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 tilla ge (CT) compared with no-tillage (NT) is related to differences in aggregat e turnover. Four soils (Haplustoll, Fragiudalf, Hapludalf, and Paleudalf), each with NT, CT, and native vegetation (NV) treatments, were separated int o aggregates. Free light fraction (LF) and intraaggregate POM (iPOM) were i solated. At one site we used C-13 natural abundance to differentiate crop- and grassland-derived C. Concentrations of coarse iPOM C (250-2000 mu m iPO M in macroaggregates), expressed on a per unit aggregate weight (g iPOM C k g(-1) aggregate), did not differ between tillage treatments. In contrast, c oncentrations of fine iPOM C (53-250 mu m iPOM in macroaggregates) were les s 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 tota l C difference between NT and CT. The concentration of free LF C was not af fected by tillage, but was on average 45% less in the cultivated systems th an NV. Proportions of crop-derived C in macroaggregates were similar in NT and CT, but were three times greater in microaggregates from NT than microa ggregates from CT. We suggest that a faster turnover rate of macroaggregate s in CT compared with NT leads to a slower rate of microaggregate formation within macroaggregates and less stabilization of new SOM in free microaggr egates under CT.