Spring Nitrous Oxide Emissions

Why is N₂O important?

• N₂O has 300 times the global warming potential of CO₂
  
• Acts as both a greenhouse gas and contributes to ozone depletion
  
• N₂O is difficult to measure, leading to low confidence in budgets and poor understanding of feedbacks

What is N₂O to an agronomist?

Before that worked at USDA-ARS North Central Soil Conservation Research Lab in Morris, MN

Worked on 4 N₂O experiments May-December

Greenhouse Gas Emission from Contrasting Management Scenarios in the Northern Corn Belt

Emissions of nitrous oxide from soils - Nature 1982, Duxbury et al.

• In alfalfa, 66% of annual emissions in 2 week period during spring
  
• 27% of annual emissions during 2 days of spring
  Duxbury et al., 1982, Nature 298:462-464

Seasonal variability in emission of nitrous oxide from soil, Geophysical Research Letters, 1980, Bremner et al.

What causes peaks in the spring?

• Big flush of labile carbon and nitrogen as frozen microbes and plant cells thaw and burst
  
• Drop in pH as soil solution is flooded with new solutes
  
• Trapped gases not a big contributer to spring peaks

What causes peaks in the spring?

More evidence from Canada

Modeled spring emissions from freezing degree days

Scaled model up to find current estimates miss ~ 28% of annual emissions

But acknowledged parameters beyond freezing degrees days are needed

USDA-ARS-GRACEnet

GRACEnet collection and sites

GRACEnet Pubs up to 2012

GRACEnet data is/was available in easily accessible public database

All available sites

Sites that experience freezing

Soil Hydrological Properties

Clay (%)
Sand (%)
Silt (%)
Fine Silt (%)
Coarse Silt (%)
Ks
\(\theta\)_s (L L¹)
\(\theta\)_r (L L¹)

Substrate Properties

Organic Carbon
Organic Matter
Total Nitrogen
C:N Ratio

Soil Chemical Properties

pH
Cation Exchange Capacity
Exchangeable Mg
Exchangeable K
Exchangeable K Saturation
Excahgeable Ca
Base Saturation

Antecedant Weather Conditions

How cold - sum of daily minimum temperatures below 0
How warm - sum of daily maximum temperatures
How sunny - sum of daily total radiation
How wet - sum of daily precipitation
How snowy - sum of daily snowfall

How do these contribute to magnitude of spring N₂O emissions?

Partial Least Squares (PLS) Regression

• Has some similarities to Principal Components Analysis (PCA)
  
• Good to use if predictors are correlated and contain similar predictive information
  
• Can produce a predictive model

Partial Least Squares (PLS) Regression

What does this mean?

• Across large spatial scales, freezing degree days is not a good predictor of spring N₂O fluxes
  
• Within a specific soil type, we currently don’t have enough data to quantify the effect of winter weather conditions on spring N₂O emissions
• Soil characteristics drive spatial variability of N₂O emissions.
  
• Organic Matter
  
• \(\theta\)_s
  
• Clay
  
• Exchangeable Mg

Early spring emissions are still important, neglected, and probably underestimated

• Need better (continuous) and more measurements

Need better modeling of this phenomenon

• Relationships are too complex for large-scale modeling
  
• N₂O timing is very dynamic
  
• Cold winters manifest in soil conditions
  
• Daily time step, mechanistic model would make best predictions
  
• I am bringing spring N₂O emissions to APSIM
  
• Then we can scale up (still dependent on good soils maps)

Nitrous oxide reduction efficiency drivers still in question

Variables listed should lead to more denitrification, but what controls nosZ?

Thank You!

• Agriculture and Food Research Initiative competitive grant number 2016-67012-25170 for the money
  
• USDA-ARS for all the data
  
• R Community for all the tools
• Integrated Cropping Systems lab (Archontoulis) for all the support