Effect of Grazing Treatment on Soil Moisture in Semi-arid ...
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Effect of Grazing Treatment on Soil
Moisture in Semi-arid Rangelands
Keith T. Weber, GISPBhushan Gokhale
Idaho State University
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Project Focus
This four-year study uses GIS and remote sensing to: Examine specific drought effects
relative to livestock grazing/rest treatments and bare earth exposure
Model and monitor rangeland condition Forecast rangeland health/condition
using cellular-automata and Markov chain analysis
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Interesting Findings…
To address our primary study questions, we… Fenced Pre-sampled Instrumented
This paper is a product of our instrumentation
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Focus of this Paper
Compare soil moisture levels at the O’Neal study area across three treatment pastures: Simulated Holistic Planned Grazing
(SHPG) Rest-rotation (RESTROT) Total Rest (TREST)
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Why Soil Moisture?
Water is the limiting factor Soil moisture typically has a direct
relationship with ground cover But, is there also a treatment effect? Or
interaction? Can grazing animals effect soil
moisture? Does the type of grazing have any
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The Study Area
The O'Neal Ecological Reserve was donated to ISU’s Department of Biological Sciences by Robin O'Neal. This 100 ha site is located about 30 miles south of Pocatello, Idaho.
Surrounding BLM land adds another 1467 ha.
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Soils in the Study Area
The entire study area is McCareyseries-McCarey variant. Shallow, well-drained soils over basalt
flows Originally formed from weathered
basalt, loess, and silty alluvium
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Pre-treatment Sampling
Vegetation cover Hi-res aerial
photography (2” resolution)
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Grazing
Prior to this experiment (i.e., up to and including 2005) No fences existed Grazed as a single rest-rotation
allotment (1467ha +) 300 head of cattle for one month (May)
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Treatment Pastures
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Grazing (cont’d)
This experiment: Rest-rotation (RESTROT): 300 head of
cattle graze for one month (May of each year)(1467ha)
Simulated Holistic Planned Grazing (SHPG): 125 head of cattle graze for six days (first week in May)(11ha)
Total Rest (TREST): Zero livestock (13ha)
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Grazing Summary
Stocking information
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Treatment Animal Days/ha
Simulated Holistic Planned Grazing (SHPG) 36
Rest Rotation (RESTROT) 6
Total Rest (TREST) 0
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Instrumentation (cont’d)
Soil moisture can be estimated using: Electrical resistance blocks Tensiometers Gravimetric calculations Neutron probes Time domain reflectrometry Capacitance sensors
Decagon ECH2O-10 10cm depth
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Instrumentation
36 Decagon soil moisture probes 12 in each pasture (three replicates of
four probes) Accuracy= +/-2% Volumetric Water
Content (VWC) (after calibration) %VWC estimates the amount of stored
water on a volumetric (not gravimetric) basis
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Instrumentation (cont’d)
Weather station on site (inside TREST pasture)
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Annual Sampling
Vegetation cover (point-intercept transects) 50 randomly located plots per treatment Two 10 meter transects per plot Transects placed perpendicular to each
other (forms a “+”) 100 observations per transect Designed to sample SPOT5 satellite
pixels
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Annual Sampling (cont’d)
Forage estimates (hoop sampling) Photo points SPOT5 Satellite imagery
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Soil Moisture is a Function of…
Soil type/structure (no difference can be attributed in this case, as the entire study area is one homogeneous soil type)
Plant cover (more plants tend to mean lower water content in the soils…its all in the plants)
Animal impact?
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Hypothesis to test
Does animal impact (trampling and breaking of soil crust) have a measurable effect on soil moisture? If so, is the effect positive or negative?
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The Data
Soil moisture (%VWC) was collected every six hours beginning in June 2006
All probes were calibrated (R2 = 0.997) Mean soil moisture was calculated for
each day and each week
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The Analysis
Data from the growing season (April 1-August 30)
Compared statistically, for example SHPG 2006 vs. RESTROT 2006 SHPG 2006 (50%) vs. SHPG 2006 (50%) Week * Year * Pasture
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Between pastures
Within pastures
All effects
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Statistical Tests
Daily means tested with single-factor ANOVA
Weekly means tested with Mixed-Procedures Models Fixed effects calculated with Prasad-
Rao-Jeske-Kackar-Harville method. Degrees of freedom follows Kenward-
Roger method.
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RESULTS
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Pre-treatment Conditions
No difference in vegetation cover (shrubs, grasses, litter, and bare ground exposure)… Save for a difference in shrub cover
between the SHPG and TREST pastures (TREST > SHPG)
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Vegetation Conditions
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During the study (2006-2008) The difference in shrub cover between
SHPG and TREST persisted No other changes were found… Except litter, which was significantly
higher in SHPG beginning in 2007 (P<0.001)
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Tests Between Pastures
SHPG %VWC higher in 2006-2008 (P < 0.001)
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x %VWC
Treatment 2006 2007 2008
SHPG 23.3 44.1 45.8
RESTROT 19.7 34.8 34.7
TREST19.2 31.9 29.8
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Between Pastures (cont’d)
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15
20
25
30
35
40
45
50
2006 2007 2008
SHPG annual %VWC
RESTROT annual %VWC
TREST annual %VWC
Started in June, dry part of
season only
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Tests Within Pastures
Compared daily mean %VWC of 6 sensors vs. 6 sensors
One pair from each set of loggers SHPG (P = 0.03) different RESTROT (P = 0.15) no difference TREST (P = 0.12) no difference
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logger
Sensor 1
Sensor 2
Sensor 3
Sensor 4
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Comparisons in Context
Within pasture differences were less than between pasture differences
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Mixed Procedures Model and Type Three Test of Fixed Effects
Significant effects (P < 0.05) WEEK (Fstat= 92) YEAR x PASTURE (Fstat= 20) WEEK x YEAR (Fstat= 6) PASTURE (Fstat= 5)
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The Big Picture
The differences in soil moisture can be attributed to: Environmental effects (for example…)
Soils in week 2 were wetter than week 18 Soils in the SHPG pasture were wetter in
2007 than in 2008 Soils in week 10 of 2007 were wetter than
soils in week 10 of 2008 Treatment effects (for example)
Soils in the SHPG pasture were wetter than the other pastures
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Animal Impact (revisited)
Does trampling and breaking of soil crust have a measurable effect? Focus on the production pastures which
Have statistically different %VWC Diverging soil moisture trends Are both grazed by cattle in May
But…
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Not all Grazing is the Same
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Time… Animal Days per hectare
Effect on litter (aka mulch)
Treatment Animal Days/ha
Simulated Holistic Planned Grazing (SHPG) 36
Rest Rotation (RESTROT) 6
Total Rest (TREST) 0
A likely “mechanism”
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Too Much of a “Good thing”
Six days of grazing at high AD/ha appears to benefit rangelands
However, 10 days or 14 days or 21 days may damage rangelands
It’s all about TIME
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Thank You
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Questions?
AcknowledgementsThis study was made possible by a grant from the National Aeronautics and Space Administration Goddard Space Flight Center. ISU would also like to acknowledge the Idaho Delegation for their assistance in obtaining this grant.