GLBRC Switchgrass Nitrogen Rate Experiment – Sampling


In use from 2008-06-01


The GLBRC Switchgrass Nitrogen Rate Experiment, formerly Nitrogen/Harvest, was established in 2008 to examine the effects of nitrogen fertilization and harvest frequency on switchgrass productivity and greenhouse gas emissions. The experiment is located in the southeast corner of the KBS GLBRC Intensive Site and is laid out in a randomized split block design with 8 N fertilization rates (F1-F8; 0, 28, 56, 84, 112, 140, 168, and 196 kg N/ha, respectively) and with 2 original harvest strategies (H1 and H2; 1 vs 2 harvests per year) comprising the split. Plots are replicated in each of 4 blocks (R1-R4) for a total of 8 × 4 = 32 plots for each harvest strategy. Plots are labeled by arrangement within the replicate block (e.g., plot 204 = Block 2, fourth plot from the west) and by N rate treatment (e.g., SWF4 = Switchgrass N rate #4 = 84 kg N/ha). The two harvests per year strategy was terminated after the 2012 harvest; since 2013 all split plots are harvested once in the fall, with yields recorded separately 2013-2022 and combined starting in 2023. See the GLBRC Switchgrass Nitrogen Rate Experiment plot map for current plot layout.

Surface soils are sampled every 2-3 years to monitor fertilizer effects on soil pH and to ascertain nutrient and liming recommendations. Surface soils were monitored in 2009-2010 for seasonal changes in soil inorganic N and deep soil cores were taken in 2013 for a specific experiment, but these are not routine measurements. Soil water (leachate) was also collected and sampled throughout the 2009-2021 growing seasons and analyzed for a variety of chemical parameters.

Plants are machine-harvested in the fall and dry matter yields recorded. Subsamples of harvested plants are dried, ground, and analyzed for total carbon and nitrogen; remaining plant material is typically archived. Plots are not sampled for aboveground net primary production (ANPP). In the spring,switchgrass stand counts may be performed to monitor for changes in population growth and recruitment.

Note that early documentation may also refer to this experiment as GLBRC N Gradient Switchgrass plots. Also note that as of May 2013, plots with an original ‘2 harvests per year’ strategy are separated by a buried one-half meter deep aluminum barrier.


Soil Sampling

Surface cores:

Since 2017, surface soil cores are collected in the fall every 2-3 years and sent to an analytical lab (e.g., MSU Plant and Soil Nutrient Lab through 2022) for determination of cation exchange capacity, pH, lime index, phosphorus, potassium, calcium and magnesium and for nutrient and liming recommendations. Using a push probe, 4 randomly placed surface cores (0-25 cm) are collected from each H1 split plot (and occasionally H2 as needed), composited by split plot, sieved though a 4-mm screen, air dried, and a subssample is sent to an analytical lab. The remaining soil may be archived as warranted.

Surface soil cores are periodically collected in the fall to measure soil pH. Using a push probe, 4 randomly placed cores (0-25 cm) are collected from each of the H1 split plots, composited by split plot in labeled plastic bags, and stored refrigerated until analysis for pH on field-moist soils. Note that pH is also included in the standard agronomic soil test described above.

Deep cores:

Deep cores (1 m deep) were taken in fall 2013 with a hydraulic probe (Geoprobe, Geoprobe Systems, Salina KS), sectioned into depth intervals (0-10 cm, 10-25 cm, 25-50 cm, and 50-100 cm) and each interval analyzed for root biomass and soil bulk density. A subsample (~30-50 g) of dried soil from each interval was pulverized for total carbon and nitrogen analysis and for archival.

Soil water:

From 2009 through 2022, soil water (leachate) samples were collected biweekly throughout the growing season (or as allowed by precipitation and soil frost) from low-tension soil water samplers (also called suction lysimeters) buried ~1.2 meters deep in the H2 split plot of every treatment (F1-F8) in replicate blocks R2-R4.

Plant Sampling

Dry matter harvest yields:

Since 2013, yields are measured by machine harvesting the middle 7.5 ft of each split plot at a cutting height of ~6 inches with a 7.5 ft wide Kemper head and biomass harvester system with fresh weights obtained by an on-board computer weighing system. Split plots (1 vs 2 harvests per year) were harvested and weighed separately through 2022 and are harvested together thereafter. Grab samples are collected from each plot replicate and weighed, then dried in 60C oven for 48 h, and re-weighed to determine moisture content of harvested material and to calculate dry matter yields. From 2010-2012, the middle 4 ft of each split plot was harvested using a 4 ft wide Kemper head.

Total carbon and nitrogen and archive:

Subsamples from the machine harvest are also dried, finely ground, analyzed for total carbon and nitrogen, and the remainder, or a portion thereof, archived for future analyses. Note that subsamples are not sorted to species; they are left “unsorted” and may contain species other than switchgrass. Since 2018, only subsamples from the original 1 harvest per year split plot were analyzed and archived; starting in 2023, subsamples are from the combined harvest of both H1 and H2 split plots.

Stand counts:

Stand counts are periodically conducted in the spring to assess the growth and recruitment of switchgrass populations.

Date modified: Tuesday, Sep 26 2023