Aggregate and Soil Organic Matter Dynamics under Conventional and No‐Tillage Systems

Tillage generally reduces aggregation and particulate organic matter (POM) content. We hypothesized that reduced C sequestration in conventional tillage (CT) compared with no‐tillage (NT) is related to differences in aggregate turnover. Four soils (Haplustoll, Fragiudalf, Hapludalf, and Paleudalf), each with NT, CT, and native vegetation (NV) treatments, were separated into aggregates. Free light fraction (LF) and intraaggregate POM (iPOM) were isolated. At one site we used 13 C natural abundance to differentiate crop‐ and grassland‐derived C. Concentrations of coarse iPOM C (250–2000 μm iPOM in macroaggregates), expressed on a per unit aggregate weight (g iPOM C kg −1 aggregate), did not differ between tillage treatments. In contrast, concentrations of fine iPOM C (53–250 μm iPOM in macroaggregates) were less in CT compared to NT macroaggregates. On a whole soil basis, fine iPOM C was on average 51% less in CT than in NT, and accounted for 21% of the total C difference between NT and CT. The concentration of free LF C was not affected by tillage, but was on average 45% less in the cultivated systems than NV. Proportions of crop‐derived C in macroaggregates were similar in NT and CT, but were three times greater in microaggregates from NT than microaggregates from CT. We suggest that a faster turnover rate of macroaggregates in CT compared with NT leads to a slower rate of microaggregate formation within macroaggregates and less stabilization of new SOM in free microaggregates under CT.