Effect of Water-Filled Pore Space on Carbon Dioxide and Nitrous Oxide Production in Tilled and Nontilled Soils

The percentage of soil pore space filled with water (percent waterfilled pores, % WFP), as determined by water content and total porosity, appears to be closely related to soil microbial activity under different tillage regimes. Soil incubated in the laboratory at 60% WFP supported maximum aerobic microbial activity as determined by CO2 production and O2 uptake. In the field, % WFP of surface no-tillage soils (0-75 mm) at four U.S. locations averaged 62% at time of sampling, whereas that for plowed soils was 44%. This difference in % WFP was reflected in 3.4 and 9.4 times greater CO2 and N2O production, respectively, from surface no-tillage soils over a 24-h period as compared to plowed soils. At a depth of 75 to 150 mm, % WFP values increased in both no-tillage and plowed soils, averaging approximately 70% for no tillage compared with 50 to 60% for plowed soils. Production of CO2 in the plowed soils was enhanced by the increased % WFP, resulting in little or no difference in CO2 production between tillage treatments. Nitrous oxide production, however, remained greater under no-tillage conditions. Substantially greater amounts of N2O were produced from the N-fertilized soils, regardless of tillage practice. Production of CO2 and N2O was primarily related to the % WFP of tillage treatments although, in several instances, soil-water-soluble C and NOf levels were important as well. Calculations of relative aerobic microbial activity between no-tillage and plowed soils, based on differences in % WFP relative to maximum activity at 60%, indicated linear relationships for COj and NjO production between WFP values of 30 to 70%. Below 60% WFP, water limits microbial activity, but above 60%, aerobic microbial activity decreases—apparently the result Of reduced aeration. Additional Index Words: aerobic microbial activity, conservation tillage, respiration, soil aeration, nitrification, denitrification. Linn, D.M., and J.W. Doran. 1984. Effect of water-filled pore space on carbon dioxide and nitrous oxide production in tilled and nontilled soils. Soil Sci. Soc. Am. J. 48:1267-1272. T no-tillage management system, often characterized by an accumulation of crop residues on the soil surface, results in greater C, N, and water contents of the surface 50 to 100 mm of soil compared to conventionally tilled (plowed) soils (Blevins et al., 1977; Campbell et al., 1976; Doran, 1980; Fleige and Baeumer, 1974). The greater numbers of microorganisms and microbial activities in the surface 75 mm of no-tillage soils are largely a reflection of these higher C, N, and water contents (Doran, 1980). Facultative anaerobes and denitrifying bacteria also are more numerous in the surface 150 mm of no-tillage soils and constitute a larger proportion of the total microbial population than in plowed soils. Thus, the greater number of organisms capable of anaerobic activity and the greater potential for denitrification (Doran, 1980;. 1 Contribution from the Agricultural Research Service, USDA, in cooperation with the Nebraska Agric. Exp. Stn., Lincoln. Published with the approval of the Director as Paper no. 7125, Journal Series. Received 9 Mar. 1983. Approved 8 June 1984. 2 Research Associate, University of Minnesota, St. Paul (formerly Reseach Technologist, Univ. Nebraska-Lincoln), and Soil Scientist, ARS, USDA, Lincoln, NE. Rice and Smith, 1982) indicate the biological environment of no-tillage soils to be less aerobic than that of plowed soils. Major factors responsible for the lessaerobic conditions in no-tillage soils are higher soil water contents and/or bulk densities which result in lower total soil porosity and greater water-filled pore space (WFP) in comparison with plowed soils (Doran and Power, 1983). Previous soil aeration research has demonstrated the importance of the soil air/water balance upon aerobic and anaerobic microbial activities. Aerobic microbial activity increases with soil water content until a point is reached where water displaces air and restricts the diffusion and availability of oxygen. Maximum rates of microbial respiration, nitrification, and mineralization occur at the highest water content (lowest tension) at which soil aeration remains nonlimiting (Bhaumik and Clark, 1948; Miller and Johnson, 1964; Parker and Larson, 1962). When soil water contents approach or exceed field capacity, the percentage of soil pore space filled with air or water are better indicators of aerobic vs. anaerobic microbial activity than either water content or water potential (Miller and Johnson, 1964; Sommers et al., 1981). The results of many studies, involving a wide range of soil types, indicate that a soil water content equivalent to 60% of a soil's water-holding capacity (WHC) delineates the point of maximum aerobic microbial activity (Table 1). The majority of studies examining interrelationships between soil water status and aeroTable 1—The relationship between percent water-holding capacity (% WHC) or percent water-filled pore space (% WFP) and microbial activities and numbers.