OXYGEN EFFECTS ON CARBON, POLYPHENOLS, AND NITROGEN MINERALIZATION POTENTIAL IN SOIL

Authors: Zibilske, Larry; Bradford, Joe

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2006
Publication Date: 1/1/2007
Citation: Zibilske, L.M., Bradford, J.M. 2007. Oxygen effects on carbon, polyphenols, and nitrogen mineralization potential in soil. Soil Science Society of America Journal. 71:133-139.

Interpretive Summary: Slowing down soil organic matter decomposition is important to maintain productive agricultural soils. Better methods are needed to manage crop residues to prevent loss. Polyphenolic compounds slow organic matter decomposition, so when plants containing high amounts of them are added to soil, decomposition is slowed and more organic matter is retained in the soil. Further slowing may be afforded by reducing oxygen supply to the microbes decomposing the residues, since oxygen is required for complete destruction of plant materials in soil. By combining the effect of using high polyphenolic plant residues and restricting the amount of oxygen in the soil, more organic matter could be retained in the soil, improving soil quality. In this experiment, plant matter was added to soil which was then incubated at 0.5%, 10%, and 21% oxygen. It was found that 0.5% oxygen prevented the complete decomposition of polyphenolics, which may result, in the long term, in larger amounts of organic matter being retained in the soil. The 21% oxygen treatment resulted in lower amounts of ammonium in the soil while the 0.5% treatment maintained three times the amount of ammonium than the 21% treatment. The 10% treatment was more similar to the 21% treatment than to the 0.5% treatment for all analyses. The results show that managing soil to keep the oxygen level below 10%, coupled with using cover crops with a high content of polyphenolics, set up a condition in which soil organic matter decomposition is slowed down, potentially increasing the accumulation of organics in the soil. The most effective way to reduce oxygen occurs when soil tillage is reduced.

Technical Abstract: Crop residue decomposition is affected by environmental factors and residue biochemical properties. These influences may be combined into management protocols that enhance the rate of soil organic matter accumulation in hot climates which have characteristically high organic matter oxidation rates. The purpose of this experiment was to determine the effects of oxygen availability on water-soluble polyphenolics (WEP), arginine ammonification (ARG) and water extractable C (WEC). Cowpea (Vigna unguiculata L.) and sorghum (Sorghum bicolor L. Moench) residues were mixed with soil and incubated in sealed jars with headspace oxygen concentration of 0.5%, 10%, or 21% (ambient) for up to 110 days. WEP and WEC were significantly (P<0.05) and negatively related to 02 concentration. WEP ranged from 0.5ug g-1 soil at 0.5% 02 to near 0 at 10% and 21% 02. WEC ranged from about 350ug C g-1 soil at 0.5% 02 to <25 ug C g-1 soil at 10% and 21% 02. ARG rates were also negatively related to 02 level with cowpea, but the effect was not as strong with sorghum. ARG and WEC were significantly and negatively correlated. ARG ranged from 6uf N g-1 soil at 0.5% 02 to <0.5 ug Ng-1 soil at 21% 02. WEC and WEP, however, appeared to vary independently throughout the incubation. Our results suggest that it is possible to improve soil organic C accumulation by, 1) using cover crops with higher polyphenolic content which could slow N mineralization and C loss and, 2) restricting soil 02 availability via reduced tillage.