A re-evaluation of the enriched labile soil organic matter fraction

Identifying 'functional' pools of soil organic matter and understanding the ir response to tillage remains elusive. We have studied the effect of tilla ge on the enriched labile fraction, thought to derive from microbes and hav ing an intermediate turnover time. Four soils, each under three regimes, lo ng-term arable use without tillage (NT), long-term arable under conventiona l tillage (CT), and native vegetation (NV), were separated into four aggreg ate size classes. Particle size fractions of macro- (250-2000 mu m) and mic roaggregates (53-250 mu m) were isolated by sonication and sieving. Subsequ ently, densiometric and chemical analyses were made on fine-silt-sized (2-2 0 mu m) particles to isolate and identify the enriched labile fraction. Acr oss soils, the amounts of C and N in the particle size fractions were highl y variable and were strongly influenced by mineralogy, specifically by the contents of Fe and Al oxides. This evidence indicates that the fractionatio n procedure cannot be standardized across soils. In one soil, C associated with fine-silt-sized particles derived from macroaggregates was 567 g C m(- 2) under NV, 541 g C m(-2) under NT, and 135 g C m(-2) under CT, whereas C associated with fine-silt-sized particles derived from microaggregates was 552, 1018, 1302 g C m(-2) in NV, NT and CT, respectively. These and other d ata indicate that carbon associated with fine-silt-sized particles is not s ignificantly affected by tillage. Its location is simply shifted from macro aggregates to microaggregates with increasing tillage intensity. Natural ab undance C-13 analyses indicated that the enriched labile fraction was the o ldest fraction isolated from both macro- and microaggregates. We conclude t hat the enriched labile fraction is a 'passive' pool of soil organic matter in the soil and is not derived from microbes nor sensitive to cultivation.