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Soil Surface GHG Fluxes

Soil Nitrous Oxide and Methane Fluxes
Daniel Ginting

Project Goal

Nitrous oxide (N2O) and methane (CH4 ) negatively impact the chemistry of Earth's stratosphere and increase atmospheric radiative force. The N2O and CH4 contribution to atmospheric radiative forcing is 290 and 21 times higher than that of CO2, respectfully. Therefore, N2O and CH4 emission can negate the positive impact of sequestering CO2 carbon in a N fertilizer, no-till, and irrigation system, depending upon the amount of applied N and upon soil conditions. Understanding the conditions that govern N2O / CH4 emission can lead to improved N and C management such that a maize cropping system can have a net positive impact upon the atmosphere.

Project Objectives

  • To determine how temporal (diurnal and seasonal) and spatial (machinary wheel tracks and landscape position) changes in soil conditions impact N2O flux.
  • To evaluate the interrelation of soil properties and N2O fluxes following urea-ammonium nitrate (UAN) injection and UAN fertigation.
  • To examine N2O and CH4 emissions (seasonal or annual) as a function of cropping system (irrigated continuous maize, irrigated maize-soybean rotation, and rainfed corn-soybean rotation).

Project Description

The core questions to be investigated are:

  • Does N2O exchange fluctuate diurnally and, if so, when is the optimal sampling time for measuring N2O diurnal fluctuations?
  • What is the impact of wheel traffic on N2O emission?
  • Partitioning N fertilizer application (by injection in the spring prior to planting and by fertigation in the summer when crop N uptake is high) presumably impacts soil N and, thus, N2O emission. How do soil properties (moisture conditions, soil temperature, and soil N) relate to N2O flux? Does seasonal fluctuation of these soil properties govern seasonal fluctuation of N2O flux?
  • How do N2O and CH4 emissions rank among the three no-till systems (i.e. irrigated continuous maize, irrigated maize-soybean rotation, and rainfed maize-soybean rotation).


Four experiments were conducted to address each of the above questions. The research description of the first three studies is presented in an article by Ginting and Eghball (2005) (Soil Sci. Soc. Am. J., Vol. 69, May–June 2005). The gas sampling collection system for measuring N2O and CH4 concentrations and fluxes was custom made (Fig.1). Air samples were taken at the non-wheel tracked (NWT) interrows and at the wheel tracked (WT) interrows (Fig. 2). Twenty milliliter air samples were transferred to 12-mL, pre-vacuumed serum vials and transported to the laboratory where N2O and CH4 gas concentrations were determined with an automated gas chromatograph. Gas flux was calculated as the change in gas concentration times chamber volume per unit area per unit time.

Fig. 1A Aluminum vented chamber consists of a base (above) and cap (shown below). The base
(69-cm i.d., 7.5-cm height) is inserted 3-4 cm into the weed-free soil between two corn rows.

Fig. 1B The aluminum chamber cap (21-cm height) is welded to a vented lid. A rubber sleeve
(5-cm width) is taped to the lower portion of the cap and seals the joint between the chamber
cap and the chamber base.

Fig. 1C A horizontal copper tube (2-mm-diam.) connects to the vent under the lid.
An electric motor and 10-cm fan is fastened 10 cm underneath the lid and 20 cm
from the sampling tube cluster.

Fig. 1D Four sampling tubes (1-mm i.d.) were inserted through a septum on the center
of the lid and lowered 10 cm. Gas samples were collected with 30-mL syringes via the
sampling tubes at 0, 5, 10, and 15 minutes after sealing the chamber. A custom made
controller turns on the fan for 30 seconds prior to sampling. Soil temperature and
chamber air temperature is measured at every sampling.



Fig. 2 A) Schema showing the placement of the chamber relative to the interrows and
urea ammonium nitrate (UAN) injection zone. B) Schema of the tractor wheel-tracked
and non-wheel-tracked interrows.

Both the WT and NWT interrows received the same rate of UAN application. The length
of the UAN band within the circular chamber was 5.1% shorter than the length of a UAN
band associated with a hypothetical rectangle of equivalent area (Fig. 2A). Thus, error
associated with a circular chamber was less than what would be expected with a
rectangular sampling chamber.

Wheel traffic paths were unchanged between sampling years and multiple passes
occurred in the WT interrow each season (i.e. spring UAN injection, planting,
and harvesting).

Diurnal N2O Fluxes and Daily Measurements

  • Diurnal fluctuation in soil temperature is seen in June measurements (Fig. 3A and B) but not in July measurements due to the degree of canopy cover and the resulting interception of solar radiation.
  • The diurnal soil temperature pattern is not reflected in the N2O flux measurements.
  • The 4 hour flux measurement values were not significantly different from the continuous measurements. Therefore, a N2O flux measurement taken at any time of the day can be considered representative of the daily flux.

Fig. 3 N2O flux and soil temperature values sampled at various 24-h cycles during the
2002 growing season. Vertical bars represent the standard error of the means (n=3)
and Dunnet's LSD shows no significant differences between discontinuous and
continuous flux measurements.

Wheel Traffic Impact on N2O Fluxes

  • Wheel traffic had no significant effect on soil bulk density, moisture, and water filled porosity. Soil bulk density in the WT (1.40 Mg m-3) and NWT (1.34 Mg m-3) interrows were not statistically different at the 0.1 probability level.
  • There was not enough evidence to suggest that N2O fluxes were different between the WT and the NWT interrows (Table 1).
  • Under the conditions in our study, topography did not significantly impact N2O fluxes.
  • The UAN injector knife produced a narrow band of loosened soil that contained high concentrations of nitrate and ammonium, the main source of N2O. This disturbance and resulting N concentration obscured the effect of wheel track on N2O flux. So, even if there had been differences in bulk density due to soil compaction in most portions of the WT the NWT interrows, differences in N2O fluxes might not have been detectable.

  Date   WT   NWT   LSDα=0.1   F   P>F  
    --------------- g N2O - N ha-1 d-1 ---------------        
  22 May   4.23   6.06   6.65   0.31   0.603  
  6 June   2.71   3.07   7.77   0.01   0.929  
  2 July   2.72   2.91   7.13   0.00   0.960  
  17 July   11.4   9.69   4.24   0.61   0.469  
  14 August   4.49   0.76   6.94   1.17   0.328  

Table 1. Soil nitrous oxide fluxes from tractor wheel tracked (WT) and non-wheel-tracked
(NWT) interrows for five dates in 2002.

Variation in Seasonal N2O Flux

From April 2001 to April 2004, daily flux measurements were made in the six plots (intensive monitoring zones or IMZs ). The irrigated continuous maize system received an annual UAN application (60% injected in the spring followed by two summer fertigations (40%)). This N application schedule reflects the worse management scenario with respect to N2O emission. In each plot, a point measurement was made in the NWT interrow at 1, 3, and 10 days after UAN injection. Subsequent flux measurements were made every 1 to 2 weeks (May to July), every other week (August to October), and once a month (November to April). Soil temperature, water filled porosity, ammonium and nitrate were measured across the interrow area. Soil ammonium and urea in the injection zone was also measured for 135 days after the 2003 UAN injection.

N2O flux did not increase above background levels until 13 days after UAN injection. N2O fluxes peaked between 13 and 44 days after the injection then decreased to background levels 60 days after UAN injection (Fig. 4D). Average daily fluxes were 3 to 5 times higher within 60 days of the UAN injection than fluxes measured 60 days after UAN injection (Table 2). Fertigation had no effect on N2O flux. It appeared that the variation in N2O flux was governed by soil nitrate N in the UAN injection zone.

Fig. 4 Fluctuation of soil nitrate and ammonium concentrations in the injection zone and
0.23 m from the injection zone, and N2O fluxes within 140 days after UAN injection in 2003.
Vertical bars are the standard errors of the N2O means (n=6) at each sampling day.

  Year Cycle   Within 60   Beyond 60   LSDα=0.1   F   P>F  
      days after   days after              
      injection   injection              
    --------------- g N2O - N ha-1 d-1 ---------------        
  May 01 - Apr 02   26.8   9.24   12.4    7.88   0.038  
  Apr 02 - Apr 03   21.2   4.05   10.0   11.96   0.018  
  Apr 03 - Apr 04   28.0   7.50   11.0   15.23   0.011  

Table 2. Means comparison of daily N2O fluxes within 60 days of injection
(the period of high soil N) vs. fluxes measured afterwards (the period of low soil N).

Elevated N2O emission coincided with high levels of soil N. Soil ammonium and nitrate in the injection zone also decreased to background levels beyond 60 d after injection. Accounting for the lag time between UAN injection and the first N2O peak, the correlation coefficient between N2O flux and soil nitrate, ammonium and soil N was 0.79, 0.70, and 0.76, respectively. Low soil N 0.23 m away from the injection zone or interrow composite soil samples (Fig. 4) failed to explain the elevated N2O flux.

The data in Figs. 4 and 5 suggest that composite samples should not be used to describe N2O dynamics when N fertilizer is applied by injection.

Fig. 5 Soil ammonium and soil nitrate concentrations measured from composite soil samples
and N2O fluxes for three growing seasons. Urea ammonium nitrate (UAN) was injected
at the start of each growing season. Vertical bars represent the standard errors
of the means (n=6) on each sampling day.

Since N2O flux variations were much greater during the period of high soil N than during the period of low soil N, more N2O flux measurements should be made during the high soil N period.

Precipitation and irrigation resulted in relatively stable soil moisture each year (0.2-0.3 g g-1 which represents a water filled porosity of 0.50 to 0.98) (Fig. 6A). When N2O fluxes were elevated, the water filled porosity varied between 0.50 and 0.85. For the rest of the year, when N2O fluxes were similar to background levels (Fig. 6C), the water filled porosity varied between 0.42 and 0.98. N2O flux was not significantly correlated with soil water filled porosity.

Fig. 6 Soil water filled porosity (WFP; A), soil temperature (B), and N2O flux (C)
for each growing season. Urea ammonium nitrate was injected into the no-till,
irrigated continuous maize at the start of each season. Vertical bars are
standard errors of the means (n=6) for each sampling day.

There appears to be no relationship between soil temperature (Fig. 6B) and N2O flux (Fig. 6C). N2O peak flux did not correspond with peaks in soil temperature. N2O flux decreased to background levels 60 days after injection when soil temperature was high. The correlation coefficients between soil temperature and N2O flux were less than 0.22.

Poor correlation between water filled porosity or soil temperature and N2O flux should not be interpreted to mean that these factors are not essential to the release of N2O. In this study, elevated N2O fluxes occurred under a wide range of water filled porosities (0.50 - 0.85) and soil temperatures (10 - 31 oC) indicating that water filled porosity and soil temperature were not limiting factors in the release of N2O.

Daniel Ginting

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