D.R. Selbie1,2,3, G.J. Lanigan1, R.J. Laughlin4, H.J. Di3, James L. Moir3, K.C. Cameron3,
T.J. Clough3, C.J. Watson4, J. Grant1, C. Somers1
& Karl G. Richards1
1 Teagasc, Environmental Research Centre, Johnstown Castle, Wexford, Ireland.2 Centre for Soil & Environmental Research, Lincoln University, Christchurch, New Zealand
3 AgResearch, Ruakura Research Centre, Hamilton, New Zealand4 Agri-Environment Branch, Agri-Food & Biosciences Institute, Belfast BT9 5PX, UK.
Co-denitrification: an important nitrogen
cycling process in grazed grasslands
60-90% ingested N returned in excreta
Urine patch loading rate 30-100 g N/m2
Gaseous emission
NH3, N2O, N2
Pasture uptake
Leaching
Background
Conceptual model of co-denitrification under urine patches in
grassland soils, commencing with urea, the dominant N substrate
found in ruminant urine.
Background 2
OBJECTIVES
1. Quantify loss of N2 from urine amended soils
2. Quantify the contribution of conventional denitrification and co-denitrification to N2 losses
METHODS - treatments
o Urine collected from dairy cows during milking
o Urine was amended with 15N urea to enrich the N content to 45 atm% excess
o Two treatments
g N m-2
o Urine 10 g N/L = 100
o Urine 10 g N/L = 100 (+DCD)
o 2 L urine applied to 0.2 m2
o Nitrification inhibitor DCD applied (1.5 g m-2) in solution form
o day after urine (late autumn), again in late winter (annual 3.0 g m-2 DCD)
METHODS
N2O + N2 emissions
o Static closed chamber
o Gas chromatography (N2O)
o IRMS N2O & N2
o Stable Isotope Laboratory, UC Davis
Calculations
The flux of N2 was calculated
using Δ29R and Δ30R to calculate
contribution due to co-
denitrification (N2CO) and true
denitrification (N2TRUE).
Stats methods
o Effect of DCD on cumulative
emissions (date)
Calculating Co-denitrification1
To calculate co-denitrification flux – need to measure the D29R and D30R
beam areas and subtract out the 29N2 arising from ‘true’ denitrification
Labelled 15N2 (50 atom%)
‘true’ denitrification 15N – 15N D30R
15N – 14N D29R
Co-denitrification 15N – 14N D29R
dCD = -D29Rp12/(-D29Rp1
2+D29Rp1p2+q1p2-q2p1)
The fraction of the total moles of N2 in the headspace from co-denitrification (dCD) is….
p1 (0.9963) and q1 (0.0037) are atom fractions of 14N and 15N in the natural
abundance pool
p2 and q2 are the atom fractions of 14N and 15N in the enriched pool
1Stevens RJ & Laughlin RJ 2001 Soil Biology & Biochemistry 331287-1289
RESULTS – Temporal emissions N2O TRUE DENITRIFICATION
Selbie et al. (2015) Nature Scientific Reports 5:17361 1-5
RESULTS – Temporal emissions N2 TRUE DENITRIFICATION
Selbie et al. (2015) Nature Scientific Reports 5:17361 1-5
RESULTS – Temporal emissions N2 CO-DENITRIFICATION
Selbie et al. (2015) Nature Scientific Reports 5:17361 1-5
RESULTS – Effect of Nitrification (30 days post urine)
P < 0.05
Selbie et al. (2015) Nature Scientific Reports 5:17361 1-5
RESULTS – Cumulative Gaseous Emissions
N2CO
55.8 g N m-2
N2TRUE
1.07 g N m-2
N2O
TRUE 0.66 g N m
-2
Selbie et al. (2015) Nature Scientific Reports 5:17361 1-5
o N losses to air via N2 was the dominant loss
pathway from urine amended soil (major loss
pathway) 56.9 g N m-2
o Co-denitrification accounted for 55.8 g N m-2,
compared to only 1.1 g N m-2 from True
Denitrification (conventional methods
underestimate total N2 loss)
o Co-denitrification was the main source of N2
(98% of total). Drivers require further
investigation.
o Large co-denitrification N2 loss suggest large
losses of native soil N.
o High emissions of N2 and loss of organic soil
N helps explain low N recoveries in grazed
grassland
CONCLUSIONS
Acknowledgements
This research is financially supported under the National Development Plan, through the Research
Stimulus Fund (07 536), administered by the Department of Agriculture, Food and the Marine,
Ireland and the Teagasc Walsh Fellowship Scheme
References
ISOTOPIC MIXING MODEL CALCULATIONS
Mulvaney RL & Boast CW (1986) Equations for determination of N-15 labelled
dinitrogen and nitrous-oxide by mass-spectrometry. Soil Science Society of
America Journal 50(2):360-363
EQUATION TO CALCULATE ENRICHMENT OF DENITRIFYING POOL FROM A
SINGLE POOL SOURCE
Arah JRM (1997) Apportioning nitrous oxide fluxes between nitrification and
denitrification using gas-phase mass spectrometry. Soil Biology & Biochemistry
29(8):1295-1299.
CALCULATING DENITRIFYING POOL ENRICHMENT FROM TWO-SOURCE
POOL
Stevens RJ & Laughlin RJ (2001) Lowering the detection limit for dinitrogen using
the enrichment of nitrous oxide. Soil Biology & Biochemistry 33(9):1287-1289.
Further questions: [email protected]
Calculating Co-denitrification
Headspace N2 isotopic signature
reflects the relative contribution of aD,
aN and atm N2
Labelled
pool aD
Atm N2
The sources of N2 are apportioned into the fraction of gas (d'D) derived from
the denitrifying pool of enrichment aD and the fraction d'N = (1 - d'D) derived
from the pool or pools at natural abundance
Natural
abundance
pool aN
Utilising D29R and D30R, we can calculate full N2 flux using isotopic mixing models
(Mulvaney & Boast 1986) which assume that 14N and 15N atoms are randomly
Distributed during N2 generation
Can calculate ‘true’ denitrification from D30R –assume that that there is only a labelled
pool and atm N2 contributing to the enrichment of the denitrifying pool – Using the
‘backsolver’ to estimate the value of D29R that could be attributed to true denitrification
The remaining D29R is sourced from co-denitrification
- - - -
NH4+
NO3-
Cation exchange
The Nitrogen Cycle(McLaren & Cameron, 1996)
Nitrification Inhibitors DCD
METHODS - Trial establishment
o Intact monolith lysimeters
o Haplic Cambisol (WRB, 2007)
o Important dairy farming soil
o Free-draining sandy loam
o Ryegrass pasture
o 30 lysimeters collected and installed at
Johnstown Castle, Wexford 2009
A (0-15 cm)
A-B (15-55 cm)
B (55-100 cm)