The effects of organic and conventional corn-soybean rotations on soil organic matter characteristics: The Rodale Farming Systems Trial experiment

Soil organic matter (SOM) is associated with soil productivity in numerous ways. Despite the fact that it is linked to desirable soil physical, chemical and biological properties there are few explicit organic matter management recommendations. This work tested the lay (farmer) hypothesis that organic management practices "build soils" by investigating whether 10 years under organic and conventional management generated differences in: (1) biologically active SOM, (2) the quantity and composition of physically and chemically isolated organic matter fractions, and (3) the fate of newly applied organic compounds.; The Rodale Institute Research Center's Farming Systems Trial (FST) was begun in 1981. It compares two organically and one conventionally managed corn-soybean rotations. The organic treatments include an animal based rotation amended with manure and hay crops and a cover crop based cash-grain rotation. The conventional rotation is cash-grain based and is amended with fertilizers and pesticides as needed.; Several indirect measures of "active" SOM were investigated to determine whether FST rotations altered biologically labile C pools. Active C fraction studies suggested soils underlying organic treatments accumulated dynamic (microbial activity, and available and mineralized N) and stable C (reduced water dispersible organic C and increased particulate SOM). The cover cropped soil accumulated the most organic matter and its C was relatively recalcitrant. SOM in the manure amended soil was the most labile; despite the high levels of organic residue addition in this treatment, underlying soils did not accumulate SOM.; Diffuse Reflectance Fourier Transform Infrared spectroscopy (DR-FTIR) was combined with chemical and physical fractionation techniques to document changes in the chemical composition of soil organic matter that are due to crop rotation effects. This work characterized the functional group composition and total C and N contents of: Humic acid, Fulvic acid, Particulate SOM, organo clays from loose and aggregated soil, and clay and fine-sand associated SOM. Peak ratios of DR-FTIR signal intensities assigned to aliphatic and aromatic functional groups showed that aliphatic compounds were enriched in the Fulvic acid and Particulate soil organic matter fractions of the organically fertilized soils. Similar peak ratios in Humic acid spectra indicated manure amendment increased the aromatic functional group compliment.; The effect of the three FST rotational histories on the fate of newly applied organic compounds was investigated using solid state CP-MAS {dollar}sp{lcub}13{rcub}{dollar}C-NMR to track added {dollar}sp{lcub}13{rcub}{dollar}C-labeled acetate, benzoate, catechol, and an unamended control. Amended soil was incubated for (1) two weeks, (2) three months, and (3) gamma sterilized and incubated for three months. Soil rotational history and compound chemistry both influenced the quantity and structural conformation of {dollar}sp{lcub}13{rcub}{dollar}C retained in newly formed organic matter in acetate amended soils.