Minnesota Watershed
Nitrogen Reduction
Planning Tool
William Lazarus
Department of Applied Economics
University of Minnesota
David Mulla
Department of Soil, Water, and Climate
University of Minnesota
David Wall
Minnesota Pollution Control Agency
Download paper at: z.umn.edu/nbmppaper
Reasons for Study
• Proposed MPCA standards for nitrate in surface waters –
adoption are a couple years away.
– “Impaired waters” status would trigger TMDL reductions for point
sources. Nutrient trading opportunities for ag?
• EPA goal - 45% reduction in N and P in the Mississippi River
basin. MN is one of 12 states developing nutrient reduction
strategies to meet it. Ultimate impact on agriculture?
• Crop agriculture is not mandated to make any changes now,
but changes will obviously be needed eventually if a
significant reduction is to be achieved.
• We want to make sure any changes are cost-effective!
PROJECT GOALS
• Assess nonpoint source nitrogen contributions to
Minnesota rivers from a) the primary land use sources,
and b) the primary hydrologic pathways under dry,
average and wet climatic conditions
• Determine the watersheds which contribute the most
nitrogen to the Mississippi River, and combination of land
uses and hydrologic factors having the greatest influences
on the elevated nitrogen
• Develop a nitrogen planning tool to estimate reductions in
N loadings to surface waters at the watershed scale with
various BMPs, and their costs
Agroecoregion Based N Database
• Point data are available for crop
acreage and livestock numbers
• County statistics are available for
crop harvest and N fertilizer use
• N transformations in soil
•
(mineralization, denitrification)
and N losses (volatilization,
leaching, drainage, etc) are
based on soil and landscape
factors (represented by
agroecoregions)
Our approach is to estimate N
inputs and outputs for
agroecoregion units and then
transform results back to
watershed units
N Loadings to Surface Water by Source
Effect of Climate on N Loadings
Watershed N Reduction Planning Tool
• The Tool is an Excel spreadsheet linked to a
database of Minnesota soils, landscapes,
cropping systems, management practices and
crop enterprise budgets
• Estimates of N reductions are based on research
meta-data and BMP specific reduction
coefficients
• Estimates are tied to site specific characteristics
such as soil, slope, climate, and baseline farm
management practices and cropping systems
N Reduction Planning Tool BMPs
•
•
•
•
•
•
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Rate and timing of N fertilizer
Controlled drainage
Bioreactors
Planting cover crops
Planting perennial grass
Installing riparian buffer strips
Installing wetlands
• Effects of individual BMPs as well as
combinations of BMPs can be evaluated
Suitable acres for BMPs
• Fertilizer rate reductions are only possible in areas where
existing application rates exceed University
recommendations
• Controlled drainage and bioreactors can be installed on
tile drained land with slopes of 0.5%, 1% or 2%
• Perennial grass can be planted on ag land with crop
productivity ratings of 60% or less (marginal land)
• Riparian buffers can be installed on ag land within 30 m of
waterways
• Wetlands can be restored on tile drained land with hydric
soils and high Compound Topographic Index values
“Effectiveness” is expressed as the N load reduction compared to the
status quo. “Cost” is expressed per pound of N removed.
Reducing N to Waters from Cropland Statewide
30%
Reduction in N
25%
$1.3
Bi l lion
20%
$81
Mi l lion
15%
$81
Mi l lion
Tile drainage BMPs
10%
5%
Vegetation changes
Cos t
s a vi ng
Cos t
s a vi ng
Cos t
s a vi ng
Optimal fertilizer
rate and timing
Fert. mgmt + tile
drainage BMPs
Fert. mgmt + tile
BMPs + vegetation
BMPs
Fertilizer mgmt.
optimized
0%
Adoption rates: Apply N at recommended rate on 90% of corn; shift 90% of fall-applied N to half
preplant and half split preplant/sidedressed; plant riparian buffers on 70% of suitable acres; adopt
controlled drainage on 90% and install bioreactors on 10% of tiled land of 1% or less slope; restore
wetlands on 78% of suitable acres; plant rye cover crops on 70% of corn and soybean acres; and
shift 27% of marginal land to a perennial crop.
Conclusions
• Total nonpoint source N loadings to Minnesota
surface waters were estimated at about 6% of the
total inputs of N on all Minnesota cropland.
• Statewide, losses of N to surface water from
agricultural sources represent 88% of total nonpoint
source losses.
• A tool was developed to assist planners evaluate
strategies for reducing N loadings to Minnesota
surface waters, by target watershed, climate, and
extent of adoption of various N reduction BMPs.
Conclusions
• Analysis so far shows that achieving N load
reductions greater than 25% would be challenging.
• Implementation of N reduction strategies by the EPA
and the states could mean some significant changes
for crop agriculture. Stay tuned!
Thank you
• Support for this research was provided by the
MN Pollution Control Agency
Download paper at: z.umn.edu/nbmppaper