Background:

The Denitrification-Decomposition or DNDC model is a process-based computer simulation model of carbon (C) and nitrogen (N) biogeochemistry. The model was initially developed for U.S. EPA to quantify greenhouse gas (GHG) emissions from the U.S. agricultural lands in the early 1990s. With the involvement of a large number of domestic and international researchers during the past more than two decades, DNDC has been calibrated and validated against GHG datasets measured worldwide. A number of farming management practices, such as crop rotation, tillage, fertilization, irrigation, grazing etc. have been parameterized in DNDC so that the model is capable of predicting impacts of changes in management on GHG emissions. The California Global Warming Solutions Act of 2006 has legislated GHG emission reductions such that 2020 emission levels are at or below 1990 levels. Mandatory GHG emission reductions are now set in law for the first time in the US. In response to this Act, a Climate Action Team (CAT) was created to identify Discrete Early Actions to reduce emissions and meet the 2020 targets. Since 2011, CA ARB (Air Resources Board) sponsored a study on model comparison and then utilizing selected model to improve GHG inventory and mitigation for CA agriculture.

DNDC Application for Inventory and Mitigation of Greenhouse Gases Emitted from Agricultural Lands in California

Changsheng Li1, Johan Six2, William Horwath2 and William Salas3

1University of New Hampshire; 2University of California, Davis; 3Applied GeoSolutions LLC

Field Measurement and Model Comparison:

Field measurements were conducted for quantifying N2O fluxes from 10 agricultural sites in CA by groups led by Dave Smart, Johan Six, Cynthia Kallenbach and William Horwath. The crops planted at the sites included grapes, almond, tomato, alfalfa, winter wheat and other row crops, which well represented the major crop types across the agricultural regions in CA. At each of the sites, alternative farming practices were applied, which included different crop rotation sequences, standard tillage vs. reduced till, furrow irrigation vs. drip irrigation, different rates of fertilizer application, with vs. without winter cover crop etc. N2O fluxes were measured at the sites with static chambers during the time period from 2003-2011. The 40 datasets resulting from the field campaigns were utilized for model comparison. Three well documented and widely applied models, DNDC, DAYCENT and IPCC Method, were selected to test their applicability for CA GHG emissions. The modeled results were compared against the same datasets of measured N2O data at daily or annual basis. The comparison tests showed that DNDC-simulated results were closer to observations. Daily Comparison Examples: Annual Comparison:

DNDC Structure

Climate Soil Vegetation Management

Temperature Moisture pH Substrates: NH4+, NO3

-, DOCEh

Denitrification Nitrification Fermentation

DecompositionPlant growthSoil climate

Ecological drivers

Soil environmental

factors

Temperature

Moisture

Litter production

Yield

Phenology

Runoff and leaching flow

Oxygen and Eh Water and N uptake

Fresh litter partitioning

Microbial assimilation

SOC turnover

CO2 production

NO3, NO2, NO, N2O, N2 production

Reduction of NO3 Oxidation of NH4

NO2, NO, N2O, NO3 production

DOC → CH4

CO2 + H2 → CH4

Observed and DNDC-Modeled Annual N2O Fluxes for 69 Agricultural Sites with

complete input data in U.S., Canada, U.K., Germany, Belgium, France, Swiss, New

Zealand, China, Japan, and Costa Rica

Regional Database and Simulations: County-based database was established to hold spatially differentiated information of climate, soil, crop types and farming management practices. DNDC was run for all the 58 counties with 54 cropping systems in CA for 2011.

The first working group for DNDC development and implementation,

Washington, DC, 1989-1992

The model structure of DNDC DNDC has been validated against N2O fluxes measured worldwide

Observed and DNDC-modeled CH4 fluxes from rice paddies in China, Thailand, Japan, Italy and the U.S.

R2 = 0.948

0

100

200

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400

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0 100 200 300 400 500 600 700 800

Modeled CH4 flux, kg C/ha/yr

Me

asu

re

d C

H4

flu

x, kg

C

/h

a/yr .

DNDC has been validated against CH4 fluxes measured worldwide

0

50

100

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200

250

300

0 50 100 150 200 250 300 350

N2O

flux

, g N

/ha/

day

Day of year 2010

DNDC and Daycent modeled daily N2O fluxes against measured data for Johan Six's Field31 in CA in 2010

Measured N2O Daycent DNDC

Comparison of DNDC- and DAYCENT -modeled daily N2O fluxes with measured N2O fluxes for a tomato field in California in 2010 (Field data from Johan Six)

Comparison of DNDC- and DAYCENT -modeled daily N2O fluxes with measured N2O fluxes for a winter wheat field with fertilizer application rate of 0 kg N/ha in California in 2010-2011 (Field data from William Horwath)

0

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N2O

flu

x, g

N/h

a/d

ay

Day

DNDC and Daycent modeled daily N2O fluxes against measured data for Kallenbach's tomato field without winter cover crop in CA in 2006

Measured N2O Daycent DNDC

Comparison of DNDC- and DAYCENT -modeled daily N2O fluxes with measured N2O fluxes for a tomato field without cover crop in California in 2006 (Field data from Cynthia Kallenbach)

y = 0.0395x + 1508R² = 0.0012

y = 0.9725x + 130.47R² = 0.7931

y = 0.1011x + 1812.6R² = 0.0052

0

1,000

2,000

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6,000

7,000

0 1,000 2,000 3,000 4,000 5,000 6,000

Mo

de

led

Em

issi

on

s (g

N-N

2O

-N/h

a)

Field Measured N2O Emissions (g N2O-N/ha) - Exponential

IPCC DNDC Daycent

Maps of maximum and minimum (a and b) bulk density (g cm-3), (c and d) clay content (%), (e and f) soil organic carbon content (%), and (g and h) pH in California.

a b

c

DNDC-simulated county-scale maximum (a), area-weighted mean (b), and minimum (c) N2O emissions (metric tons N yr-1) from agricultural lands in California.

DNDC simulated county-scale (a) maximum CH4 emissions, (b) area-weighted mean CH4 emissions, and (c) minimum CH4 emissions (metric tons C yr-1), (d) maximum SOC change, (e) area-weighted mean SOC change, and (f) minimum SOC change (109g C yr-1) from agricultural lands in California.

Updated CA Agricultural GHG Inventory:

Next Project: From Inventory to Mitigation: “Improving DNDC Modeling Capability to Quantify Mitigation Potential of N2O from California Agricultural Soils”, 2014-2017, PI: Changsheng Li (UNH), sponsored by CA Air Resources Board.

Acknowledgements: We thanks CA ARB for its support for the former and current projects.

Greenhouse gas N2O CH4 CO2* Sum

Total emission Direct: 0.0085±

0.0048Tg N

Indirect:

0.0040±0.0002 Tg N

0.037± 0.085Tg C -2.72±1.41Tg C

GWP** (TgCO2

equivalent yr-1

)

Direct: 4.14 ± 2.33

Indirect: 1.94±0.12

1.23± 2.83 -9.97±5.17 -4.60±10.33

(-2.66±10.21***)

Major contributors Major emitter: corn

(28%), lettuce

(11%), grape (11%),

cotton(8%), rice

(6%)

Major emitter: rice

100%

Major emitters:

tomato (27%),

cotton (37%);

Major sequesters:

corn (36%), alfalfa

(35%), grape (17%)