Soil Microsites as a Source of Denitrification Variability1

The spatial variability exhibited by soil denitriflcation rates is high. As is typical for natural denitriflcation rate measurements, the individual rates of most samples of a given data set are low; however, a few samples often exhibit extremely high rates. Such data are characterized by highly skewed sample frequency distributions. This study was initiated to investigate the underlying mechanisms responsible for these observations. It was found that "hot-spots" of high specific denitriflcation activity were associated with particulate organic C material in the soil. The high specific activities of these hot-spots (incubated under aerobic conditions with no amendments) were similar to the denitrification activity of the bulk soil measured under conditions of anaerobiosis with added glucose and NO3. This observation served as the basis of a computer model that evaluates the influence of the density and dispersion pattern of these high activity sites on the measured rates of denitrification. Histograms generated from computer simulations are very similar to histograms obtained for real data, supporting the concept that the patchy dispersion of particulate organic material in soil is a major factor influencing the variability of natural denitrification rates. Additional Index Words: lognormal, skewed distributions, particulate C, CO, production. Parkin, T.B. 1987. Soil microsites as a source of denitrification variability. Soil Sci. Soc. Am. J. 51:1194-1199. S VARIABILITY of soil properties has been a topic of high interest, as evidenced by the increased application of novel statistical methods in the analysis of soils data (Nielsen and Bouma, 1985). Due 1 Contribution from the USDA Soil Nitrogen and Environmental Chemistry Lab. , Beltsville Agric. Res. Ctr. , Beltsville, MD. 20705. Received 22 Sept. 1986. Research Microbiologist, USDA, Soil Nitrogen and Environmental Chemistry Lab. , Beltsville Agric. Res. Ctr. Beltsville, MD 20705. to their dynamic nature, microbial processes and soil properties directly influenced by soil microorganisms typically display high variability and often exhibit skewed frequency distributions that can be approximated by the lognormal distribution family. This recognition has been an important step in dealing with variability in that statistical techniques designed for lognormal distributions can be applied to yield better parameter estimates (Finney, 1941; Aitchinson and Brown, 1957; Sichel, 1966). Whereas consideration of the frequency distribution provides better information about variability than consideration of just point estimates of location and scale (Fluhler et al., 1976; Nicot et al., 1984), this approach has not yielded insights into the underlying mechanisms responsible for the variability. Insight can be gained as to the underlying mechanisms controlling variability by considering the factors that result in skewed distributions. The law of proportionate effects predicts that a variable will exhibit a lognormal distribution when the factors controlling the variable are combined in multiplicative manner (Aitchinson and Brown, 1957). Multiplicative effects have been proposed as the source of the lognormal distributions exhibited by populations of bacteria in the rhizosphere and on leaf surfaces (Loper et al., 1984; Hirano et al., 1982). In a study of community structure, Ugland and Gray (1982) suggest that multiplicative effects yield patchy dispersions of species in nature, which is the ultimate cause for the skewed distributions observed. The concept of a patchy distribution may also apply to the natural denitrification process since, in unsaturated soils, denitrification is presumed to occur in anaerobic microsites (Dowdell and Smith, 1974; Fluhler et al., 1976; Tiedje et al., 1984). The highly skewed frequency distributions of dePARKIN: SOIL MICROSITES AS A SOURCE OF DENITRIFICATION VARIABILITY 1195 nitrification indicate that most samples of a given data set exhibit low rates but a few samples have very high rates. It is the high rates of these few samples that result in the high observed variability. This study was undertaken to determine the underlying mechanisms controlling the variability of denitrincation in soil. MATERIALS AND METHODS Field Site and Sampling. Samples were collected from a field site located on the Univ. of Maryland Plant Res. Farm, Beltsville, MD. The site had established plots of no-till continuous corn (12 yr). The soil at this site was a Beltsville silt loam (Typic Fragiudult) having a pH of 6.5 and total organic N (Kjeldahl) and C (persulfate digestion) contents of 0.8 and 5.1 g kg~', respectively. Soil cores were obtained by pounding steel tubes, which were fit with hardened cutting bits, into the ground to a depth of 16 cm. The intact soil cores contained within the steel tubes were then slid into stoppered plastic tubes. The soil cores fit loosely in the plastic tubes to facilitate gas diffusion into and out of the soil Upon returning to the laboratory, denitrification rates were measured on the intact soil cores. Soil samples were collected with five different sized soil cores ranging from 1.73 to 5.4 cm in diameter as part of a larger study to investigate the influence of sample size on denitrification (Parkin et al., 1987). Rate data of all but the smallest core size were analyzed as a single data set since they were not significantly different as determined with the Kolmogorov-Smirnov test and Tukey's test on both untransformed and log-transformed rate data (p < 0.05). Natural Denitrification Rate Measurements Denitrification rate measurements were begun immediately upon returning to the laboratory. Natural denitrification rates of the intact soil cores were estimated using an C2H2 block technique. First, the gas pressure in the cores was brought to atmospheric levels by venting the cores with a needle. After the cores were vented, the appropriate volume of C2H2 was added to each core to achieve a final C2H2 partial pressure of approximately 10 kPa in each of the different sizes of soil cores. All C2H2 gas used for the denitrification incubations was generated by reacting CaC2 with distilled water immediately prior to use. The pressure increase resulting from the C2H2 addition was then observed in each core using a pressure transducer equipped with a 55.2 kPa (8 psi) bellows (Unimeasure, Inc., Grants Pass, OR). The pressure readings were used to calculate the total gas filled volume in each of the samples as described by Parkin et al. (1984). Gas in the soil cores was mixed to distribute C2H2 throughout the soil pores. Mixing was accomplished by alternately drawing and releasing a vacuum on the samples using a 60-cm syringe. A device was constructed to facilitate this procedure, whereby four soil cores could be mixed at once (Parkin et al., 1987). The mixing procedure and the loose fit of the intact soil cores in the tubes facilitated both C2H2 distribution into and N2O distribution out of the soil pores. Following the gas mixing, the overpressure of gas in the soil cores resulting from the initial C2H2 injection was vented. Cores were incubated at 24 to 26°C and gas samples withdrawn at 3-, 6-, and 18-h time points following the C2H2 mixing. Gas samples were obtained by adding 5 mL of air to each core, mixing the gas in the cores, then removing a 5-mL gas sample. The 5-mL gas samples were stored in 33 Mention of a product by company or name is not an expressed or implied endorsement by the USDA. mL evacuated vials (Beckton Dickinson Co., Rutherford, NJ) for later N2O analyses.