Aggregation and soil organic matter accumulation in cultivated and native grassland soils

Tillage intensity affects soil structure and the loss of soil organic C and N. We hypothesized that no-tillage (NT) and conventional tillage (CT) differentially affect three physically defined particulate organic matter (POM) fractions. A grassland-derived Haplustoll was separated into aggregates by wet sieving. Free light fraction (LF) and intra-aggregate PORI (iPOM) were isolated. Natural abundance C-13 was measured for whole soil C, free LF C, and iPOM C. The mean residence time of soil C under CT (44 yr) was 1.7 times less than in NT (73 yr). The amount of free LF C was 174, 196, and 374 g C m(-2) for CT, NT, and NS, respectively. Total iPOM C amounts in CT, NT, and NS vc ere 193, 337, and 503 g C m(-2), respectively. The level of fine iPOM C (53-250 mu m) level in macroaggregates (250-2000 mu m) obtained after slaking,vas five times greater in NT vs. CT and accounted for 47.3% of the difference in total POM C between NT and CT. The amount of coarse iPOM C (250-2000 mu m) was only 2.4 times greater and accounted for only 21% of the difference in total PORI C. Sequestration of iPOM was observed in NT vs. CT, but free LF was not influenced by differential tillage, We conclude that differences in aggregate turnover largely control the difference in fine iPOM in CT vs. NT and consequently SOM loss is affected by both the amount of aggregation and aggregate turnover.