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a Institute of Ecology, Univ. of Georgia, Athens, GA, 30602
b Dep. of Agronomy and Range Science, Univ. of California, Davis, CA, 95616
* Corresponding author (hbossuyt{at}arches.uga.edu)
No-tillage (NT) management can result in higher soil organic matter (SOM) levels than conventional tillage (CT) practices. The objective was to investigate the underlying mechanisms in which C is protected under NT management, using 14C-labeled plant residue as a tracer. Samples were collected from the Horseshoe Bend Research area in Athens, GA. 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. Compared with CT, NT practices resulted in higher total C and 14C in all aggregate-size classes of the 0- to 2.5- and 2.5- to 5-cm layers, except for 14C in the <53- and 250- to 2000-µm aggregate-size classes at the 2.5- to 5-cm layer. At the 5- to 15-cm depth, more 14C was found in the >2000-µm aggregate-size class under NT than CT. In contrast, more 14C was found in the 53- to 250-µm and <53-µm size classes under CT than NT. Unprotected C and 14C pools, microaggregate-protected and micro within macroaggregate-protected C and 14C pools were significantly higher for the 0- to 2.5- and 2.5- to 5-cm layers under NT than CT. Carbon-14 pools were generally higher in CT than in NT at the 5- to 15-cm depth, while total C did not differ between tillage treatments at this depth. The results indicate that (i) more young C (14C) is accumulated in the subsurface soil of 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.
Abbreviations: CT, conventional tillage • NT, no-tillage • SOM, soil organic matter • WSA, water-stable aggregates
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