Development of an Improved Method for Measuring Nitrous Oxide Emissions from Soils in the Southeast 1Natalie Ross*, 1Chris Reberg-Horton, 1,2Julie Grossman, and 1Wayne Robarge 1Department of Soil Science, NCSU, 2Department of Horticultural Science, UMN

Introduction

•  Nitrous oxide (N2O) is a greenhouse gas trapping approximately 300 times more atmospheric heat than carbon dioxide (CO2).

68% Agricultural Soils

300 CO2

1 N2O

*Email: NRoss2@ncsu.edu

Nitrous Oxide Production A microbial process called denitrification produces N2O when water from precipitation or irrigation fills soil pores and creates an anaerobic environment. Through denitrification, nitrate (NO3

-) can be converted to and released as N2O, if not further reduced to harmless N2 gas.

No rain

N2O

Rain N2O

Soil

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0 1 2 3 4 5 6 7

Rel

ativ

e N

2O F

lux

Time (days)

Figure 1: N2O Temporal Emissions Curve

Point 1, assumed

Rain on day 1

Point 2, actual

•  Assuming measurements are being made at the peak of emissions leads to uncertainty when calculating cumulative emissions.

Approximately 95% of N2O emission measurements use the static chamber method: individual enclosed chambers over limited surface area for short time periods . Release of N2O from agricultural soils, however, exhibits high temporal and spatial variability, both of which contribute to uncertainty in quantifying cumulative N2O emissions per crop. The estimate of cumulative emissions is the most important parameter for comparing N2O emissions between agro-ecosystems.

Spatial variability can be addressed by increasing the number of chambers sampled. Temporal variability is less straightforward to address. Even with daily static chamber sampling (typically ~ 1 hour), the 23 hours in-between measurements is enough time to fail to properly quantify a significant emissions event.

•  Agricultural soils account for 68% of U.S. anthropogenic N2O emissions.

Objective •  To reduce uncertainty in cumulative N2O flux estimates due to temporal

variability

Approach:

•  Use static chamber method for quantitative N2O flux values

•  Use a continuous-flow chamber method to record an index of the temporal emissions curve that will be used to qualitatively inform adjustments static-chamber measurement based cumulative N2O flux

Researchers often measure N2O emissions 12 to 72 hours after a rain event. It is often assumed measurements are being made at the peak of the temporal emissions curve. However, the magnitudes and patterns of N2O temporal emissions curves are largely unknown.

Static Chamber Method A semi-permanent PVC anchor is placed in the soil at least 24 hours prior to sampling.

An air-tight lid with a septum and a vent is placed over the anchor.

Five gas samples are extracted over 40 minutes using a two-way needle and pre-evacuated glass serum vials with red rubber septa.

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0 10 20 30 40

N2O

(ppb

v)

Minutes

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0 1 2 3 4 5 6 Days

Flux

(mg

N2O

h-1

m-2

)

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Samples are transported to the lab and analyzed on a gas chromatograph (GC).

Flux, or the rate of change of N2O in the chamber headspace per unit area per unit time, is then calculated.

Flux: 2 mg N2O m-2 h-1

Cumulative Flux Calculation

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5

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Flux

(mg

N2O

h-1

m-2

)

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1 3 5 7 9 11 13 15 17

Interpolate between flux data points. Each point represents one flux estimate derived from five static chamber measurements.

Integrate the area under the curve for the cumulative flux value. This represents an ideal situation, where the temporal pattern in emissions is well characterized.

Realistically, there is often poor characterization of the temporal emissions curve (Fig. 1), leading to uncertainty in cumulative flux estimates.

We need a way to characterize the temporal emissions curve and reduce uncertainty.

Flow-Through Chamber Method

Air is pumped from each chamber…

… to the gas analyzer in the trailer.

•  A qualitative index of the temporal emissions curve was recorded, which was then used to inform adjustments to cumulative flux calculations from static chamber data.

•  An infrared gas analyzer was housed in a trailer at the field site to continuously measure the concentration of N2O in the gas stream drawn from flow-through chambers positioned at multiple group locations in a no-till, conventional agro-ecosystem.

•  Sampling cycled between four chambers and one ambient air sample every 2.5 hours.

•  Chamber lids were moved after rain events to fresh monitoring location anchors installed prior to the start of the monitoring period.

Results & Discussion

•  Each point is one static-chamber flux value (n=120 observations).

•  The highest flux was observed immediately after the passage of tropical storm Ana and corresponded with a recent N fertilizer application.

•  For the remainder of the season no further peak fluxes were detected.

Static Chambers

•  This is an example of the flow-through chamber data from one chamber

•  The net concentration of N2O in the headspace peaked and diminished following the same trend as measured with the static chamber N2O.

Flow-through Chambers

Note: For all this work, you get ONE data point.

Adjustments to Cumulative Flux Calculations

•  Using only static chamber data and linear interpolation and integration to calculate cumulative flux potentially overestimates total flux.

•  The index of the temporal emissions curve from the continuous flow-through chamber provided an estimate of when the emissions event started.

•  Adding a “static chamber” measurement at the start of the emissions event reduced the tendency to overestimate flux, but it resulted in potential underestimation of cumulative flux.

Thanks

We used three methods to derive an estimate of cumulative N2O loss from April 30 to Sept. 3, 2015

1

2

3

•  To estimate a peak flux value, we modeled the decay in emissions using a simple exponential decay curve:

Static Chamber Flux = A e(-Bx)

where A = peak flux, B is constant and x = time since peak flux

•  Using natural logs, B is the slope of continuous flow-through chamber N2O concentration vs. time

•  For n = 4 continuous flow-through chambers, B = 0.0384 ± 0.0046 (12% RSD)

•  With derived B value, the A-term was calculated from a static chamber flux measurement taken 18.6 hours after peak

•  The defined start of event (method 2, left) and the derived exponential decay curve calibrated with the static chamber measurement were then used to estimate cumulative N2O flux.

•  This method corrected for the tendency to overestimate cumulative flux in method 1 and to underestimate cumulative flux in method 2.

Sincere thanks to Dr. Wayne Robarge, Dr. Chris Reberg-Horton, Dr. Shuijin Hu, Andrew Meier, CEFS, BASF, Najwa Alnsour, JiJY Sooksa-nguan, and the Greenhouse Gas team. Funded by a USDA-NIFA Grant.

Conclusion

3 1 2 Cumulative Flux Calculation Method

Error bars represent one standard deviation for n = 4 observations.

•  Using a continuously recorded index of the temporal emissions curve to adjust quantitative estimates of the cumulative N2O flux from static chamber measurements did not produce absolute differences between flux calculation methods.

•  However method 3 corrected for the tendency to overestimate cumulative flux from method 1 and the tendency to underestimate flux in method 2.

1009 ± 827

609 ± 379

768 ± 540

Future Work •  Use automated chamber lids to decrease

effects on chamber environment

•  Quantitatively measure flux with the continuous flow-through system