Soil Sampling

Surface cores:

Since 2018, surface soils are sampled annually in the fall for inorganic nitrogen. Using a push corer (2 cm diameter), six random cores (0-25 cm depth) are collected per split plot and composited by split plot for all plots. Samples are stored in plastic bags and refrigerated until the soil is sieved through a 4-mm mesh screen and all subsamples are processed for inorganic nitrogen and soil moisture. The remaining soil is prepared for potential nutrient analysis and/or archival.

Since 2015, surface soils are sampled approximately every 3 years in the fall and analyzed for a standard suite of agronomic soil parameters (pH, lime index, phosphorus, potassium, calcium, magnesium, cation exchange capacity) by an analytical laboratory (MSU Plant and Soil Nutrient Laboratory through 2022). Since 2018, these analyses are conducted on the same samples collected for inorganic nitrogen; samples before then were collected similarly. Prior to 2015, sampling and analysis of Wisconsin sites was done by UW staff and the UW Nutrient lab on surface soils (0-15 cm) sieved through a 2mm screen.

Deep cores:

Deep cores samples are typically taken every 10 years to primarily monitor for changes in soil organic carbon and nitrogen. All sites and treatments were sampled in fall 2013. One intact core (7.6 cm dia., 1 meter deep) was taken per plot with a hydraulic probe (Giddings probe, Giddings Machinery Co, Windsor, Colorado). Cores were partitioned by depth (0-10 cm, 10-25 cm, 25-50 cm, and 50-100 cm) and each interval analyzed for soil bulk density. A subsample (~100 g) of dried soil from each interval was pulverized for total carbon and nitrogen analysis. A dried subsample from each interval was also prepared for a standard suite of agronomic analyses (pH, lime index, phosphorus, potassium, calcium, magnesium, and cation exchange capacity) by the MSU (Michigan sites) and UW (Wisconsin sites) Nutrient Labs and for archival. In addition, samples were frozen for DNA extraction and microbial community analysis.

Plant Sampling

Annual Net Primary Production (ANPP):

Since 2018, ANPP is measured in early fall at peak biomass. For herbaceous treatments (all but G8 poplars), all plants rooted within the boundary of a 1m2 quadrat (0.5 m x 2m) are hand clipped at ground level and placed into labeled paper or burlap bag, depending on sample size and handling. Two samples per split plot are collected; one at opposite lengthwise ends of the split plot (N and S or E and W depending on plot orientation). Sampling locations are pre-determined (rotating distance and direction from a plot corner, as specified on plot sampling map) to avoid the harvest yield strips and the G5 rainout shelter and associated areas. The sampling quadrat is oriented perpendicular to planted row crops to allow assessment of both row and inter-row communities. Samples are not sorted to species and are processed in one of two different ways, depending on travel logistics, timing, staffing and oven capacity. Either (1) the sample bag is weighed and the fresh weight of plant sample recorded; the sample is then run through a shredder, and the two samples from each split plot are combined, keeping the split plots separate. A subsample of the shredded material is placed into a 10# paper bag, weighed and the fresh weight of the subsample determined. Subsamples are then oven dried at 65C for a minimum of 4 days, weighed, and the dry mass of subsample and moisture content of plant material determined to calculate the dry weight of original sample. Or (2) the entire contents of the bag is oven dried at 65C for a minimum of 4 days, weighed, and the dry weight of the original sample determined by subtracting the bag weight.

For the poplar treatment (G8), woody tree biomass and annual woody production are estimated by allometric equations using annual measurements of stem diameter. In late fall to early winter, the basal tree diameter (mm) is measured with a caliper at 15 cm above the soil surface on 10 randomly selected trees in the center 2 rows of each split plot.

Yields:

Since 2014, machine harvested yields of herbaceous crops are measured in the fall of each year within two weeks after the first killing frost to maximize nutrient resorption. The yield of each split plot is determined individually. For treatments G5-G7 and G9-G10, a 7.5-foot-wide strip is harvested lengthwise down the center of each split plot (32 feet long in G5 and offset a bit to accommodate rainout shelter) with a Kemper harvester at a cut height of 4-6 inches. The harvested material is offloaded through a chopper and collected in a forage weigh wagon. The fresh weight of harvested material is recorded. A subsample from each split plot is collected, oven dried at 60C until a constant dry weight, weighed and the moisture content of subsample determined as above. The subsample, or a portion thereof, is then finely ground using a grinding mill for total carbon and nitrogen analysis and archival.

The historical treatment (G11) is a control and is not harvested annually. Yields for G11 are estimated by hand harvesting. A hedge clippers is used to cut the vegetation within a randomly placed 1 m2 quadrat (0.5m x 2m) at 6 inches above ground level to mimic the harvester cutting height. Two samples are taken per split plot (since 2018). Samples are weighed and dried as above to determine moisture content and dry matter yields. The two samples from the same split plot are combined and subsamples are processed for total carbon and nitrogen analysis and for archival as given above. Note that in 2015-2017, three samples were taken per split plot.

Yields for the poplar treatment (G8) are measured by clear-cutting on an approximate 6-year rotation. Twenty-two trees are harvested from the center two rows of each split plot using a chain saw. All harvested material is fed through a chipper, collected in a container, and weighed. A grab sample of chipped material is collected, weighed and dried to determine moisture content, and processed for CN analysis and archival as described above.

Species composition:

Since 2017, canopy species composition and height are measured annually in late summer to early fall at or near maturity of the dominant plant species using the line-point intercept method. The first species intercepted by a vertically lowered pointer and its height is recorded at regular intervals (1 to 5 m) along 4 transects per plot: 2 transects in each of the split plots.