Characteristics of Abiological Carbon Monoxide Formation from Soil Organic Matter, Humic Acids, and Phenolic Compounds Ralf Conrad" and Wolfgang Seller
Max-Planck-Instiiut fur Chemie, 0-6500Mainz, Federal Republic of Germany The abiological CO formation from soil and humic acids is a thermally sensitized process following the Arrhenius equation. The magnitude of the Arrhenius constant (activation entropy) and the activation energy decreased with increasing moisture content and pH, i.e., conditions favoring the solubilization of the humic acid polymers. CO was also formed from various phenolic compounds provided that water or preferably alkali was added. The CO formation was stimulated by an oxygen atmosphere but was not inhibited by the addition of chemicals quenching singlet oxygen, superoxide, or hydroxyl radicals. Introduction
It is well established that soils contain microorganisms which are able to oxidize CO (1-5). Some of these microorganisms have such a high affinity for CO that they are able to utilize atmospheric CO ( 6 , 7 )and thus provide a removal mechanism for the CO that is produced by natural and anthropogenic processes (8,9). However, little attention has been paid to the fact that CO not only is consumed but is also produced in soil (3). Under arid conditions, e.g., in deserts or savannahs, CO production rates often exceed CO consumption rates so that these soils even act as a net source of atmospheric CO (10,11). CO formation in soil was shown to be an abiological process (3, 10). It was shown by field measurements that CO emission from soil into the atmosphere was dependent on soil temperature, soil moisture, and organic carbon content and that most of the emitted CO was produced in the upper soil layers by a light-independent reaction (11). The intention of this work was to characterize the abiological CO-producing reaction in soil by laboratory experiments. We present data on temperature dependence of CO formation in soils which were treated in different ways, as well as in purified quartz sand to which humic acids and other phenolic compounds were added. Materials and Methods
Soil samples were taken from the top 10-cm layer of different field sites which were under arid conditions. The field sites and the characteristics of their soils have already been described (11-13). The soil samples were passed through a 4-mm mesh screen and stored in polyethylene bottles at ambient temperature for several months. NaOH, HC1, NaN3, L-alanine, and D-mannitol were obtained from Merck (Darmstadt),humic acid ( M , 600-1000) was from Fluka (Switzerland),and all the other chemicals were from Aldrich (EGA Chemie, Steinheim). N2 (99.99%) and 0013-936X/85/0919-1165$01.50/0
technical O2were purchased from Linde, Germany. Soil (100 g) was filled in 1-L glass flasks which were placed in a water bath kept at a constant temperature (h0.5 "C).The flasks were gassed with air, N2,or O2which was purified from traces of CO by passing the gas stream over Hopkalit catalyst (Drager Werke, Liibeck). The gas stream was moistened by passing it through an HzO bubbler kept at the temperature of incubation. After 15 min of temperature adaptation, the flasks were pressurized to 1.3 bar and closed. Gas samples were taken from the flasks at 5-min intervals for a total of 30-45 min and analyzed immediately for CO by using an instrument based on the HgO-to-Hg vapor conversion technique (9,14). CO production rates were determined from the linear temporal increase of the CO mixing ratio (vol/vol) in the headspace of the flask. When 100 g dry weight (d.w.) of soil was used, the lower detection limit was 0.05 nL of CO h-' g-' d.w. Maximum rates of C600 nL of CO h-l (g d.w. of soil)-' were sometimes observed. These rates resulted in CO mixing ratios of
