ORGANIC-MATTER FRACTIONS FROM ORGANICALLY AND CONVENTIONALLY MANAGED SOILS .1. CARBON AND NITROGEN DISTRIBUTION

Chemical and physical fractionation techniques were used to study the effects of organic and conventional management practices on soil organ ic matter (SOM) characteristics. We isolated SOM from: (i) organic ani mal-based, manure-amended (ORG-AN), (ii) organic cash-grain-based, cov er cropped (ORG-CC), and (iii) conventional, cash-grain-based (CONV) r otations. We quantified the total C and N contents of humin (HN) and h umic substances (HS), and light (LF), heavy (HF), fine organo-clay, an d organo-sand fractions. Management changed the quantity and distribut ion of C and N in physically and chemically isolated fractions. The OR G-CC soil had significantly greater quantities of C in its HN, HS, LF, and HF fractions than the CONV soil. Management impacted the distribu tion of C and N only in the LF fraction. The ORG-CC had a larger perce ntage and proportion of C and N in its LF than the other soils. The pe rcentage of soil N in HS was higher in the ORG-AN and ORG-CC than in t he CONV son. Even though the quantity of N in the CONV's HN equaled th e other soils, it had the highest percentage and proportion of N alloc ated to this fraction. Farming systems did not significantly alter the C and N contents of HS or the quantity of C associated with selected particle-size fractions; however, the C content of fine clay isolated from aggregated soil was significantly greater than the C content of c lay isolated from loose soil. By isolating macroorganic matter (LF) in concert with HS and HN fractions, we were able to assess management p ractice impacts on SOM characteristics.