Steady‐State Aerobic Microbial Activity as a Function of Soil Water Content

Soil physical properties often regulate aeration-dependent microbial activities important to nutrient cycling, soil fertility and environmental quality. Microbial activity depends on soil water content and is maximum at a water content where the limiting effects of substrate diffusion and O2 supply are equal. The mechanism whereby this occurs and predictions of the soil water content where aerobic microbial activity is a maximum were the objectives of this study. In particular, this study predicted the shape of the microbial activity vs. water content function from soil physical concepts. Soil physical processes are assumed to influence microbial activity by limiting the steady flux of a required substrate or O2 to sites of microbial activity. Steady-state flux relations are used to define the activity function. The dependence of diffusion coefficient on water content or air-filled porosity is assumed. With these assumptions, it is possible to show that a maximum in the activity function exists. The predicted shape of the activity curve is consistent with experimental observations. The relationship between aeration-dependent microbial activity and soil water content facilitates evaluating the indirect effects of soil management practices, such as tillage, on microbial activity. S MICROBIAL ACTIVITY is strongly influenced by soil water. A clear understanding of the mechanisms by which this occurs is useful to explain a variety of phenomena including the transition between aerobic and anaerobic processes, and the effect of tillage practices on microbial processes. Changes in tillage practices tend to result in rather dramatic changes in the soil physical environment (Mielke et al., 1986). This environment in turn controls microbial processes such as mineralization and denitrification that directly impact nutrient leaching or plant growth (Doran, 1980). The mechanisms through which soil water affects microbial activity have not been completely elucidated. A more detailed conceptualization would assist in the careful design of experiments as well as the interpretation of results. This research was undertaken to provide a means to conceptualize and evaluate physical mechanisms whereby soil water governs soil microbial processes. The emphasis is on aerobic microbial activity with O2 and substrate availability as limiting processes. Soil respiration, specifically CO2 production, is the empirical measure of microbial activity. Several factors exist whereby physical processes influence the activity of soil microorganisms. CollisGeorge (1959) summarized these physical factors as (i) water availability, (ii) gas exchange, (iii) temperature, and (iv) spatial constraints. Collis-George (1959) and McLaren and Skujins (1968) emphasized that water availability is best described by the soil matric potential or equivalent quantities (e.g., water activity J. Skopp and M.D. Jawson, Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583-0915. J.W. Doran, USDA-ARS, Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583. Joint contribution of the Nebraska Agric. Exp. Stn. and the USDA-ARS. Journal Series no. 9303. Received 8 Jan. 1990. "Corresponding author. Published in Soil Sci. Soc. Am. J. 54:1619-1625 (1990). or relative humidity). Most recent efforts studying the influence of soil water on microbial activity have focused on microorganism response and classification based on soil matric potential (Papendick and Campbell, 1981; Harris, 1981; Griffin, 1981). This emphasis on soil matric potential or water activity developed because of the usefulness of water activity to describe microbial growth (Reid, 1980) and the availability of water to plants. However, this work tends to emphasize the lowest matric potential (ie., driest conditions) at which growth will occur. Furthermore, the prediction of microbial growth and activity (nonequilibrium processes) from physical principles does not appear possible using only the water activity (an equilibrium property). The high water activity of most soils (Papendick and Campbell, 1981) suggests that other processes also regulate microbial activity by limiting the diffusion of substrates and O2. Substrate and O2 diffusion coefficients are typically characterized using soil water content relationships. Thus, whereas soil matric potential may be viewed as an important measure of water availability, soil water content may be a more useful measurement where water availability is not limiting. Greaves and Carter (1920) were one of the first to document a consistent relationship between soil water content and microbial activity. Microbial activity was described by measuring NH4 or NO3 at the end of an incubation period. For 22 soils of varying texture, maximum activity was observed at approximately 60% of water-holding capacity. Seifert (1962) observed NO2 production in a variety of aggregate sizes. The samples were not amended with substrate but were incubated for 14 d prior to analysis. All aggregate sizes showed maximum activity at water contents near 40%. However, it is not stated how water content was determined, although probably it was a gravimetric procedure. Linn and Doran (1984) performed a similar study where bulk density was also determined. They found maximum aerobic microbial activity occurred at a volumetric water content (0V) equal to 0.60 times the value of the total porosity (e). In later studies, the maximum aerobic microbial respiration occurred between 0V = 0.55 and 0.61 times e with 16 soils of varying texture (Doran et al., 1988). Complementing the work on the influence of soil water on microbial activity are general studies of soil aeration. Soil physical studies of aeration focus on gasphase diffusion coefficients without regard to interactions with microbial activity (Smith, 1977; Currie, 1984; Sallam et al., 1984). Empirical approaches commonly use a limiting gas flux or oxygen diffusion rate (ODR) to characterize soil aeration (Armstrong and Wright, 1976; Glinski and Stepniewski, 1985). Microbial studies of aeration tend to focus on threshold values controlling the shift from aerobic to anaerobic activity (Bridge and Rixon, 1976). The role of O2 and substrate diffusion in limiting microbial activity has previously been recognized