Soil organic matter fractions of no-tilled and tilled soils and their reactivity with herbicides

Properties of soil humic fractions were determined on surface and 7.5-15.0 cm soil samples of continuously (7-year) no-tilled and tilled cotton, corn, and soybean plots in West Tennessee. Soil humic and fulvic acid were extracted by standard methods and the humic acid was characterized by ¹³C--NMR spectroscopy, titration of total acidity and carboxyl groups, and infrared and elemental analysis. Humic acid NMR spectra were divided into six regions (0-40, 40-62, 62-105, 105-150, 150-170, 170-190 ppm) and peak areas were compared. Humic acid composition differed by depth and crop. Small differences were observed between tillage systems. Humic acid aliphatic and aromatic carbons ranged from 48 to 65% and 25 to 40% of total peak area, respectively. The humic acids extracted from soils with larger amounts of carbon (surface no-tilled treatments) had larger aliphatic to aromatic ratios, indicating less decomposed organic matter. Carboxyl groups of the humic acids ranged from 9 to 13% and samples from tilled soil had slightly greater amounts of carboxyl and aromatic groups. Carboxyl group determinations by ¹³C--NMR, compared more closely with total acidity determinations by titration than with carboxyl determinations by titration. All infrared spectra were similar. Elemental composition of humic acid averaged C, 52.7%: H, 5.6%; N, 4.8%; and O, 36.9%. A greenhouse herbicide bioassay was conducted on a cotton-cropped tilled soil and three no-tilled soils, where cotton was cropped with vetch, rye, and crimson clover cover crops. Total C ranged from 9.9 g kg^-1 in the tilled soil to 13.5, 16.6, and 23.5 g kg^-1 in the no-tilled soil with rye, vetch, and crimson clover covers, respectively. The tilled soil had the largest fraction of extractable C and fulvic acid, relative to total soil C. Sorghum growth was measured to indicate soil effects on activity of herbicides, metribuzin and oxyfluorfen. Herbicide activity was inversely related to soil C, extractable C, carboxyl groups of humic acid, and fulvic acid C of soils. Fulvic acid C best predicted herbicide phytoxicity. Results indicated that C was more reactive in tilled than in no-tilled soils.