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: karl.richards@teagasc.ie

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)