Soil-atmosphere CO exchanges and microbial biogeochemistry of CO transformations in a Brazilian agricultural ecosystem

Abstract

Although anthropogenic land use has major impacts on the exchange of soil and atmosphere gas in general, relatively little is known about its impacts on carbon monoxide. We compared soil-atmosphere CO exchanges as a function of land use, crop type, and tillage treatment on an experimental farm in Parãna, Brazil, that is representative of regionally important agricultural ecosystems. Our results showed that cultivated soils consumed CO at rates between 3 and 6 mg of CO m(-2) day(-1), with no statistically significant effect of tillage method or crop. However, CO exchange for a pasture soil was near zero, and an unmanaged woodlot emitted CO at a rate of 9 mg of CO m(-2) day(-1). Neither nitrite, aluminum sulfate, nor methyl fluoride additions affected CO consumption by tilled or untilled soils from soybean plots, indicating that CO oxidation did not depend on ammonia oxidizers and that CO oxidation patterns differed in part from patterns reported for forest soils. The apparent K(m) for CO uptake, 5 to 11 ppm, was similar to values reported for temperate forest soils; V(max) values, approximately 1 micro g of CO g (dry weight)(-1) h(-1), were comparable for woodlot and cultivated soils in spite of the fact that the latter consumed CO under ambient conditions. Short-term (24-h) exposure to elevated levels of CO (10% CO) partially inhibited uptake at lower concentrations (i.e., 100 ppm), suggesting that the sensitivity to CO of microbial populations that are active in situ differs from that of known carboxydotrophs. Soil-free soybean and corn roots consumed CO when they were incubated with 100-ppm concentrations and produced CO when they were incubated with ambient concentrations. These results document for the first time a role for cultivated plant roots in the dynamics of CO in an agricultural ecosystem.

FIG. 1.

Time course of headspace CO concentrations for samples of soils from a nontilled soybean plot (○) and an unmanaged woodlot (•). Data represent means ± 1 standard error for triplicate (soybean) or quadruplicate (woodlot) assays.

FIG. 2.

Headspace CO concentrations during uptake assays of surface soils from a 20-year-old nontilled soybean plot. Samples were preincubated with ambient air (○) or 10% CO (•). All data are means ± 1 standard error for triplicate assays.

FIG. 3.

Atmospheric CO uptake rates for surface soils from a 20-year-old nontilled soybean plot. Samples were preincubated with ambient air and amended with deionized water sequentially to vary water content. Data for soils with varied water contents (•) are means ± 1 standard error for triplicate assays; the value for controls (○) is the mean of rates determined in triplicate at five intervals, during which control water contents were varied.

FIG. 4.

Headspace CO concentrations during assays of freshly collected, excised soil-free soybean roots (in parts per billion) (A) and after the addition of CO to initial values of approximately 100 ppm (in parts per million) (B). Different symbols indicate individual replicates.