Aggregate‐Protected and Unprotected Organic Matter Pools in Conventional‐ and No‐Tillage Soils

TLDR

No‑tillage practices increase soil aggregation and organic matter relative to conventional tillage, yet the underlying mechanisms remain unclear. The study aims to characterize the size and quality of biologically active aggregate‑associated SOM pools in long‑term conventional and no‑tillage soils of the southeastern United States. Researchers sampled replicated CT and NT plots, sieved soils into four aggregate size classes, and measured potential mineralizable C, N, and N₂O emissions from 20‑day incubations of intact and crushed macro‑ and microaggregates. Unprotected SOM pools were 21–65 % larger in NT soils, macroaggregate disruption increased mineralization in NT but not CT, and protected SOM comprised a larger fraction of total mineralizable C and N in NT than CT, indicating macroaggregates in NT soils protect SOM from mineralization.

Abstract

Abstract No‐tillage (NT) practices can result in greater soil aggregation and higher soil organic matter (SOM) levels than conventional‐tillage (CT) practices, but the mechanisms for these effects are poorly known. Our objectives were to describe the size and quality of biologically active pools of aggregate‐associated SOM in long‐term CT and NT soils of the southeastern USA. Samples were collected from replicated CT and NT plots on a Hiwassee sandy clay loam (clayey, kaolinitic, thermic Rhodic Kanhapludult) and separated into four aggregate size classes (>2000, 250–2000, 106–250, 53–106 µm) by wet sieving. Potentially mineralizable C and N and N 2 O emissions were measured from 20‐d laboratory incuhations of intact and crushed macroaggregates (>250 µm) and intact microaggregates (<250 µm). Three primary pools of aggregate‐associated SOM were quantified: unprotected, protected, and resistant C and N. Aggregate‐unprotected pools of SOM were 21 to 65% higher in surface soils of NT than of CT, with greater differences in the macroaggregate size classes. Disruption of macroaggregates increased the mineralization of SOM in NT but had little effect in CT. Rates of mineralization from protected and unprotected pools of C were higher in surface soils of CT than of NT. Macroaggregate‐protected SOM accounted for 18.8 and 19.1% of the total mineralizable C and N (0–15 cm), respectively, in NT but only 10.2 and 5.4% of the total mineralizable C and N in CT. Our results indicate that macroaggregates in NT soils provide an important mechanism for the protection of SOM that may otherwise be mineralized under CT practices.