B53C-0682. Seasonal Patterns of Denitrification and Trace Gas Emissions in a Northern Hardwood Forest
Jennifer L. Morse; Jorge Durán; Peter M. Groffman
Cary Institute of Ecosystem Studies, Millbrook, NY, USA
Background
Environmental variables
We measured environmental variables, trace gas
fluxes, and soil denitrification rates in high and
low elevation sites in a northern hardwood forest
in New Hampshire, USA, over two years.
This study aimed to identify seasonal patterns
and correlations between environmental
variables and trace gas emissions
We estimated soil denitrification rates using an
intact core direct flux (N2, N2O) method (Burgin et
al. 2010, Soil Science Society of America Journal
74(5):1826-1833) and compared N2O fluxes
measured in the field and during lab incubations.
Approach
high
Field trace gas fluxes
A)
Lab gas fluxes
A)
A)
Snowmelt
B)
B)
B)
C)
C)
C)
Snowmelt
Six plots in Hubbard Brook
Experimental Forest, NH, USA:
low
Hubbard Brook, NH
-3 at high elevation (539-595 m)
-3 at low elevation (375-511 m)
Environmental variables with
Campbell Scientific data
loggers (hourly means at 5 cm
depth):
- Soil oxygen (Apogee sensors)
- Soil moisture and temperature
(Decagon 5TM probes)
Gas flux measurements using
closed chambers (CO2, N2O, CH4)
- During fall, snowmelt, spring, and
summer (Nov 2010-August 2012)
- Measured with GC-ECD, and GC-FID
Surface soil (0-10 cm) incubated at
5%, 10%, and 20% O2 in Helium
- 5% and 20% not shown
- N2-free atmosphere
- Measured N2 and CO2 with GC-TCD
and N2O with GC-ECD
Snowmelt
Snowmelt
Snowmelt
Snowmelt
Key Findings
Seasonal patterns
• Clear seasonal patterns were seen
in soil temperature, but not in soil
O2 and soil moisture.
• Field CO2 and CH4 fluxes followed
soil temperature patterns
• Soil N2O fluxes had no clear
seasonal pattern
• Snowmelt periods had the highest
denitrification rates (both N2 and
N2O) in lab incubations at 10% O2.
Elevation patterns
• Soils at low elevation sites were
warmer, drier, and more oxic, with
higher CH4 consumption, compared
to high elevation sites
• No consistent elevation patterns
were found in field N2O fluxes or in
lab incubations at 10% O2
Denitrification N gas fluxes
Table 1. Mean annual denitrification N gas fluxes and N2O yields
% O2
Fig 1. A) Soil temperature; B) soil moisture; and
C) soil O2 concentrations from Dec 2010 to Aug
2012, with snowmelt periods shown in blue.
Fig 2. Field-measured fluxes of A) CO2; B) N2O;
and C) CH4 from Nov 2010 to Aug 2012, with
snowmelt periods shown in blue.
Conclusions
• We found clear seasonal patterns for soil temperature and fluxes of CO2 and CH4
• In lab incubations, we measured higher N2O fluxes at 5% O2 versus 10% O2 and 20% O2
• We measured the highest N2 and total N gas fluxes and lowest N2O yields at 10% O2,
suggesting a tight link between nitrification and denitrification.
• Using Q10-based corrected mean fluxes over 184 days, we estimated denitrification N
losses range from 0.4 kg N ha-1 y-1 at 20% O2 to 2.8 kg N ha-1 y-1 at 10% O2
Fig 3. Fluxes of A) CO2; B) N2O; and C) N2 from lab
incubations of soils collected from Nov 2010 to
Aug 2012, with snowmelt periods shown in blue.
5%
10%
20%
Total N gas
Elevation
% N2O
-1 -1
-1 -1
-1 -1
(kg N ha y ) (kg N ha y ) (kg N ha y )
Low
High
Low
High
Low
High
N2O flux
N2 flux
0.71
0.30
0.21
0.17
0.16
0.21
2.07
2.28
2.62
2.62
0.38
0.11
2.78
2.58
2.83
2.79
0.54
0.32
25.54
11.63
7.42
6.09
29.63
65.63
(estimated from Q10-corrected 24-h lab incubations, and upscaled using a 184-day "year")
Acknowledgments
This research was supported by the National Science Foundation (DEB #0949664 and
DEB #0919047). This research was conducted at the Hubbard Brook Experimental Forest,
which is operated by the Northeastern Research Station, USDA Forest Service, Newtown
Square, PA. This paper is a contribution to the Hubbard Brook Ecosystem Study.
Contact Information
J.L. Morse: Cary Institute of Ecosystem Studies, PO Box AB, Millbrook, NY 12545;
[email protected]; 1-845-677-7600, ext. 234