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 stabili ty of soil aggregates. We investigated the effect of cultivation intensity on aggregate distribution and aggregate C in three soils dominated by 2:1 c lay mineralogy and one soil characterized by a mixed (2:1 and 1:1) mineralo gy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. Slaked (i.e., sir-dried and fast-rewetted) and ca pillary rewetted soils were separated into four aggregate-size classes (<53 , 53-250, 250-2000, and >2000 pm) by wet sieving. In rewetted soils, the pr oportion of macroaggregates accounted for 85% of the dry soil weight and wa s similar across management treatments. In contrast, aggregate distribution from slaked soils increasingly shifted toward more mirroaggregates and few er macroaggregates with increasing cultivation intensity. In soils dominate d by 2:1 clay mineralogy, the C content of macroaggregates was 1.65 times g reater 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 mi xed mineralogy, aggregate C did not increase with increasing aggregate size . At all sites, rewetted macro- and microaggregate C and slaked microaggreg ate C differed in the order NV > NT > CT. In contrast, slaked macroaggregat e C concentration was similar across management treatments, except in the s oil with mixed clay mineralogy, We conclude that increasing cultivation int ensity leads to a loss of C-rich macroaggregates and an increase of C-deple ted microaggregates in soils that express aggregate hierarchy.