Nitrate transport and fluxes during storm-event discharge from a 12 ha tile-drained dryland agricultural field (B51F-0623)

By: Kelley1*, Christopher J., C. Kent Keller1, Erin Brooks2, Jeffrey L. Smith3, Cailin Huyck Orr1, R. Dave Evans4

1School of the Environment, Washington State University, Pullman, WA 99164 *email: christopher.kelley@email.wsu.edu

2Department of Biological and Agricultural Engineering, University of Idaho, Moscow, ID 838443USDA-ARS at Washington State University, Pullman, WA 99164

4School of Biological Science, Washington State University, Pullman, WA 99164

1) Washington State University Cook Agronomy Farm

II) Study Area & Methods:Samples were collected from 2000 to 2012 from the tile-drained section of the

Washington State University Cook Agronomy Farm (CAF). CAF is approximately 11 km Northwest of Pullman, Washington in the Palouse Region of Eastern Washington State (Figure 1). The climate of the region is semi-arid with winter dominated precipitation ranging from 31-58 cm yr-1. The soils in this region are silt loam Mollisols that are mapped as part of the Palouse-Thatuna Association soil series (USDA, 1978). Current crops grown at the CAF consist of a rotation of winter wheat, spring wheat, and garbanzo beans. Historically, discharge was recorded with a bucket and stop watch, and [NO3

-] was measured only on grab samples. In the summer of 2011 a flume and pressure transducer were installed at the mouth of the tile line to recorded discharge at 15 minute intervals. An ISCO auto sampler was also installed and set to collect a sample at flume depth changes of 0.5 cm. Precipitation data if from the AgWeatherNet (2012).

I) Introduction:Tile drains shortcut natural soil hydrology and decrease the capacity of soils to buffer

water and nutrient fluxes during storm events. Previous research at the Cook Agronomy Farm found seasonal patterns for both nutrient and water fluxes, larger during the winter and smaller during the summer. The objective of this study was to determine the effects storm events have on tile-drain water and nutrient fluxes.

Our first hypothesis is that winter storm events activate shallow soil-water flow paths, resulting in rapid transport of precipitation and younger soil pore-water through the tile-drain system. Our second hypothesis is that the observed increase in discharge during storm events do not decrease nitrate concentrations in discharge, because the storm-event flow paths also transport additional nitrate from the upper soil profile through the tile-drain system.

VI) References:• AgWeatherNet. 2012. Pullman NE weather station. Available Online at

http://weather.wsu.edu/awn.php. Accessed October 10, 2012.• Kelley, C. C.K. Keller, R.D. Evans, C.H. Orr, J.L. Smith, B.A. Harlow. In Press. Nitrate-nitrogen and

oxygen isotope ratios for identification of nitrate sources and dominant nitrogen cycle processes in a tile-drained dryland agricultural field. Soil Biology & Biochemistry.

III) Results• Over the 2011-2012 year [NO3

-] ranges from 4 - 20 mg/L (Figure 3), similar to the historic yearly fluctuations (Figure 2) of 4 – 30 mg/L.

• Tile-drain discharge responds to precipitation/ snowmelt events within ~1 day (Figures 4 & 5).

• During storm-events EC immediately decreasing with even small increases in discharge (Figure 4).

• Water temperature does not respond instantly with increasing discharge or with small increases in discharge (Figure 4 & 5).

• Like EC, [NO3-] responds instantly with increasing discharge, even for small

increases in discharge (Figure 4).

IV) Discussion/ Conclusions:• During storm-events tile drain discharge and [NO3

-] increase while EC and temperature decrease, compared to pre and post storm-event discharge.

• Historical sampling methods did not have the temporal resolution to detect short-term increases in discharge and [NO3

-] (observed in Figures 4 & 5)• Based on our interpretations NO3

- is flushed from the upper soil profile during storm-event driven discharge, based on low EC (indicator of younger soil pore water) and high [NO3

-] discharge is from fall N-fertilizer applications (Kelley et al., In Press).

• We suggest, rapid mobilization and transport of NO3- from the unsaturated

zone above the tile drain can significantly increase N-fluxes during storm-event discharge (Figure 6) compared to pre and post storm event base flow conditions.

1/18/12 1/19/12 1/20/12 1/21/12 1/22/12 1/23/12 1/24/122.5

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Water TemperatureWater EC

Wat

er T

empe

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re (

C)

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DischargePrecipitation[Nitrate]

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recip

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)

[NO

3--N

] (m

g/L)

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203) 2011-2012 CAF Tile-Drain DischargePrecipitationDischargeNitrate-N

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[NO

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ater

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DischargePrecipi-tation[Nitrate]

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p. (c

m)

[NO

3--N

] (m

g/L)

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35 2) CAF Tile-Drain Discharge [NO3--N]

[NO

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] (m

g/L)

6) Conceptual N-transport models

Base flow (unsaturated conditions) Storm flow (saturated conditions)

Soil Particles

High [NO3

-] water

Soil Particles

High [NO3

-] water