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Abstract
Understanding soil organic matter (SOM) dynamics and their interactions with different management practices and soil faunal activities is essential in trying to develop sustainable agroecosystems. In this study, the effects of tillage and earthworm activity on SOM and carbon (C) protection were investigated. In the first experiment, the objective was to study the mechanisms by which C is protected under no-tillage (NT) management, using 14C-labeled plant residue. Aggregate-size distribution, total C, and 14C were measured together with different pools of aggregate-associated C and 14C from 21-d laboratory incubations of intact and crushed macro-and microaggregates. The results indicated that (i) more young C (14C) is accumulated in the subsurface soil of conventional tillage (CT) than NT, but this C is not stabilized in the long term, and (ii) short- and long-term stabilization of C is higher in the soil surface layers under NT compared with CT. This C stabilization occurs mainly at the microaggregate level. The objectives of the next set of experiments were to investigate the effects of different earthworm species (Aporrotedea caliginosa and Lumbricus rubellus) on aggregation, aggregate-associated C pools and the formation of stable microaggregates within macroaggregates. Two incubations were set up. The first incubation consisted of soil samples crushed < 250 m to break up all macroaggregates with three treatments: (i) control soil; (ii) soil + 13C-labeled residue; and (iii) soil + 13C-labeled residue + earthworms. After 20 days, aggregate size distribution was measured and microaggregates (53-250 m) were isolated out of the formed macroaggregates (> 250 m). A second incubation was conducted to determine protected versus unprotected total C and 13C from 21-day laboratory incubations of intact and crushed macro- and microaggregates. The results indicated that microaggregates are rapidly formed within earthworm casts and showed the direct involvement of earthworms in inducing an important protection of soil C at the microaggregate level. The results also suggested that important interactions between earthworm species take place affecting the incorporation of fresh residue-derived C and the formation of stable microaggregates when fresh residue was placed on the surface.