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University of Toronto (2007)

Formation and stabilization of aggregates in a Vertisol from Northeastern Mexico

Bravo-Garza, Maria del Refugio

Titre : Formation and stabilization of aggregates in a Vertisol from Northeastern Mexico

Auteur : Bravo-Garza, Maria del Refugio

Université de soutenance : University of Toronto

Grade : Doctor of Philosophy (PhD) 2007

Formation and stabilization of soil aggregates involves the interaction of physical, chemical, mineralogical and biological factors, all of which are highly variable within and between soils. Soil organic matter is the most important factor controlling aggregation processes in many soils, but studies on clayey soils have reported contradictory results. This study explored aggregation processes in a Vertisol dominated by smectitic clays from a semi-arid region in Northeastern Mexico. Changes in aggregation were studied in relation to the decomposition of added organic material under controlled laboratory weathering conditions. The study evaluates the role and relative importance of particulate organic matter (POM), microbial communities, and wetting and drying. Results showed that all three factors studied and their interrelationship influenced aggregation. However, their relative influence varied depending on aggregate size. Large macroaggregate (>2.00 mm) formation was largely controlled by the addition of organic residues increasing the amount of these macroaggregates by five to six-fold. Furthermore, POM formed nucleation points for adhesion of mineral particles, and also served as a source of energy for microbial activity. Wetting and drying increased large macroaggregate formation by 26% after organic amendment and enhanced the stability of the large macroaggregates formed. In addition, formation of small macroaggregates (0.25—2.00 mm) was increased by 20% in unamended soil compared to amended samples after one wetting and drying event. However, repeated wetting and drying events also accelerated small macroaggregate turnover. The enhancement of aggregate formation and stabilization with wetting and drying correlated with increased microbial biomass, and fungal and bacterial populations, indicating the important role of soil structure on microbial protection. Furthermore, the variability on the microbial community composition due to wetting and drying was greater for the fungal community than for the bacterial community. Reduction of the amount of C evolved as CO2 from added organic residues with wetting and drying, compared to constantly wet soil, indicated a decline on the decay of added organic matter. This result suggests that amended Vertisols undergoing shrinking and swelling conditions could play an important role on soil C sequestration due to promotion of physico-chemical reactions that slow the decay of added organic matter

Mots clés : Carbon sequestration, Particulate organic matter, Biogeochemistry, Smectic clays, Soil aggregation, Soil sciences Biological sciences, Earth sciences, Vertisol


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