AGGREGATE-PROTECTED AND UNPROTECTED ORGANIC-MATTER POOLS IN CONVENTIONAL-TILLAGE AND NO-TILLAGE SOILS

No-tillage (NT) practices can result in greater soil aggregation and h igher 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 activ e pools of aggregate-associated SOM in long-term CT and NT soils of th e southeastern USA. Samples were collected from replicated CT and NT p lots on a Hiwassee sandy clay loam (clavey, kaolinitic, thermic Rhodic Kanhapludult) and separated into four aggregate size classes (>2000, 250-2000, 106-250, 53-106 mum) by wet sieving. Potentially mineralizab le C and N and N2O emissions were measured from 20-d laboratory incuba tions of intact and crushed macroaggregates (>250 mum) and intact micr oaggregates (<250 mum). Three primary pools of aggregate-associated SO M were quantified: unprotected, protected, and resistant C and N. Aggr egate-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 p rotected 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 resu lts indicate that macroaggregates in NT soils provide an important mec hanism for the protection of SOM that may otherwise be mineralized und er CT practices.