Influence of content and nature of organic matter on the structure of some sandy soils from West Africa
In the Soudano-Sahelian area, the expansion of more intensively cultivated fields has detrimental effects on both the stock of organic matter and the soil structure. Tropical sandy soils from Mali and Burkina Faso (West Africa) are characterized by low amounts of clay (<20%) and organic matter (< 2%) which partly explains their weak structure and intense soil surface structural crusts formation under rainfall. The study of 20 topsoil samples shows the striking relationship which exists between soil aggregate water-stability and organic matter content. When the structural stability was relatively high, soil structure was more compact with self-similar aggregates of different sizes. When it was low, quartz was often coated and cemented by organo-mineral material in all the sandy soils studied. This study also reports investigations about the nature of these organic materials in two pairs of soil samples of contrasting structural stability in water, tensile strength and organic matter content. Organic matter in all the selected soils was characterized by its high degree of humification, with a high proportion of humin fraction (75 to 90%) and low fulvic acids/humic acids ratio. A strong organic-clay association and the incorporation of nitrogen in humic materials were confirmed by the resistance of carbon and nitrogen to acid hydrolysis (70 to 80% of C and N were unhydrolysable). Humic acids were very rich in oxygen-containing functional groups, and poorly condensed, with close similarities to fulvic acids from other soil types. These general organic patterns should be attributed mainly to bioclimatic environment and partly to the expansion of continous cultivation. Results supported that the structural stability of the sandy top-soils studied could be the result of both the special chemical nature of organic matter with numerous functional acid groups and its strong association with goethite-kaolinite within the clay-size fraction. Low soil organic matter contents were mainly explained by the low amount of clay mineral-organic matter associations and limited even more by intensive agricultural practices. The most stable soils were characterized by large contents of humin, uronic acids, osamines and polyphenols, which would be related to soil microbial activity. It was finally suggested that, among these substances, uronic acids produced the strongest aggregative effects.
| Main Authors: | , , , , |
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| Format: | article biblioteca |
| Language: | eng |
| Published: |
Elsevier
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| Subjects: | P30 - Sciences et aménagement du sol, P34 - Biologie du sol, sciences du sol, matière organique, sol sableux, teneur en matière organique, structure du sol, matière organique du sol, http://aims.fao.org/aos/agrovoc/c_7188, http://aims.fao.org/aos/agrovoc/c_5387, http://aims.fao.org/aos/agrovoc/c_6781, http://aims.fao.org/aos/agrovoc/c_5388, http://aims.fao.org/aos/agrovoc/c_7196, http://aims.fao.org/aos/agrovoc/c_35657, http://aims.fao.org/aos/agrovoc/c_4540, http://aims.fao.org/aos/agrovoc/c_8081, |
| Online Access: | http://agritrop.cirad.fr/599256/ http://agritrop.cirad.fr/599256/1/599256.pdf |
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| Summary: | In the Soudano-Sahelian area, the expansion of more intensively cultivated fields has detrimental effects on both the stock of organic matter and the soil structure. Tropical sandy soils from Mali and Burkina Faso (West Africa) are characterized by low amounts of clay (<20%) and organic matter (< 2%) which partly explains their weak structure and intense soil surface structural crusts formation under rainfall. The study of 20 topsoil samples shows the striking relationship which exists between soil aggregate water-stability and organic matter content. When the structural stability was relatively high, soil structure was more compact with self-similar aggregates of different sizes. When it was low, quartz was often coated and cemented by organo-mineral material in all the sandy soils studied. This study also reports investigations about the nature of these organic materials in two pairs of soil samples of contrasting structural stability in water, tensile strength and organic matter content. Organic matter in all the selected soils was characterized by its high degree of humification, with a high proportion of humin fraction (75 to 90%) and low fulvic acids/humic acids ratio. A strong organic-clay association and the incorporation of nitrogen in humic materials were confirmed by the resistance of carbon and nitrogen to acid hydrolysis (70 to 80% of C and N were unhydrolysable). Humic acids were very rich in oxygen-containing functional groups, and poorly condensed, with close similarities to fulvic acids from other soil types. These general organic patterns should be attributed mainly to bioclimatic environment and partly to the expansion of continous cultivation. Results supported that the structural stability of the sandy top-soils studied could be the result of both the special chemical nature of organic matter with numerous functional acid groups and its strong association with goethite-kaolinite within the clay-size fraction. Low soil organic matter contents were mainly explained by the low amount of clay mineral-organic matter associations and limited even more by intensive agricultural practices. The most stable soils were characterized by large contents of humin, uronic acids, osamines and polyphenols, which would be related to soil microbial activity. It was finally suggested that, among these substances, uronic acids produced the strongest aggregative effects. |
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