a hierarchical order of aggregates exists in soil where SOM is the major binding agent; (2) microaggregates are formed within macroaggregates; (3) root-derived POM plays an

Acknowledgements

We would like to thank Martin Carter for organizing the Advances in Soil Structure Research Workshop, 2002, Prince Edward Island, Canada and letting us contribute to the resulting special publication in Soil & Tillage Research. This work was supported by a grant from the National Science Foundation (IBN 9987996).

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    This publication is dedicated to Ted Elliott who personally taught many of the concepts described here and is in many ways “the grandfather” of many of the ideas expressed here.

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