Review
The effects of organic inputs over time on soil aggregate stability – A literature analysis

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Abstract

Since the beginning of the last century, many studies have reported evidence describing the effects of organic inputs on soil aggregate stability. In 1965, Monnier proposed a conceptual model that considers different patterns of temporal effects on aggregate stability depending on the nature of the organic inputs: easily decomposable products have an intense and transient effect on aggregate stability while more recalcitrant products have a lower but longer term effect. We confronted this conceptual model with a literature review of experimental data from laboratory and field experiments. This literature analysis validated the conceptual model proposed by Monnier and pointed out gaps in our current knowledge concerning the relationships between aggregate stability and organic inputs. Noticeably, the experimental dataset confirmed the biological and temporal effects of organic inputs on aggregate stability as proposed in the model. Monnier's model also related the evolution of aggregate stability to different microbial decomposing agents, but this relationship was not made clear in this literature analysis. No direct or universal relationship was found between the aggregative factors induced by organic input decomposition (binding molecules or decomposers of biomass) and temporal aggregate stability dynamics. This suggests the existence of even more complex relationships. The model can be improved by considering (i) the direct abiotic effect of some organic products immediately after the inputs, (ii) the initial biochemical characteristics of the organic products and (iii) the effects of organic products on the various mechanisms of aggregate breakdown. For now, no trend is evident in the effect of the rate of organic inputs or the effect of the soil characteristics (essentially carbon and clay contents) on aggregate stability.

Introduction

Aggregate stability is a keystone factor in questions of soil fertility and environmental problems. Resistance of the aggregate to physical stresses determines soil sensitivity to crusting and erosion (Le Bissonnais, 1996a), germination and rooting of cultivated plants (Lynch and Bragg, 1985, Angers and Caron, 1998) and the ability of a soil to store carbon through the physical protection of organic molecules (Jastrow and Miller, 1997). Factors that influence aggregate stability have been frequently reviewed (Le Bissonnais, 1996b, Kay, 1998, Bronick and Lal, 2005, Amezketa, 1999). Texture, clay mineralogy, cation content, aluminium and iron oxides, and soil organic matter are the major soil properties influencing aggregate stability. Organic matter (OM) is a major factor affecting aggregate stability because its abundance and characteristics can be modified by agricultural practices. In many cultivation systems, fresh OM is periodically returned to the soil as litter or crop residues but the amounts and quality of the organic input are variable. This agricultural practice would appear to be an interesting solution for maintaining soil fertility and for soil rehabilitation in degraded situations.

Over years or even over seasons, aggregate stability in the field increases or decreases due to climatic conditions, agricultural practices (tillage or no tillage) and the decomposition of fresh organic input. In several conceptual models the increase of aggregate stability after organic additions to the soil is related to the decomposition dynamics of the inputs. In 1965, Monnier proposed a conceptual scheme (Fig. 1) that describes the effect of different reference products on soil aggregate stability across time scales varying from weeks, months to years after the incorporation. Some products, such as green manure had intense effects within a month on aggregate stability, while less readily decomposable materials, such as decomposed manure, had small initial effects but relatively large effects at the annual scale. Monnier proposed that the short-term effects of organic matter on aggregate stability were due to the turnover of microbial products and cells while the long-term effects were due to humified compounds.

Other authors proposed conceptual models that support this scheme (Guckert et al., 1975, Golchin et al., 1994, Chenu et al., 1998, Six et al., 2000). Formation and stabilization of aggregates by organic matter (Six et al., 2004) and interactions between aggregative factors and aggregate structures (Lynch and Bragg, 1985) have been reviewed already.

However, Monnier's scheme is the only one that simultaneously takes into account the effects of different products and the effect of aggregating factors on aggregate stability following organic inputs.

This review is aimed at identifying the well described although still unclear relationships between the decomposition of organic inputs to soil and aggregate stability. A specific objective was to discuss the validity of the conceptual model proposed by Monnier (1965). For this, we collected literature data about the effects of different organic inputs on soil aggregate stability and compared them with Monnier's scheme. We compared the relationships between binding agents and aggregate stability after organic additions. Then, we examined different points that are not explicitly taken into account in this scheme: the quality (intrinsic characteristics) and the quantity (rate of addition) of the organic product, the direct physico-chemical effect of the product immediately after the input and the effect of soil type in response to an organic product.

Section snippets

Material and methods

We based this literature analysis on the scheme proposed by Monnier in 1965 (Fig. 1). Monnier proposed a conceptual model where the effects of aggregate stability in terms of intensity, time and persistence of the measured effect depend on the organic products. In his model, Monnier presented aggregate stability in three phases identified as zones A, B and C. Zone A corresponds to the initial weeks after the input. During this phase, a maximal intensity effect of organic products on aggregate

Evidence of organic input effects on aggregate stability

Studies focusing on the impacts of organic inputs on aggregate stability are summarized in Table 1. Several studies used model molecules such as monosaccharides (e.g. Martin, 1945a), casein, lignin (e.g. Martin and Waksman, 1942, Martin, 1945b) or humic compounds (Fortun et al., 1989). A large range of complex organic products were also investigated, from different crop residues (e.g. Martens, 2000) to manures (e.g Gerzabek et al., 1995, Paré et al., 1999), composts (e.g. Martin, 1942,

The relationships between soil aggregate stability and binding agents during the decomposition of organic products

Several biological binding agents have been recognised as responsible for aggregation and aggregate stability. Monnier's model emphasises that the peak in aggregate stability is related to microbial decomposing agents. Here we recorded relationships between aggregate stability and binding agents over time after the addition of organic matter to the soil (Table 2). Good correlations were observed, in some cases, with microbial biomass over time (Kushwaha et al., 2001, Sonnleitner et al., 2003,

Underlying questions of Monnier's conceptual model

Several points are not clearly explained or predicted by Monnier's model, but should be taken into account to estimate the effects of an organic product on aggregate stability.

Conclusion

Poor soil structural stability is a serious and increasing problem in several areas of the world. An appropriate management of organic matter additions to soils may increase aggregate stability and thus reduce crusting and erosion problems. With this in mind it is necessary to select the quality, quantity and timing of organic matter additions to achieve the expected increases in aggregate stability over time. The conceptual scheme proposed by Monnier (1965) provides a framework for classifying

Acknowledgements

This study was funded by the French agency for environment and energy (ADEME). The authors would like to thank Sandhya Chennu, Ivan Woodhatch and Michael Corson for their judicious comments and decisive help in smoothing the text. We would also like to acknowledge the useful comments of the anonymous reviewers. Samuel Abiven was funded by the University of Zurich and the Swiss national foundation for Science while writing the manuscript.

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