Carbon and Nitrogen Dynamics of Soil Organic Matter Fractions from Cultivated Grassland Soils
1994
Cambardella, C. A. | Elliott, E. T.
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.
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