Toby Ahrens

26 Oct 2004

Linking Spatial Variability of Soil N Retention Mechanisms to

Landscape-level Fates in Yaqui Valley, Mexico

Goals

Quantify degrees of N contamination under different management regimes

Investigate the value of spatial data sets varying in resolution

Integrate 10+ yrs of biogeochemistry research in the Valley

What do I need to do to get there?

Link process and transport models

Improve process models Include abiotic retention mechanisms

Develop spatial data sets Eg. soil type, texture, mineralogy,

management, leaching potential

Coastal eutrophication

Declassified Keyhole satellite image

March 8, 1978

(Thanks Mike!)

Nitrate-contaminated groundwater

<10 ppm10-40 ppm116 ppm

Hungate, unpublished data, 1997-98

Two Q’s guiding my efforts:

1. What soil characteristics control N availability, retention and loss in OM-deficient ag soils?

2. Can the spatial variability of these characteristics be linked to aqueous N fates throughout the Valley?

Model schematic…

soil properties

NLOSS

soil mineralogy

crop yield (+N use)

Solute transport

un/saturated boundary conditions

management unit

crop type

Output maps:• Leaching vulnerability• Aquifer contamination• Coastal N sources

groundwater depth

Lee Addams’s modelIrrigation Drains Irrigation Losses Main Canal Infiltration

250m-1km

“AQUIFER HORIZON”

“CONNECTOR HORIZON”

“SHALLOW HORIZON”

WellScreens

Irrigation Drains Irrigation Losses Main Canal Infiltration

250m-1km

“AQUIFER HORIZON”

“CONNECTOR HORIZON”

“SHALLOW HORIZON”

WellScreens

Major modeling efforts:

Soil submodel including sorption isotherms and mineral fixation

Solute transport component to saturated hydrology model

Saturated/unsaturated boundary layer conditions

Spatial data referencing

Applied N:250 kg/ha

Plant uptake:31%

Leached:2-5%

(14-26%)

Gaseous losses:

NO+N2O: 2-5%NH3: __%N2: __%

?

?

?

?

Drainage canals:NO3

- + NH4+: 2-5%

NO+N2O: <0.1%

References:

Riley et al. 2001Harrison 2003Matson et al. 1998Ortiz-Monasterio, pers. comm.

Applied N:250 kg/ha

Plant uptake:31%

Leached:2-5%

(14-26%)

Gaseous losses:

NO+N2O: 2-5%NH3: __%N2: __%

?

?

?

?

Drainage canals:NO3

- + NH4+: 2-5%

NO+N2O: <0.1%

Field experiments:

• abiotic fixation

• link leaching potential to actual fates

Future directions and broader implications…

Does better spatial resolution increase our ability to predict SW/GW vulnerability? And the degree of how mechanistic submodels

are?

Identify thresholds for N retention Or pesticides…

Investigate management scenarios Different irrigation or fertilization regimes

Resolution of input data:Field based…

Legend

ARCILLA

5.820000 - 27.820000

27.820001 - 38.940000

38.940001 - 49.300000

49.300001 - 59.460000

59.460001 - 87.300000

Legend

ARCILLA

5.820000 - 27.820000

27.820001 - 38.940000

38.940001 - 49.300000

49.300001 - 59.460000

59.460001 - 87.300000

Policy implications

The Danish and Dutch experiences: N control policies aimed at reducing N

leaching (SW + GW) and NH3 and N2O emissions

Importance of model validation