Soil macroaggregate turnover and microaggregate formation: a mechanism forC sequestration under no-tillage agriculture

Soil disturbance from tillage is a major cause of organic matter depletion and reduction in the number and stability of soil aggregates when native ec osystems are converted to agriculture. No-till (NT) cropping systems usuall y exhibit increased aggregation and soil organic matter relative to convent ional tillage (CT). However, the extent of soil organic matter changes in r esponse to NT management varies between soils and the mechanisms of organic matter stabilization in NT systems are unclear. We evaluated a conceptual model which links the turnover of aggregates to soil organic matter dynamic s in NT and CT systems; we argue that the I:ate of macroaggregate formation and degradation (i.e. aggregate turnover) is reduced under NT compared to CT and leads to a formation of stable microaggregates in which carbon is st abilized and sequestered in the long term. Therefore, the link between macr oaggregate turnover, microaggregate formation, and C stabilization within m icroaggregates partly determines the observed soil organic matter increases under NT. (C) 2000 Elsevier Science Ltd. All rights reserved.