Aggregate‐Protected Carbon in No‐tillage and Conventional Tillage Agroecosystems Using Carbon‐14 Labeled Plant Residue

Abstract

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