Marginal Land Experiment – Sampling

Soil Sampling

Surface cores:

Since 2018, surface soils are sampled annually in the fall for inorganic nitrogen. Using a push corer (2 cm diameter), take six random cores (0-25 cm depth) per split plot and composite by split plot for all plots. Store samples in plastic bags and refrigerate until soil is sieved through a 4-mm mesh screen and all subsamples are processed for inorganic nitrogen and soil moisture. Air dry the remaining soil 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 (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), hand clip all plants rooted within the boundary of a 1m2 quadrat (0.5 m x 2m) at ground level and place into labeled paper or burlap bag, depending on sample size and handling. Take two samples per split plot; 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) that avoids the harvest yield strips and the G5 rainout shelter and associated areas. Orient the sampling quadrat perpendicular to planted row crops to allow assessment of both row and inter-row communities. Do not sort to species. Samples are processed in one of two different ways, depending on travel logistics, timing, staffing and oven capacity. Either (1) weigh the sample bag and record fresh weight of plant sample, then run plant sample through a shredder, combining the two samples from each split plot but keeping the split plots separate, place a subsample of the shredded material into a 10# paper bag, weigh and obtain a fresh weight of the subsample, oven dry subsamples at 65C for a minimum of 4 days, weigh, and determine dry mass of subsample and moisture content of plant material and use that percentage to determine dry weight of original sample. Or (2) oven dry the entire contents of the bag at 65C for a minimum of 4 days, weigh, and determine the dry weight of the original sample 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, measure the basal tree diameter (mm) 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. Determine the yield of each split plot individually. For treatments G5-G7 and G9-G10, harvest a 7.5-foot-wide strip 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. Offload harvested material through a chopper and collect in a forage weigh wagon. Record fresh weight of harvested material. Collect a subsample from each split plot, oven dry at 60C until a constant dry weight, weigh and determine moisture content of subsample as above. Then use a grinding mill to finely grind the subsample, or a portion thereof, 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. Use a hedge clippers 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. Take two samples per split plot (since 2018). Weigh and dry as above to determine moisture content and dry matter yields. Combine samples from the same split plot and process for total carbon and nitrogen analysis and 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. Harvest 22 trees from the center two rows of each split plot using a chain saw. Feed all harvested material through a chipper, collect in a container, and weigh. Take a grab sample of chipped material, weigh and dry to determine moisture content and process for 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. Record the first species intercepted by a vertically lowered pointer and its height at regular intervals (1 to 5 m) along 4 transects per plot: 2 transects in each of the split plots.