Eidgenössisches Departement für Wirtschaft,

Bildung und Forschung WBF

Agroscope

www.agroscope.ch I gutes Essen, gesunde Umwelt

Process-based model estimation of N2O Emission

factors for urine patches in a Swiss grazing system Kate Kuntu-Blankson (1,2), Lena Barczyk (1,2), Johan Six (2), Christof Ammann (1), and Pierluigi Calanca (1)

1 Climate and Agriculture group-Agroscope, Reckenholzstrasse 191, 8046 Zürich2 ETH Zürich, Institute of Agricultural Sciences (IAS) – Sustainable Agroecosystems, Tannenstrasse 1, 8092 Zürich

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Background

Understanding the processes driving grassland N2O emissions is of paramount

importance for developing national GHG emissions inventories

Controls on N2O production processes

Climate (precipitation, temperature)

Soil N and properties affecting O2 availability

Management: timing of grazing

Grazing related N2O emissions

Cattle excreta returned during grazing has ~ 80% of N intake by cattle

Spot covered by a urine patch has high N load (500 to 1000 kg N ha-1)

N load usually in excess of pasture needs

Urine patches become hotspots for N2O emissions

Rates and extends of N2O emissions

depend on the complex interactions

between soil conditions, short-term

weather and management

Unlike mineral fertilizer, excreta return

is heterogeneous in nature creating a

spatial variability in the field

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Background

Quantifying grazing related N2O emissions: IPCC (2006)

Emission factor (EF) guidelines

Tier 1 method used for GHG inventory in Switzerland

Global default value of 1% is assumed for uniform N

inputs whiles 2% is assumed for grazing related N input

Tier 2 uses country-specific EF inferred from scientific

evaluations

Studies using Tier 2 show country-specific EF much lower

than 2%

Tier 3 involves the use of process-based models to

estimate non-constant EF

Objectives:

Use ecosys (model) to simulate N2O emissions from artificially applied urine patches and investigate the pathways

taken by the high N load.

Examine the effects of seasonal variations in the environmental drivers on N2O EF.

Process-based modeling of N2O emissions

Provide opportunity to improve N2O inventories

However, most grassland N2O emissions

modeling studies refer to one-dimensional, multi-

layered runs that assume a uniform return of

nutrient across the field and do not account for

field heterogeneity

Explicit representation of urine patches

is necessary for accurate estimation of

grazing-related N2O EFTier 1 EF does not take into account spatial

variability of N2O controlling factors,

therefore, Tier 2 and 3 approaches preferred

for EF estimation

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Method

Ecosys: Canadian Ecosystem Model

A comprehensive process-based mathematical model that simulates C, N and P dynamics.

Runs on an hourly time-step to allow accurate tracking of N2O pathways and makes it easier to identify

environmental interactions that cause surge in emissions

It can integrate spatial scales from mm to km and can run as 1-. 2- or 3-D model allowing for spatial explicit

simulations at the landscape level

Model setup and Experiments

Model already tested wrt N2O emissions from mineral/organic

fertilizers for a grassland site in Switzerland (Grant et al., 2016)

A similar setup is used here

Experiments

Urine is simulated as urea with a fast hydrolysis rate

200g of urea diluted with 20 mm of water is applied to an area of

1 m-2 to mimic cattle urine

Model is run in 1-D (exchange processes only in the vertical)

Urine patch emissions measured in the framework of the REFGRASS

field experiment in Tänikon will serve as benchmarks for testing the

model (see pres. …)

Ecosys allows to explicitly model

spatial heterogeneity of urine patches

Ecosystem-atmosphere exchanges and subsurface transfers of heat,

gases, water, C, N, and P simulated in ecosys. Grant (2001)

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Preliminary Results

N in excess of plant

uptake remains in the

system for ~100 days

Total N2O emissions

from the urine patch over

a 100 day period:

~6 g N m-2 or 3 %

The simulation results presented here show N2O

emissions from

A. fertilizer source with 3 g NH4NO3 m-2 and

B. urine patch with 200 g N m-2

both of which were applied on day 123 (3rd May) of

the year

A

B

Total N2O emissions

from mineral fert:

~0.03 g N m-2 or 1 %.

Peak emission driven by

rainfall event

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Preliminary Conclusions & Outlook

Conclusions

The dynamics of N2O emissions from urine patches differs from emissions due to mineral N

applications

EF for the simulated urine patch is ~ 3%.

This value is higher than the IPCC default but the latter is valid for the whole field scale

Outlook

Devise an upscaling strategy for infer EF for the field scale from the simulations

Perform 3-D simulations to see the effects of lateral diffusion on N2O emissions

Verify simulated urine patches emissions (and other relevant quantities) against experimental data

Conduct sensitivity analyses to determine model response to varying N input rates

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THANK YOU !