CARBON AND NITROGEN DYNAMICS OF SOIL ORGANIC-MATTER FRACTIONS FROM CULTIVATED GRASSLAND SOILS

The amount of organic matter present in soil and the rate of soil organic matter (SOM) turnover are influenced by agricultural management practices. Because SOM is composed of a series of fractions, management practices will also influence the distribution of organic C and N among SOM pools. Our study examined SOM fractions that are occluded within the aggregate structure. Aggregates were disrupted by sonication and the disrupted soil suspensions were passed through a series of sieves to isolate size fractions. Densiometric separations were carried out on the size fractions, creating size-density fractions. Fine-silt-size particles having a density of 2.07 to 2.22 g/cm3 isolated from inside macroaggregates contained the highest percentage of total soil C and N for all cultivation treatments and, because of its properties, will be referred to as the enriched labile fraction (ELF). As cultivation intensity was reduced, the amount of N in the ELF increased from 110 mg N/kg in the bare fallow treatment to 405 mg N/kg in the no-till treatment. About 5% of the N in the ELF was mineralized during a 28-d laboratory incubation, averaged across treatments. The proportion of N mineralized from the ELF (4.7%) was significantly higher than from intact macroaggregates (2.1%), which suggests this fraction may be protected from decomposition within the aggregate structure. We postulate that the ELF is a byproduct of microbial activity and that it contributes to binding microaggregates into macroaggregates in cultivated grassland soils.