Comparison of Field Denitrification Rates Determined by Acetylene-Based Soil Core and. Nitrogen-15 Methods1. TIMOTHY B. PARKIN, ALAN J. SEXSTONE ...
Comparison of Field Denitrification Rates Determined by Acetylene-Based Soil Core and Nitrogen-15 Methods1 TIMOTHY B. PARKIN, ALAN J. SEXSTONE AND JAMES M. TiEDjE2 ABSTRACT A soil core method to measure field denitrification rates was evaluated by comparing the results with denitrification rates measured by the "N difference method. In the core method, acetylene amended air was recirculated through an undisturbed soil core and short-term (ca. 1 h) nitrous oxide production monitored. Denitrification rates were obtained on soil cores from two field sites and a set of sieved, repacked soil cores in a laboratory incubation. Cumulative N-losses were estimated by interpolating between sample dates. Denitrification rates obtained by the core method were found to approximate a log normal distribution at both sites studied and at all sampling times. Statistical calculations appropriate for this distribution resulted in improved estimates of the mean rates and confidence intervals. For the 15N difference method, I5N enriched nitrate was added 1 Contribution from the Dep. of Crop & Soil Sciences and of Microbiology & Public Health, Michigan State Univer., East Lansing, MI 48824-1114. Published as Journal Article no. 11211 of the Michigan Agric. Exp. Stn. This work was supported by National Science Foundation Grant DEB-80-12168, USDA Cooperative Agreement no. 58-32U4-1-329 and USDA Regional Research Project2 NE-39. Received 4 Nov. 1983. Approved 6 Aug. 1984. Former Research Associate, Graduate Student and Professor of Soil Microbiology, respectively. Present addresses: T.B.P., Soil Nitrogen & Environmental Chemistry Lab., Beltsville Agric. Res. Center, Beltsville, MD 20705 and A. J.S., Div. of Plant & Soil Sciences, West Virginia Univ., Morgantown, WV 26506.
to microplots of 30 cm diameter and the total gaseous losses were estimated by considering the change in the total nitrate pool, the isotope ratio of that pool and the "N not recovered from the microplots. Experiments were conducted over periods of 32 to 45 d on sandy loam and clay loam soils, respectively. Denitrification rates from the acetylene-core method were not significantly different than the estimates by the "N method. Denitrification rates as measured by both methods were highly variable. Additional Index Words: nitrate, nitrogen balance, nitrous oxide, log-normal distribution, spatial variability Parkin, T.B., A.J. Sexstone, and J.M. Tiedje. 1985. Comparison of field denitrification rates determined by acetylene-based soil core and nitrogen-15 methods. Soil Sci. Soc. Am. J. 49:94-99.
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ESPITE THE GENERALLY RECOGNIZED importance
of denitrification, field rates of N loss by this process are poorly quantified. Methods that are accurate, reliable and simple are needed to measure these rates, to quantify the effect of environmental factors on these losses and to characterize the high temporal and spatial variability that are characteristic of this process. The acetylene inhibition methods used with
PARKIN ET AL.: DENITRIFICATION RATES DETERMINED BY SOIL CORE AND NITROGEN-15 METHODS
soil cores appear to fill this need, but have not yet been evaluated against other methods. The most widely accepted methods to measure denitrification in the field use 15N as a tracer. The 15N methods include attributing unrecovered 15N to denitrification (e.g. Broadbent and Clark, 1965; Olson, 1980), measurement of 15N gases produced by 15N amended soils (e.g., Rolston et al., 1978; Focht and Stolzy, 1978; Siegel et al., 1982) and dilution of added 15 N-N2 by denitrified soil N (Limmer et al., 1982). The acetylene methods use two basic approaches: (i) measurement of N2O accumulation under in situ soil covers after acetylene addition to soil (e.g. Ryden et al., 1979), and (ii) measurement of N2O production rates in acetylene amended soil cores (e.g. Aulakh et al., 1982; Rice and Smith, 1982; Parkin et al., 1984). Soil core methods have been very useful for comparisons of denitrification rates among treatments. However, before the rates obtained by these methods are used to estimate actual N losses from a sampled area, experimental evaluation of the accuracy of the N loss estimates is needed. In this paper we compare the cumulative N loss obtained by repeated field sampling using our gas-flow soil core method (Parkin et al., 1984) 15 with the N difference method. 15 The N method is most commonly used for assessment of fertilizer-N losses and not for measurements of total denitrification. In order to compare the core and 15N methods, the 15N method must be adapted to measure total denitrification. We also present here the approach used and the inherent limitations of measuring total denitrification rates by the 15N difference method. Because of the large number of soil cores required to quantify the N loss, we were also able to evaluate the statistical distribution of denitrification rates. Ryden and Dawson (1982) report coefficients of variation for denitrification in grassland in the range of 10 to 30%. In other denitrification studies, however, variability was higher and coefficients of variation of 50 to 100% were observed (Aulakh et al., 1982; Rice and Smith, 1982; Parkin et al., 1984). Although high variability of denitrification rates has been observed, few studies have investigated the variability in detail, or considered the most appropriate methods for statistical summarization of the data. MATERIALS AND METHODS Experimental Plots Two field sites were established on the Michigan State University farms, East Lansing, Michigan. Site I was a Capac clay loam (Aerie Ochraqualfs) of 34% clay, 30% sand, 36% silt, pH 6.8. Site II was a Spinks sandy loam (Psammentic Hapludalfs) of 13% clay, 71% sand, 16% silt, pH 6.5. The sites were fallow during the experimental periods, but previously had been planted in corn (Zea mays L.) and soybeans (Glycine max L.) in alternate years. Plots were approximately 20 m by 10 m and were rototilled prior to the start of each experiment. Denitrification Rates Measured by the Soil Core Method Denitrification rates were measured using the core method described by Parkin et al. (1984), in which acetylene amended
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air was recirculated through intact soil cores. Soil cores were contained in rubber-stoppered plastic tubes 4.7 cm in diam and 20 cm long. The assay was carried.out at 22°C which was not substantially different from in situ soil temperatures. Linear rates of N2O production, obtained within 1 to 2 h of incubation, were used to estimate the denitrification rates which were calculated on a per gram dry weight of soil basis (Parkin et al., 1984). Cumulative denitrification N losses from the sites were then calculated by integration of the daily mean rates using the trapezoid rule implemented by a BASIC computer program (Poole and Borchers, 1979). The first field evaluation was conducted during May and June of 1981 on the clay loam site. Nearly every day during this experiment, 4 to 36 soil cores were collected and denitrification rates determined. The second field evaluation was carried out on the sandy loam site during June and July, 1982. For this experiment, cores were collected at 3-to 4-d intervals during relatively dry periods and at 6-to 12-h intervals after rainfall. At each sampling during the second field trial, 6 to 56 cores were collected. Denitrification rates in the first field experiment were measured with the soil core technique using air + acetylene (1.82 X 10~2 MPa O2, 18%) as the recirculating gas. In the second field experiment, the in situ gas composition of soil macropores was measured with Gortex-tubing gas samplers (Parkin and Tiedje, 1984) throughout the course of the experiment. The O2 concentration in the gas recirculated through the soil cores was adjusted to match the in situ O2 content of the soil macropores. Soil oxygen concentration ranged from 1.41 X 10~2 MPa (14%) to 2.0 X 10~2 MPa (20%) O2. The denitrification data from both sites are presented in Fig. 4 and 6 of the accompanying manuscript (Sexstone et al., 1985). Denitrification Rates Measured by the 1SN Difference Method Rates of denitrification N loss were determined in galvanized steel cylinders (30 cm in diam by 95 cm long) driven into the ground to a depth of 90 cm. Compaction was < 5%. These microplots were allowed to stabilize for 14 to 21 d before experiments I5were started. At the start of the first experiment 39 mg of N (99 atom %) as KNO3 was added to soil in each cylinder in 175 mL of deionized water. This resulted in a 2 mg/kg increase of the in situ NOf pool over the 0-to 10-cm depth. In the second field experiment15a water solution of 100 mg N as NH/'NOj (2 atom % N) was added to each cylinder. Nonenriched KN03 or NH4NO3 was applied to the surrounding field at the same1 concentration as in the microplots, 4.4 and 14.2 kg N ha" for Sites I and II, respectively. At 7 to 14 d intervals after addition of labeled N, two to four cylinders were sacrificed and soil was removed in 10 cm layers. This soil was sieved (
