Enregistrement scientifique n : 1363 Symposium n : 2 Présentation : orale. ANGERS Denis A.

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1 Enregistrement scientifique n : 1363 Symposium n : 2 Présentation : orale Water-Stable Aggregation of Québec Silty Clay Soils: Some Factors Controlling its Dynamics. Stabilité structurale des sols argilo-limoneux du Québec : Quelques facteurs déterminant son évolution. ANGERS Denis A. Centre de Recherche et de Développement sur les Sols et les Grandes Cultures, Agriculture et Agroalimentaire Canada, 2560 Boul. Hochelaga, Sainte-Foy (Québec) G1V 2J3 Canada. INTRODUCTION Soil structure varies in time and space as a function of soil properties, climatic conditions and land management practices. Through complex interactions, these factors will affect both the physical and biological controls of soil structure. The heavy soils of the St.Lawrence lowlands of eastern Canada are characterized by a high proportion of nonphyllosillicate clay and silt particles, and despite a relatively high organic matter (OM) content, are susceptible to structural degradation in the form of compaction, surface crusting and sheet erosion. The resistance of soils to structural degradation is largely controlled by the presence and abundance of water-stable macroaggregates. Several agronomic field studies have allowed to intensively monitor the changes in water-stable aggregation of Québec fine-textured soils through time. These trials involved various combinations of management practices such as organic additions (e.g. manures, wood-derived residues), crop rotations (e.g. cereal vs. perennial forages) and reduced tillage (e.g. no-till, chisel plowing) and have been in place for 3 to 10 yr. The soils under study were generally developed on marine deposits, and as previously mentioned present a fine texture (clay loams, silty clays and clays), and mixed mineralogy (De Kimpe et al., 1979; Simard et al., 1990). They are generally classified as Humic Gleysol in the Canadian soil classification system and Humaquept in the USDA system. The objective of this brief review is to summarize some of our findings on management factors controlling the dynamics of soil structure in Québec silty clay soils and to present a hypothesis to explain the high responsiveness of the stable macroaggregation of these soils to changes in management practices. 1

2 SEASONAL VARIATIONS The proportion of soil present as water-stable macroaggregates (>250 µm or >1 mm) was used in many studies on Québec soils to monitor the effects of management and climate on the formation and degradation of stable structural units. This parameter was generally measured on whole soil sampled at several dates during the snow-free season and involved wet-sieving of directly immersed soil as described by Angers and Mehuys (1993). Water-stable macroaggregation of Québec clay soils usually showed large temporal variations (e.g. Angers et al., 1993). A significant proportion of the short-range variation (days or weeks) was attributable to climatic conditions as revealed by the high positive correlation between soil water content at time of sampling and aggregation (e.g. Angers et al., 1993). The positive correlation illustrates the increased susceptibility of soil aggregates to slaking as water content decreases since more rapid water entry at low water content induces a greater internal air pressure leading to aggregate breakdown. The relationship between water content and macroaggregation which was found in most of our studies can vary with management systems. Soils under no-till (Angers et al., 1993), alfalfa (Angers, 1992) or perennial grasses (Chantigny et al, 1997) showed greater resistance to slaking at low water content than their annually tilled counterparts. This has practical significance as it is clearly shown that resistance to slaking is related to susceptibility to erosion and surface crusting (Le Bissonnais and Arrouays, 1997). Shortrange temporal variations in macroaggregation can also be induced by the direct mechanical impact of tillage as revealed by the decrease in water-stable aggregation observed after fall and spring tillage operations (Lafond, J., D.A. Angers and D. Pageau, unpublished). ORGANIC MATTER We found that for our soils, long-term trends (>2 years) in stable macroaggregation were often a function of organic matter management i.e. placement of the residues in the soil profile and the amount and quality of the residues. This effect of carbon management was reflected by correlations between aggregation and estimated C inputs (Fig. 1). Stable aggregates >1 mm (%) Clover (MP-DLM) Clover (CP-DLM) Clover (CP-MIN) Clover (MP-MIN) Barley (MP-MIN) Barley (MP-DLM) Barley (CP-DLM) Barley (CP-MIN) r = 0.91** C input (kg C ha -1 yr -1 ) Figure 1 Relationship between the estimated C input from different cropping systems and the proportion of water-stable aggregates in a silty clay (Normandin, Québec) CP=Chisel plowing, MP=moldboard plowing, MIN=mineral fertilization, DLM=fertilization with dairy liquid manure. (Adapted from Angers et al., 1994). 2

3 However, when looking at complex farming systems, the effect of residue inputs can be difficult to separate from those of tillage or other management factors. For instance, the use of different rotations usually results in different C inputs derived from the plants but also in different tillage practices, especially in the case of perennial forages. The direct effect of C inputs was more clearly seen in experiments where the only variable was C amendments in the form of animal manure (e.g. N'dayegamiye and Angers, 1990). In such experiments with appropriate experimental design, the direct effect of manure can be separated from its indirect effect on crop yields and therefore on crop residue inputs (Aoyama, M, D.A. Angers and A. N'dayegamiye, unpublished). The quality of the residue also has an impact on aggregation. Incubation studies have shown that the addition of humified materials such as a humic peat to a Québec marine silty clay had a slow but long-lasting effect on its stability whereas a more decomposable material (fibric peat) had a greater effect on the resistance to slaking (Dinel et al., 1991). The response of the slaking-resistant macroaggregation of Québec silty clay soils to a range of organic amendments has been shown to be closely and positively related to the decomposability of the organic material (Lafond et al., 1994). Most of the research performed on the clay soils of the St.Lawrence lowlands has shown that the proportion of water-stable macroaggregates changes rapidly (within a few years) under the influence of tillage, rotations or organic amendments. Field and laboratory studies have investigated the various OM fractions that could control the dynamics of stable macroaggregation in these soils. Polysaccharides have been proposed in the case of alfalfa-induced changes (Angers and Mehuys, 1989) and fungal biomass in the case of perennial grasses (Chantigny et al., 1997). The lipidic fraction of the soil OM also contributes to stabilizing aggregates of these clay soils following the addition of humic and waxy materials (Dinel et al., 1991 and 1992). In several recent studies from various locations world wide, the involvement of particulate or macroorganic matter in the dynamics of soil aggregation has also been invoked (e.g. Cambardella and Elliott, 1993). Particulate OM is composed of different forms of OM at various stages of decomposition, including plant remains, fungal hyphae and microfauna, and has been found to respond rapidly to changes in management under Québec conditions and elsewhere (Bolinder M.A. et al., unpublished). Decomposing plant residues represent nuclei around which aggregate formation could take place, probably by enmeshment of soil particles induced by fungal growth and/or the mucilages produced by the decomposing microflora (Tiessen and Stewart, 1988; Oades and Waters, 1991; Cambardella and Elliott, 1993; Angers and Chenu, 1997). Particulate OM may also be localised within aggregates through various mechanisms including tillage and root growth (Angers and Chenu, 1997). Once present within the aggregate structure, particulate OM may act in different ways to provide stability. It has been proposed that OM may stabilize aggregates by obstructing pores and thereby reducing water entry rate and reduce slaking (Caron et al., 1996). It is concevable that small particulate OM could play this role. The role of fine roots and fungal hyphae in macroaggregation is also well accepted (Tisdall and Oades, 1982), and these would be largely recovered in the sandsize fraction. As mentioned, particulate OM present in aggregates may also provide stability by providing an energy source to the microbes producing binding materials (Golchin et al., 1994; Besnard et al., 1996; Jastrow, 1996). 3

4 In a study involving several soil types from Québec under either perennial forage or continuous corn (Zea mays L.), Elustondo et al. (1990) have found a close relationship between the proportion of stable macroaggregates and soil clay content (Fig. 2) but also and in a similar way between aggregation and clay-associated OM. This confirmed the dominant role of the clay particles in determining soil structure. Clay particles provide surface for mineral-to-mineral and mineral-to-organics interactions (Rengasamy et al., 1984). However, the relationship found by Elustondo et al. (1990) was different for soils under continuous maize than for those under perennial forages with higher values for the soils under forages. Stable aggregates >1 mm (%) Maize Alfalfa r = 0.89*** (Alfalfa) Clay (%) r = 0.78*** (Maize) Figure 2 Relationships between clay content and the proportion of water-stable aggregates for different textured soils from Québec under alfalfa or maize cropping (Adapted from Elustondo et al., 1990). Elustondo et al. (1990) also found that the quantity of OM associated with clay particles increased with clay content (Fig. 3) but was not affected by land use treatment whereas that of the sand-size OM was more abundant in the forage than in the maize soils. Those and similar observations from soils from other locations world wide have led Hassink (1997) to propose that the silt and clay fractions of these soils were saturated in OM, even the cultivated soils. Under such conditions, any additional C provided to the soil by additional inputs or decreased decomposition would be recovered in the larger size fractions and in particular in the sand. The saturation in OM of clay size particles in Québec soils could be explained by the wet and cold climatic conditions which favour OM accumulation through high primary productivity and slow decomposition. This saturation would in turn be responsible for the very stable microaggregation of these soils. Indeed, despite having >80% of their primary particles as silt or clay, less than 10 % of the soil mass is dispersed and recovered in the <50 µm fraction after applying slaking and wet-sieving in these soils (Chantigny M.H. et al., unpublished). Our hypothesis is that if, as suggested by Hassink (1997), the sites on the clay and silt particles are saturated in OM, the microaggreagtes should be very stable, and any soil OM increase would be observed in the sand-size fraction and result in an increase in stable macroaggregation. 4

5 Clay-associated C (g C kg-1 soil) Maize Alfalfa r = 0.87*** (Alfalfa) Clay (%) r = 0.68** (Maize) Figure 3 Relationships between clay content and amounts of C associated with the clay particles for different textured soils from Québec under alfalfa or maize cropping (Adapted from Elustondo et al., 1990). This hypothesis is corroborated by the results of an experiment performed on a low-om silty soil from Northern France (Angers et al., 1997). The field experiment consisted of amending the soil with the equivalent of 8 Mg ha -1 of wheat (Triticum aestivum L.) straw. The monitoring of the changes in slaking-resistant aggregation showed that no increase in macroaggregation (>250 um) was observed during the 18 months of the study. The only increase in water-stable aggregation occurred in the µm fraction. This may be explained by the fact that according to Hassink's (1997) model, this soil has a saturation deficit in OM bound to silt and clay particles. We propose that the additional C brought by the straw admendment affected the microaggregation first and not the macroaggregation because of this saturation deficit. In other words, the microsites have to be saturated and microaggregates stabilized before stable macroaggregates can be formed and stabilized biologically (Tisdall and Oades, 1982). In conclusion, our model suggests that for Québec silty clay soils the clay surfaces are relatively saturated in OM which leads to highly stable microaggregation. Any additional OM accumulation in the surface soil provided by reduced- or no-till conditions, perennial forage crops or manure additions would lead to increased macroaggregation fraction and be related to the accumulation of particulate OM. This hypothesis helps to explain the relatively high responsiveness of soil structure, and in particular stable macroaggregation, to changes in cropping systems in these soils. REFERENCES Angers, D.A Changes in soil aggregation and organic carbon under corn and alfalfa. Soil Sci. Soc. Am. J. 56 : Angers, D.A. and Chenu. C Dynamics of soil aggregation and C sequestration. In R. Lal et al. (Ed.) Carbon Sequestration in Soils. Advances in Soil Science. (In press). Angers, D.A. and Mehuys, G.R Effects of cropping on carbohydrate content and water-stable aggregation of a clay soil. Can. J. Soil Sci. 69:

6 Angers, D.A. and Mehuys, G.R Aggregate stability to water. Chapter 61. P In M.R. Carter (Ed.). Manual on soil sampling and methods of analysis. CRC Press, Boca Raton, Florida Angers, D.A., Samson, N. and Légère, A Early changes in water-stable aggregation induced by rotation and tillage in a soil under barley production. Can. J. Soil Sci. 73: Angers, D.A., Duval, M., Simard, R.R. and Laverdière, M.R Soil structure, carbon inputs and management practices. P In Caron, J. and Angers, D.A. (Eds.) Proceedings of the Second Eastern Canada Soil Structure Workshop. Mont St- Anne, Aug , Université Laval, Québec, Canada. Angers, D.A., Recous, S. and Aita. C Fate of carbon and nitrogen in water-stable aggregates during decomposition of wheat straw in situ. Europ. J. Soil Sci. 48: Besnard, E., Chenu, C., Balesdent, J., Puget P. and Arrouays, D Fate of particulate organic matter in soil aggregates during cultivation. Europ. J. Soil Sci. 47: Cambardella, C.A. and Elliott, E.T Carbon and nitrogen in aggregates from cultivated and native grassland soils. Soil Sci. Soc. Am. J. 57: Caron, J., Espindola, C.R. and Angers, D.A Soil structural stability during rapid wetting: influence of land use on some aggregate properties. Soil Sci. Soc. Am. J. 60: Chantigny, M.H., Angers, D. A., Prévost, D., Vézina L.P., Chalifour, F.P Soil aggregation, and fungal and bacterial biomass under annual and perennial cropping systems. Soil Sci. Soc. Am. J. 61: De Kimpe, C.R., Laverdière, M.R. and Martel, Y.A Surface area and exchange capacity of clay in relation to the mineralogical composition of Gleysolic soils. Can. J. Soil Sci. 59: Dinel, H., Mehuys, G.R., Lévesque, M Influence of humic and fibric materials on the aggregation and aggregate stability of a lacustrine silty clay. Soil Sci. 151: Dinel, H., Lévesque, P.E.M., Jambu, P. and Righi, D Microbial activity and longchain aliphatics in the formation of stable soil aggregates. Soil Sci. Soc. Am. J. 56: Elustondo, J., Angers, D.A., Laverdière, M.R. and N'dayegamiye, A Influence de la culture de maïs et de la prairie sur l'agrégation et la matière organique de sept sols de Québec. Can. J. Soil Sci. 70: Golchin, A., Oades, J.M., Skjemstad, J.O. and Clarke, P Soil structure and carbon cycling. Austr. J. Soil Res. 32:

7 Hassink, J The capacity of soils to preserve organic C and N by their association with clay and silt particles. Plant Soil 191: Jastrow, J.D Soil aggregate formation and the accrual of particulate and mineralassociated organic matter. Soil Biol. Biochem. 45: Lafond, J., Angers, D.A. and Laverdière, M.R Water-stable macroaggregation in soils amended with various organic materials. P In Caron, J. and Angers, D.A. (Eds.) Proceedings of the Second Eastern Canada Soil Structure Workshop. Mont St- Anne, Aug , Université Laval, Québec, Canada. Le Bissonnais, Y. and Arrouays, D Aggregate stability and assessment of soil crustabiltiy and erodibility: II. application to humic loamy soils with various organic carbon contents. Europ. J. Soil Sci. 48: N'dayegamiye, A. and Angers, D.A Effets de l'apport prolongé de fumier de bovins sur quelques propriétés physiques et biologiques d'un loam limoneux Neubois sous culture de maïs. Can. J. Soil Sci. 70: Oades, J.M. and Waters, A.G Aggregate hierarchy in soils. Aust. J. Soil Res. 29: Rengasamy, P., Greene, R.S.B. and Ford, G.W The role of clay fraction in the particle arrangement and stability of aggregates a review. Clay Res. 3: Simard, R.R., J. Zizka, J. and De Kimpe, C.R Le prélèvement du K par la luzerne (Medicago sativa L.) et sa dynamique dans 30 sols du Québec. Can. J. Soil Sci. 70: Tiessen, H. and Stewart, J.W.B Light and electron microscopy of stained microaggregates: the role of organic matter and microbes in soil aggregation. Biogeochem. 5: Tisdall, J. M. and Oades, J.M Organic matter and water-stable aggregates. J. Soil Sci. 33: Keywords : soil, structure, organic matter, management, aggregate stability, aggregation Mots clés : sol, structure, matière organique, gestion, stabilité structurale, aggrégation 7