Published October 31, 2014
Original Research
Soil Bulk Density and Moisture Content Influence Relative Gas Diffusivity and the Reduction of Nitrogen-15 Nitrous Oxide
Roland R. Klefoth, Tim J. Clough,* Oene Oenema, and Jan-Willem Van Groenigen Soil bulk density and moisture influence N2O movement and its reduction. This isotope study shows the sensitivity reduction of N2O, a greenhouse gas, to soil physical properties and their effect on gas diffusion. Increasing soil bulk density and water content promoted N2O reduction.
Nitrous oxide is a greenhouse gas and contributes to stratospheric ozone depletion. Soil physical conditions may influence N2O reduction and subsequent N2O emissions. We studied how soil water-filled pore space (WFPS) and soil bulk density (r b) affect N2O reduction and surface fluxes. Columns were repacked with soil and arranged in a factorial design at three levels of WFPS (60, 75, and 90%) and three levels of soil r b (0.94, 1.00, 1.07 Mg m−3). Over 19 d, 15N-enriched N2O was introduced at the base of the soil columns and N2O fluxes were measured. Relative gas diffusivities (Dp/Do) were also calculated. Soil r b and WFPS interacted to affect the recovery of N2O-15N and the antecedent inorganic-N contribution to surface fluxes. Reduction rates of N2O-15N ranged from 0.15 to 0.47 mg N2O-N g−1 soil d−1. Calculated Dp/Do values correlated (P < 0.01) with soil NH4+ –N (r = −0.73), NO3− –N (r = 0.93), cumulative N2O-N flux (r = 0.76), and N2O-N 15N enrichment (r = 0.80) and were affected by a soil WFPS ´ soil r b interaction. Soil N transformations and the net surface N2O flux is dependent on the soil’s Dp/Do, and WFPS alone does not suffice to discriminate between N2O emission sources. Consequently, the soil surface N2O flux may be comprised of N2O originating from deeper soil layers transported upward and/or from production in the topsoil. Abbreviations: PVC, polyvinyl chloride; WFPS, water-filled pore space.
Nitrous oxide is a greenhouse gas with a global warming potential of 298 during
R.R. Klefoth, O. Oenema, and J.-W. Van Groenigen, Dep. of Soil Quality, Wageningen Univ., Wageningen, the Netherlands; T.J. Clough, Faculty of Agriculture and Life Sciences, Lincoln Univ., Lincoln, New Zealand; and O. Oenema, Alterra, Wageningen Univ. and Research Centre, Wageningen, the Netherlands. *Corresponding author (
[email protected]). Vadose Zone J. doi:10.2136/vzj2014.07.0089 Received 13 July 2014.
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a 100-yr time period (Ciais et al., 2013). In addition, N2O has become the most important anthropogenic compound depleting stratospheric ozone (Ravishankara et al., 2009). Globally, the dominant source of anthropogenic N2O emissions is fertilized soils (Ciais et al., 2013), where it is produced predominately via microbial processes such as nitrification, denitrification, and nitrifier denitrification (Kool et al., 2011; Wrage et al., 2001). Under O2–limited conditions, N2O formation is dominated by denitrification, the stepwise reduction of NO3− to N2 , with N2O as an obligate intermediary that is reduced to N2 (Duxbury et al., 1982; Zhu et al., 2013). The N2O reduction rate is influenced by soil NO3− and microbially available C concentrations, soil pH, soil O2 status, and temperature (Butterbach-Bahl et al., 2013; Firestone and Davidson, 1989). It has been estimated that approximately 97% of the estimated annual global production of N2O-N (545 Tg) in soils, sediments, and surface waters is reduced to N2 and that only 3% is emitted as N2O into the atmosphere (Gruber, 2008). Once formed in the soil, N2O will diffuse to zones of lower concentration. Because the diffusion of gases through water is 10 4 times slower than in air, the effective diffusion coefficient for N2O in soil is proportional to the volume fraction of soil that is water filled (Farquharson and Baldock, 2008). Soil water-filled pore space (WFPS) has been used as a predictor for determining the occurrence of N2O production and reduction (Smith et al., 1998). However, the use of total porosity in the WFPS determination means that a comparison of soils varying in bulk density (r b) becomes problematic due
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to the distortion of the relationship between WFPS and N2O fluxes derived at a single soil r b (Farquharson and Baldock, 2008). Balaine et al. (2013) showed that relative gas diffusivity (Dp/Do) was a good predictor of soil surface N2O fluxes when comparing soils varying in both water content and soil r b because it directly reflects the potential for gas movement in the soil. Schjønning and Rasmussen (2000) indicated that soil structure and aggregation, like soil r b, were important considerations in characterizing diffusion through soil and hence denitrification rates. Previous studies have reported changes in N2O concentrations within a soil profile and the reduction of N2O during its diffusion through the soil (Clough et al., 1998; Klefoth et al., 2012; Russow et al., 2002; Van Groenigen et al., 2005). The potential to use 15N-enriched N2O to simultaneously measure both reduction and production of N2O has also been previously shown (Clough et al., 2006). However, there remains a dearth of information relating to N2O consumption rates within soil profiles, and there is still a need for systematic investigation into the many factors involved in the simultaneous consumption and production of N2O (Ball, 2013; Chapuis-Lardy et al., 2007; Clough et al., 2005). Such information is needed to improve N models and methods that seek to understand N transformations relating to N2O movement within soils and N2O emissions from soils (Del Grosso et al., 2000; Maier and Schack-Kirchner, 2014). The objective of this study was to assess the effect of varying both soil r b and WFPS on the reduction of N2O. We modified a chamber method (Klefoth et al., 2012) to let N2O-15N diffuse into repacked soils of varying r b and WFPS to trace the fate of the N2O-15N. Using N2O-15N allowed the relative contributions to the surface N2O flux to be distinguished. The surface flux comprised either the N2O-15N added or the N2O produced in situ. In addition N2O-15N use also allowed consumption rates of N2O to be calculated.
66Materials
and Methods
Experimental Design and Setup
A silt loam soil (a Typic Endoaquoll [Soil Survey Staff, 1999] with 12% sand, 74% silt, and 14% clay) was collected from a pasture soil at a depth of 0 to 20 cm near Christchurch, New Zealand (43°22.811 S, 172°40.567 E). Soil analysis conducted according to Blakemore et al. (1987) showed that the soil pH was 5.9, total C and N contents were 45 and 4.6 mg g−1, respectively, and the anaerobically mineralizable N content was 212 mg g−1. The soil was air dried at 20°C, sieved (2 mm), and homogenized before use. Sieved soil was packed according to soil r b treatment (see below) into polyvinyl chloride (PVC) columns (22 cm long by 8.6-cm diameter) that were fitted with PVC bases. Even compaction was achieved by taking aliquots of the sieved soil and placing these inside the PVC columns and then gently tapping the column, with an equal
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number of taps between soil aliquots, with a rubber mallet (Clough et al., 2003). The columns were divided into three compartments: a void at the base (2 cm deep) acted as a subsoil gas reservoir, while the soil core itself formed the middle compartment (15 cm deep). A metal mesh at the base of the middle compartment supported a 1-mm-thick silicone sheet (AlteSil silicone sheet, Altec Product Ltd.), permeable to N2O, which divided the subsoil gas reservoir and middle compartments. Finally, a headspace (8.5 cm high) was created above the soil during gas sampling when the chamber was fitted with a gas-tight PVC lid equipped with an O-ring and two rubber septa, which facilitated the sealing and gas sampling of the chamber headspace, respectively. Treatments consisted of three levels of soil moisture (60, 75, and 90% WFPS) and three levels of soil r b (0.94, 1.00, and 1.07 Mg m−3), subsequently referred to as low, medium, and high, respectively, arranged in a factorial design with four replicates per treatment, yielding a total of 36 soil columns. Soil moisture content was corrected for evaporative losses every 2 d using deionized water. Soil porosities were calculated using the soil r b and an assumed particle density of 2.65 Mg m−3 (Nimmo, 2004), with treatment soil r b values resulting in total porosities of 0.66, 0.64, and 0.62 m3 pores m−3 soil for the soil r b treatments of 0.94, 1.00, and 1.07 Mg m−3, respectively. Relative Dp/Do for the various soil treatments was calculated using a soil density corrected gas diffusivity model (Chamindu Deepagoda et al., 2011): æ e ö3 æeö = 0.02çç ÷÷÷ + 0.004 çç ÷÷÷ ÷ ç ç Do èfø è f ÷ø
Dp
where e is air-filled porosity (cm3 air-filled pores cm−3 soil pores) and f is total porosity (cm3 pores cm−3 soil). This was the only model available specifically derived to calculate Dp/Do for varying soil r b (Chamindu Deepagoda et al., 2011) One third of the soil water required to meet the WFPS treatment level was added to the soil before packing, while the remaining water required was applied slowly to the soil surface and allowed to infiltrate while avoiding surface ponding. Subsequently, the packed soil columns were kept at room temperature (20°C) for 4 d before the experiment to allow the soil moisture to equilibrate. It is conceivable that an artificial soil moisture gradient may have existed following equilibration and throughout the experiment; however, this was unavoidable because the experimental system did not permit the matric potential to be manipulated. The experiment was also conducted at room temperature. Soil subsamples (10 g dry soil equivalent) were taken before column packing and from every soil column at the end of the experiment to determine NH4+–N, NO2−–N, and NO3−–N concentrations using a 2 mol L−1 KCl extraction method, where 10 g of soil was shaken with 100 mL of KCl for 1 h and then filtered through Whatman no. 42 filter paper (Blakemore et al., 1987). p. 2 of 8
Nitrogen-15 Nitrous Oxide Production and Application Production of 15N-enriched N2O was achieved by taking a known mass of 15N-enriched NH4 NO3 (15NH414 NO3) and thermally decomposing this according to the method of Cotton and Wilkinson (1980). The 15N-enriched N2O produced was collected in a glass syringe and transferred to a pre-evacuated Tedlar gas bag (air sample bags, SKC Ltd.) and further diluted with N2 to achieve an N2O concentration of 10%. Production of 15N-enriched N2O was performed every 2 d to prevent the gas mixture residing for too long in the Tedlar bags. The average 15N enrichment of the N2O produced, across 10 batches, was 9.70 ± 0.59 (SD) atom% 15N. The 15N enrichment and concentration of the N O produced 2 was determined using continuous-flow isotope ratio mass spectrometry (IRMS) on a Sercon 20/20 (Sercon Ltd.) at Lincoln University, according to Stevens et al. (1993). Four days after the soil treatments were established, a gas mixture comprising 10% 15N-enriched N2O and 90% N2 was periodically injected into the gas reservoir at the bottom of the soil columns using a glass syringe fitted with a three-way stopcock. Injections (2 mL) were made three times, every 2 h during a 6-h period, on Day 0 and then every 12 h for the remaining 19 d of the experiment. This maintained an elevated N2O concentration in the subsoil compartment. At the end of the experiment, concentrations of dissolved N2O were determined using the appropriate Bunsen coefficient (Weiss and Price, 1980) and the method of Davidson and Firestone (1988).
Nitrous Oxide Fluxes and Nitrogen-15 Nitrous Oxide Recovery Nitrous oxide fluxes at the soil surface were determined before the injection of 15N-enriched N2O and during the course of the experiment. To facilitate gas sampling, the headspace was sealed with the PVC cap for 30 min before gas sampling. Two gas samples were taken after this 30-min period. The first (15 mL, 3.0% of the headspace volume) was taken using a glass syringe fitted with a three-way stopcock, placed into a 12 mL Exetainer, and sent to the University California, Davis, isotope laboratory for 15N analysis on an IRMS (Sercon 20/20, Sercon Ltd.). The second gas sample (7 mL, 1.4% of the headspace volume), sampled as described above, was placed into a 6-mL Exetainer and analyzed on a gas chromatograph to determine its N2O concentration as described by Clough et al. (2006). Only one N2O sample was taken for gas chromatography to minimize the potential for sampling-induced disturbance of gas diffusion, and it was therefore assumed that N2O in the gas sampling headspace increased in a linear fashion. Previous experiments, using similar headspace volumes and with high rates of N application, have previously shown linear increases in headspace N2O concentrations (Uchida et al., 2008). The headspace cap was removed after gas sampling was complete. Fluxes of N2 O-N (mg m−2 h−1) were determined using the increase in headspace N2 O concentration with time while
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assuming, as noted above, that increases in headspace N2 O concentrations followed a linear trend. The ambient N2 O concentration was sampled and used as the time-zero N2O concentration. Cumulative fluxes were calculated by integrating the area under the plot of N2O flux vs. time using a trapezoidal method. Recovery of the N2O-15N applied was calculated for the sum of the cumulative N2O-15N flux and the N2O-15N recovered in the soil atmosphere and water phases and in the reservoir at the end of the experiment. Recovery of the N2O-15N applied was expressed as a percentage and determined using the equation of Cabrera and Kissel (1989). Non-recovery of applied N2O-15N was assumed to be due to reduction of N2O-15N to N2 . The contribution of antecedent N to the total N2O flux was calculated by determining the ratio of moles of unenriched N2O evolved to the total moles N2O evolved (unenriched and 15N enriched) and then expressing this as a percentage. After the last N2O flux measurement (Day 19), the N2O remaining entrapped within the soil columns was determined. Twelve hours after the last injection of 15N-enriched N2O into the reservoir gas samples, one 15- and one 7-mL sample, for 15N-enriched N2O and N2O gas chromatography measurements, respectively, were taken from the reservoir via the injection port. The headspace chambers were then placed on the soil columns, which were manually shaken until the soil was loose and friable and the entrapped N2O was released (Clough et al., 2001) and thoroughly mixed with the headspace atmosphere. Gas samples were then taken via the headspace septa as described above, with concentrations of N2O and 15N enrichment corrected for dilution by ambient air contained in the headspace.
Statistical Analyses Statistical analyses were performed using SPSS (IBM Corporation). The N2O flux data were tested for normality and found to be skewed and hence were logarithmically transformed. Two-way analysis of variance was used to determine treatment (WFPS and soil r b) effects and their interactions on N2O reduction and soil surface fluxes. When significant treatment effects occurred, differences between means were evaluated using Tukey’s test. Pearson correlation coefficients were determined to evaluate linear relationships between variables.
66Results
Nitrous Oxide Flux Measurements
Average daily N2O-N fluxes ranged from a minimum of 0 mg N2O-N m−2 h−1 in the low soil r b treatment at 90% WFPS to 2.94 mg N2O-N m−2 h−1 in the high soil r b treatment at 60% WFPS (Fig. 1). The average daily N2O-N fluxes were relatively constant with time within a given treatment (Fig. 1). An interaction (P < 0.05) occurred between soil r b and WFPS. The N2O-N fluxes were unaffected by soil r b at either 60 or 90% WFPS, averaging p. 3 of 8
Table 1. Cumulative N2O-N fluxes, N2O-15N enrichment, and 15N recoveries as surface fluxes of N2O-15N applied at three soil bulk densities (r b) and three water contents determined as water-filled pore space (WFPS). Soil r b Low
Medium
High
WFPS
Cumulative N2O-N
Mean N2O-15N enrichment
15N recovery as N2O-N flux
% 60 75 90 60 75 90 60 75 90
mg m−2 915 ± 49 a† 878 ± 65 a 6±1b 890 ± 24 a 473 ± 248 ab 17 ± 6 b 912 ± 22 a 252 ± 227 b 34 ± 21 b
atom% 10.72 ± 0.07 a 9.71 ± 0.35 ab 0.39 ± 0.02 c 10.76 ± 0.03 a 5.09 ± 2.50 bc 0.38 ± 0.01 c 10.75 ± 0.07 a 2.88 ± 0.35 c 0.39 ± 0.02 c
% 63.8 ± 3.7 a 54.5 ± 6.2 ab 0.0 ± 0 c 62.3 ± 1.8 ab 26.8 ± 15.4 bc 0.0 ± 0 c 63.8 ± 1.5 a 14.5 ± 14.5 c 0.0 ± 0 c
† Means ± SE (n = 4); means within a column followed by different letters are significantly different (P < 0.05) for the interaction between WFPS and soil r b by Tukey’s test.
Fig. 1. Nitrous oxide fluxes during the experimental period at either 60, 75, or 90% water-filled pore space (WFPS) for soil columns at low, medium, and high soil bulk densities of 0.94, 1.00, and 1.07 Mg m−3, respectively. Error bars denote the standard error of the mean (n = 4).
1.89 ± 0.66 (SD) and 0.04 ± 0.14, respectively. At 75% WFPS, however, the N2O-N fluxes decreased with increasing soil r b, averaging 1.68 ± 0.09, 0.92 ± 0.16, and 0.49 ± 0.14 in the low, medium, and high soil r b treatments, respectively (Fig. 1). Cumulative N2O-N fluxes ranged from 6 to 915 mg N2O-N m−2 (Table 1), with a treatment interaction between WFPS and soil r b significant at P = 0.06. Higher cumulative N2O-N fluxes occurred at 60% WFPS, while the lowest cumulative fluxes occurred at 90% WFPS. At 75% WFPS, however, cumulative N2O-N fluxes decreased as soil r b increased (Table 1). Atom% 15N enrichment of the N2O-N fluxes, while constant with time within a treatment (Table 1), also varied as a result of a WFPS ´ soil r b interaction (P < 0.05). In the 60% WFPS treatment, atom% 15N enrichment remained elevated regardless of the soil r treatment, b averaging 10.74 ± 0.50 (SD). In the 75% WFPS treatment, N2O-N atom% 15N enrichment declined with increasing soil r b, with values of 9.71 ± 0.53, 5.09 ± 0.73, and 2.88 ± 0.21 atom% 15N in the low, medium, and high soil r b treatments, respectively. At 90% WFPS, N2O-N 15N enrichment remained low (average 0.38 ± 0.14 atom%), regardless of soil r b treatment (Table 1).
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The 15N enrichment of the N2O-N flux at 90% WFPS was equal to the soil 15N enrichment, indicating that the relatively small cumulative N2O fluxes at 90% WFPS (Table 1) were totally derived from soil N. Conversely, at 60% WFPS, 15N enrichment of the N2O fluxes showed no contribution from antecedent soil N, with 15N enrichment equal to the 15N enrichment of the N O injected. 2 At 75% WFPS, however, the contribution of antecedent soil N to the N2O flux increased with increasing r b, with antecedent soil N comprising, on average, 0, 53, and 70% of the N2O flux in the low, medium, and high soil r b treatments, respectively. Recovery of the N2O-15N applied, as a surface N2O flux, varied from 0 to 63.8%, with the highest recoveries at 60% WFPS (P < 0.01) and with an interaction (P = 0.07) between soil r b and WFPS affecting N2O-15N recovery. This interaction followed that observed for the cumulative N2O-N flux, with the highest 15N recoveries at 60% WFPS, the lowest 15N recoveries at 90% WFPS, and increasing 15N recoveries with decreasing soil r b at 75% WFPS (Table 1).
Reservoir and Soil Entrapped Nitrous Oxide
Some of the N2O-15N applied was recovered in the reservoir, where the N2O-15N was initially injected each day, and in both the soil atmosphere and soil water phases. In the reservoir, the recovery of the N2O-15N applied averaged £0.83%, with treatment effects limited to WFPS, where higher recoveries occurred in the reservoir at 75% WFPS (Table 2). The recovery of the applied N2O-15N from the soil’s air-filled pore space was highest at 75% WFPS, with mean recoveries increasing as soil r b decreased, while at 60 and 90% WFPS the N2O-15N recoveries were static at £0.05 and 0%, respectively (Table 2). Recovery of N2O-15N applied as dissolved N2O in the soil water reflected the air-filled trends, with mean dissolved N2O-15N recoveries at 75% WFPS increasing from 0.1 to 0.9% as soil r b decreased. Mean dissolved N2O-15N recoveries at p. 4 of 8
60 and 90% WFPS were again static at £0.05 and 0%, respectively (Table 2). The mean total of the N2O-15N recovered in the soil column and reservoir was a maximum of 2.2%, which occurred at 75% WFPS and low soil r b (Table 2).
Soil Inorganic Nitrogen and Relative Gas Diffusivities
Table 2. Nitrogen-15 recoveries in the reservoir and soil gas and water phases at three soil bulk densities (r b) and three water contents determined as water-filled pore space (WFPS). Soil r b Low
Soil atmosphere
Dissolved in soil water
WFPS
Reservoir
60 75 90 60 75 90 60 75 90
————————————————% ———————————————— 0.05 ± 0.01 a† 0.05 ± 0.01 b 0.05 ± 0.008 b 0.15 ± 0.02 b 0.83 ± 0.12 a 0.47 ± 0.08 a 0.90 ± 0.15 a 2.20 ± 0.33 a 0a 0b 0b 0b 0.04 ± 0.01 a 0.04 ± 0.01 b 0.03 ± 0.01 b 0.11 ± 0.02 b 0.55 ± 0.33 a 0.10 ± 0.06 b 0.19 ± 0.11 b 0.86 ± 0.49 b 0.01 ± 0.01 a 0b 0b 0.01 ± 0.01 b 0.04 ± 0.01 a 0.02 ± 0.01 b 0.02 ± 0.01 b 0.08 ± 0.01 b 0.42 ± 0.42 a 0.05 ± 0.05 b 0.10 ± 0.09 b 0.58 ± 0.57 b 0.02 ± 0.01 a 0b 0b 0.02 ± 0.01 b
Total
Initial soil NH4 +–N, NO2−–N, and NO3−–N Medium concentrations at packing were 0.2, 0.1, and 5.9 mg g−1 soil, respectively. When the soil columns High were destructively analyzed, soil inorganic-N concentrations differed due to a treatment interaction (P < 0.01). Concentrations of NH4 +–N were † Means ± SE (n = 4); means within a column followed by different letters are significantly different (P < 0.05) for the interaction between WFPS and soil r b by Tukey’s test. £0.1 mg g−1 soil at 60% WFPS, regardless of soil r b, while at 75% WFPS, soil NH4 +–N concentrations increased from
