『Abstract
Denitrification is known as an important pathway for nitrate
loss in agroecosystems. It is important to estimate denitrification
fluxes to close field and watershed N mass balances, determine
greenhouse gas emissions (N2O), and help
constrain estimates of other major N fluxes (e.g., nitrate leaching,
mineralization, nitrification). We compared predicted denitrification
estimates for a typical corn and soybean agroecosystem on a tile
drained Mollisol from five models (DAYCENT, SWAT, EPIC, DRAINMOD-N
II and two versions of DNDC, 82a and 82h), after first calibrating
each model to crop yields, water flux, and nitrate leaching. Known
annual crop yields and daily flux values (water, nitrate-N) for
1993-2006 were provided, along with daily environmental variables
(air temperature, precipitation) and soil characteristics. Measured
denitrification fluxes were not available. Model output for 1997-2006
was then compared for a range of annual, monthly and daily fluxes.
Each model was able to estimate corn and soybean yields accurately,
and most did well in estimating riverine water and nitrate-N fluxes
(1997-2006 mean measured nitrate-N loss 28 kg N ha-1
year-1, model range 21-28 kg N ha-1 year-1).
Monthly patterns in observed riverine nitrate-N flux were generally
reflected in model output (r2 values ranged from 0.51
to 0.76). Nitrogen fluxes that did not have corresponding measurements
were quite variable across the models, including 10-year average
denitrification estimates, ranging from 3.8 to 21 kg N ha-1
year-1 and substantial variability in simulated soybean
N2 fixation, N harvest, and the change in
soil organic N pools. DNDC82a and DAYCENT gave comparatively low
estimates of total denitrification flux (3.8 and 5.6 kg N ha-1
year-1, respectively) with similar patterns controlled
primarily by moisture. DNDC82h predicted similar fluxes until
2003, when estimates were abruptly much greater. SWAT and DRAINMOD
predicted larger denitrification fluxes (about 17-18 kg N ha-1
year-1) with monthly values that were similar. EPIC
denitrification was intermediate between all models (11 kg N ha-1
year-1). Predicted daily fluxes during a high precipitation
year (2002) varied considerably among models regardless of whether
the models had comparable annual fluxes for the years. Some models
predicted large denitrification fluxes for a few days, whereas
others predicted large fluxes persisting for several weeks to
months. Modeled denitrification fluxes were controlled mainly
by soil moisture status and nitrate available to be denitrified,
and the way denitrification in each model responded to moisture
status greatly determined the flux. Because denitrification is
dependent on the amount of nitrate available at any given time,
modeled differences in other components of the N cycle (e.g.,
N2 fixation, N harvest, change in soil N
storage) no doubt led to differences in predicted denitrification.
Model comparisons suggest our ability to accurately predict denitrification
fluxes (without known values) from the dominant agroecosystem
in the midwestern Illinois is quite uncertain at this time.
Keywords: Crop yields; Mollisol; N2O; Nitrate;
Soil moisture』
Introduction
Methods
DAYCENT
SWAT
DNDC
SRAINMOD-N II
EPIC
Results and discussion
Modeling results
Modeled denitrification in response to environmental variables
Conclusions
Acknowledgments
References