Marginal Land Rainfall Exclusion Experiment – Sampling
The GLBRC Marginal Land Rainfall Exclusion Experiment was established in 2018 in the Switchgrass treatment (G5) of the Marginal Land Experiment (MLE) to examine the effects of drought on plant production and composition. Rainfall exclusion (rainout) shelters are deployed over the growing season in the fertilized (plus_n) split plot of each of the four G5 replicates (R1-R4) at every active MLE site except Wisconsin Central –Hancock, which since 2015 has only three replicates (R2-R4). Rainout shelters are also deployed in Switchgrass treatment (G5; R2-R5) of the Biofuel Cropping System Experiment (BCSE) at the GLBRC Intensive site; plots and shelters are managed and sampled in the same manner as those in the MLE and thus serve as reference for drought effects on active croplands with higher soil fertility.
Rainout shelters are constructed of a 12’ long x 12’ wide x 6.7’ tall (at side posts with 8" caster wheels) frame of galvanized steel rectangular tubing with vertical supports every 4’ for reinforcement and covered by a pitched (3":12") roof made of overlapping corrugated, clear polycarbonate roofing panels (Amerilux Greca Lexan) which allow ~90% light transmission above 385 nm. Gutters with hoses are installed around the perimeter of the roof and divert rain to outside the plot. Test trials show the shelters exclude nearly 100% of rainfall but very little sunlight.
Under each shelter is a centered 8′ × 8′ subplot where most sampling occurs; samples from this area are cataloged as “rainout”. Two additional subplots (both 6′ × 10′) are located outside the sheltered area but still within the fertilized split plot: an “ambient” subplot that is exposed to ambient weather conditions and a “reserved” subplot in case additional agronomic activities (e.g., irrigation) are implemented.
Surface “rainout” soils are sampled for inorganic nitrogen and soil moisture annually on the same day as the fertilized and unfertilized split plots; these samples are also archived and every three years a subsample is sent to an analytical lab for testing and nutrient/liming recommendations. Plants under the rainout shelters are not machine harvested; instead yields of “rainout” subplots are determined by hand-harvesting and are compared to hand-harvested yields of “ambient” subplots. “Rainout” yield samples are archived on MSU main campus by Kurt Thelen’s lab. Annual net primary production (ANPP) under the rainout shelters is not measured.
In two replicate G5 plots per site, two Campbell Scientific CS655 soil moisture probes (https://www.campbellsci.com/cs655) are buried horizontally near the center of each of the rainout, ambient, and reserved subplots. One sensor is buried at 10 cm, the other at 25 cm. Hourly measurements are saved using a Campbell Scientific CR800 data logger.
Surface soil under the rainout shelters are sampled on the same day as surface soils in the fertilized/unfertilized split plots. Under each rainout shelter, take four cores using a push corer (0-25 cm), composite into a single sample, sieve through a 4 mm screen to remove debris and homogenize the sample, and store refrigerated until subsamples are processed and analyzed for inorganic nitrogen and soil moisture. Then air dry the sieved soil and archive. Roughly every three years, send a subsample of the air-dried sample to an analytical laboratory (e.g., MSU Soil and Plant Nutrient Lab through 2022) for nutrient and liming recommendations. To examine the effects of drought on these soil measurements, compare results of “rainout” samples with those taken from the fertilized split plot.
To determine switchgrass yields underneath the rainout shelter, ensure that the survey whiskers installed at the 4 corners of the rainout shelter are still present before removing the shelter from plot. Mark out the 8′×8′ center under the rainout shelter and use a hedge clipper to cut all plants inside the square at 6 inches above ground level to mimic machine harvesting. Collect the plant material in a burlap bag, oven dry until a constant weight, and record dry weight of entire sample by subtracting the bag dry weight. Run the sample through a chipper/shredder and subsample for archival. Locate the 6′ × 10′ ambient (and/or reserved, if implemented) sampling subplot outside the shelter and hand-harvest and process as above.
Subsamples of hand-harvested yields from under the rainout shelter are ground and stored on MSU main campus by Kurt Thelen’s lab for potential future analyses. Subsamples are not saved from the “ambient” hand-harvested yields.
Date modified: Tuesday, Feb 28 2023