Soil structure and organic matter: I. Distribution of aggregate-size classes and aggregate-associated carbon

Cultivation reduces soil C content and changes the distribution and stability of soil aggregates. We investigated the effect of cultivation intensity on aggregate distribution and aggregate C in three soils dominated by 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) mineralogy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. Slaked (i.e., sir-dried and fast-rewetted) and capillary rewetted soils were separated into four aggregate-size classes (<53, 53-250, 250-2000, and >2000 pm) by wet sieving. In rewetted soils, the proportion of macroaggregates accounted for 85% of the dry soil weight and was similar across management treatments. In contrast, aggregate distribution from slaked soils increasingly shifted toward more mirroaggregates and fewer macroaggregates with increasing cultivation intensity. In soils dominated by 2:1 clay mineralogy, the C content of macroaggregates was 1.65 times greater compared to microaggregates. These observations support an aggregate hierarchy in which microaggregates are bound together into macroaggregates by organic binding agents in 2:1 clay-dominated soils. In the soil with mixed mineralogy, aggregate C did not increase with increasing aggregate size. At all sites, rewetted macro- and microaggregate C and slaked microaggregate C differed in the order NV > NT > CT. In contrast, slaked macroaggregate C concentration was similar across management treatments, except in the soil with mixed clay mineralogy, We conclude that increasing cultivation intensity leads to a loss of C-rich macroaggregates and an increase of C-depleted microaggregates in soils that express aggregate hierarchy.