Organic matter fractions from organically and conventionally managed soils: I. Carbon and nitrogen distribution

Chemical and physical fractionation techniques were used to study the effects of organic and conventional management practices on soil organic matter (SOM) characteristics. We isolated SOM from: (i) organic animal- based, manure-amended (ORG-AN), (ii) organic cash-grain-based, cover cropped (ORG-CC), and (iii) conventional, cash-grain-based (CONV) rotations. We quantified the total C and N contents of humin (HN) and humic substances (HS), and light (LF), heavy (HF), fine organo-clay, and organo-sand fractions. Management changed the quantity and distribution of C and N in physically and chemically isolated fractions. The ORG-CC soil had significantly greater quantities of C in its HN, HS, LF, and HF fractions than the CONV soil. Management impacted the distribution of C and N only in the LF fraction. The ORG-CC had a larger percentage and proportion of C and N in its LF than the other soils. The percentage of soil N in HS was higher in the ORG-AN and ORG-CC than in the CONV soil. Even though the quantity of N in the CONV's HN equaled the other soils, it had the highest percentage and proportion of N allocated 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 clay isolated from loose soil. By isolating macroorganic matter (LF) in concert with HS and HN fractions, we were able to assess management practice impacts on SOM characteristics.