Aggregate-protected carbon in no-tillage and conventional tillage agroecosystems using carbon-14 labeled plant residue

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 C-14-labeled plant residue as a tracer. Samples were collected from the Horseshoe Bend Research area in Athens, GA. Aggregate-size distribution, total C, and C-14 were measured together with different pools of aggregate-associated C and C-14 from 21-d laboratory incubations of intact and crushed macro and microaggregates. Compared with CT, NT practices resulted in higher total C and C-14 in all aggregate-size classes of the 0- to 2.5- and 2.5- to 5-cm layers, except for C-14 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 C-14 was found in the >2000-mum aggregate-size class under NT than CT. In contrast, more C-14 was found in the 53- to 250-mum and <53-μm size classes under CT than NT. Unprotected C and C-14 pools, microaggregate-protected and micro within macroaggregate-protected C and C-14 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 (C-14) 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.