R. N. Lerch1, C. H. Lin2, K. W. Goyne3, R. J. Kremer1, and S. H. Anderson3

Vegetative Buffer Strips for Reducing Herbicide Transport in Runoff:

Effects of Season, Vegetation, and Buffer Width

1USDA-ARS, Cropping Systems and Water Quality Research Unit, Columbia, MO2 Center for Agroforestry, University of Missouri

3Department of Soil, Environmental and Atmospheric Sciences, University of Missouri

Rationale and Objectives• Rationale:

– The ability of vegetative buffer strips (VBS) to reduce the transport of herbicides has not been evaluated for high runoff potential soils such as those in the Central Claypan Region of northeastern Missouri.

– Data to needed to support design criteria for implementation relative to contaminant of interest, choice of grass species, and buffer width

• Objectives:1) Assess the effects of season, vegetation, and buffer width on runoff

transport of herbicides 2) Develop design criteria for buffer widths3) Assess soil quality in the buffers since their implementation in 2002.

Study Location

• Plot Layout– Mexico silt loam, 5% slope, eroded– Twelve 1.5 m by 16 m plots (4 treatments

replicated 3 times)• Experimental Design

– Grass treatments (3 reps/treatment):1) Tall fescue (TF); 2) Tall fescue with a switchgrass hedge (Hedge +

TF)3) Warm-season grasses (mainly eastern

gamagrass and indian grass) (Native)4) Continuous cultivated fallow (Control)5) Poplar trees + Native grasses (Tree)

– Buffer width Runoff collectors at: -1 m; 1 m, 4m, and 8m

– Seasons (2 reps/trt) – Spring, Summer, Fall• Herbicides (rate, kg/ha)

– Atrazine (2.2), glyphosate (1.5), and metolachlor (1.7)

Materials and MethodsExperimental Design

8 m

1.0 m

4 m

Slope(5%)

Fescue

Runoff sampling troughs

Fescue

T1

Native

T2 T3 T4

TilledArea

Switchgrass hedge width is 0.7 m

8 m

TilledArea

TilledArea

TilledArea

TilledArea

1.5 m

1 M

4 M

Source Area-1 M

8 M

Materials and MethodsExperimental Design – Vegetation Treatments

Control

Tall Fescue

Native

Hedge + Tall Fescue

Tree-Grass

• Plot Preparation– 1.5 m by 8 m area above the VBS was roto-tilled to ~10 cm

(source area)– Plots brought to saturation 24 hours before runoff collection– Herbicides broadcast with a backpack sprayer 16 hours before

runoff collection (not incorporated)• Runoff Collection

– Rainfall rate = 50 mm hr-1

– Collected beginning with initiation of flow at the 8 m sampler– Collected every 10 minutes for 60 minutes; composited to create

one sample for each buffer width (i.e., -1, 1, 4, and 8 m)• Runoff Sample Analyses

– Water and sediment analyzed for herbicides– Suspended sediment

• Soil Quality Assessments– Soil samples collected in May 2011– Enzyme activities – dehydrogenase, β-glucosidase, and

fluorescein diacetate hydrolysis (0-10 cm)– Atrazine degradation – 56 day incubations (0-10 cm)– Saturated hydraulic conductivity (0-10 cm and 10-20 cm)

Materials and MethodsExperimental Design

• Relative Load−Normalized Load = contaminant mass at each

sampling position normalized to the total mass at the -1m samplers (input)

• Statistics− 3-way ANOVA for runoff data Factors - Season, Vegetation, Width PROC Mixed procedure with repeated measures Significance level, α = 0.05 PDIFF for LS mean comparisons

– 1-way or 2-way ANOVA for soil quality data Significance level, α = 0.05 F-LSD0.05 for mean comparisons

Materials and MethodsComputations and Statistics

-1m(Input)

1m 4m 8m

X X X XC1 C2 C3 C4

M1 M2 M3 M4M1/M1 M2/M1 M3/M1 M4/M1

( N o r m a l i z e d t o I n p u t )

Q1 Q2 Q3 Q4

– Non-linear regression used to correlate relative load reduction to buffer width General form of the 1st-order decay equation: y = a + be-kx

y = relative load; x = buffer width;k – rate constant; a,b – model coefficients

ResultsRainfall

Error bars indicate +/‐SD

ResultsRunoff

ResultsRunoff

ResultsAbsolute and Relative Loads

Error bars indicate +/‐SD

ResultsInput Normalized Loads

ResultsVegetation X Season Interactions

Atrazine Metolachlor

ResultsDissolved vs Sediment-Bound

ResultsGlyphosate - Dissolved vs Sediment-Bound

47%

36%

27%24%

ResultsLoad Reductions as a Function of Buffer Width

ResultsLoad Reductions as a Function of Buffer Width

Grass Buffer DesignAnticipated Field‐Scale Results

Collecting runoff samples Drainage to Buffer Area Ratio8:1 2:1 1:18:0

Results – Runoff ExperimentsMechanisms for Reducing Transport

• Infiltration– Runoff volume reductions: 62% for TF 54% for Hedge+TF 54% for Native 7-8% reduction per meter of buffer

• Sediment Trapping– Sediment load reduced by: 88% for TF 85% for Hedge+TF 80% for Native ~10% reduction per meter of buffer

Soil Quality Assessments

Degradation of contaminantsMicrobial activitySoil hydrologic propertiesSorption Properties

Atrazine Degradation

Statistical Differences

Vegetation Treatment

Control TF Hedge+TF Native Tree

Atra

zine

Rem

aini

ng o

r Min

eral

ized

(%

of A

pplie

d)

0

1

2

3

4

5

10

15

20

25

30

Atra

zine

Hal

f-Life

(day

s)

0

2

4

6

8

10

12

14Atrazine RemainingAtrazine Mineralized Half-Life

a a aa

a

a b b b b

a a a b ab

Soil Enzyme Activities

Control TF Hedge+TF Native Tree

Act

ivity

(nm

ol o

r m

ol g

-1h-1

)

0.0

0.2

0.4

0.6

5.0

10.0

15.0

20.0Glucosidase Dehydrogenase FDA

a abb b b

Hydraulic Conductivity

Treatment differences within a depth were NS Species by Depth interaction was NS

Control TF Native Tree Depth Means

Ksa

t (m

m h

r-1)

0

50

100

150

200

250

300

350 0-10 cm10-20 cm

a

b

Summary and ConclusionsWater Quality• Vegetation treatment ‐ highly significant Load reductions in the range of 60‐90% for the vegetated treatments

• Buffer width – highly significant Exponential decrease in load with increasing width for the vegetation treatments

• Season – minor effect Only Hedge+TF treatment showed an effect for atrazine and metolachlor in summer

• Buffer Design  Regression equations provide practical design criteria – accounts for contaminant, drainage‐to‐buffer area ratios, and vegetative cover type Potentially achieve desired reductions with less land taken out of production

Soil Quality• After 9 years, VBS had minimal effect on microbial activity or soil hydrologic properties

• Microbial adaptation to atrazine common in all treatments – t1/2 = 4.5 to 9.7 d