Soil Sampling

Surface cores:

Since 2009, surface soils are sampled on a near annual basis in the fall to early winter for determination of soil moisture and inorganic nitrogen. Ten push cores (2 cm diameter; 0-25 cm depth) are collected at each of the 10 sampling stations per field and composited by station, for a total of 10 composited samples per field. Composited samples are subsampled for soil moisture and inorganic N. Every year subsamples from corn fields (L1, M1) and every 3 years subsamples from all fields are also sent to an analytical laboratory (e.g., MSU Soil and Plant Nutrient Laboratory through 2021; A&L Laboratory starting in 2022) for standard agronomic analyses and fertilizer recommendations; the remainder, or a portion thereof, is placed in a jar and archived.

Surface samples were also collected in the spring from 2009-2013 and analyzed for inorganic nitrogen and soil moisture as above.

Deep core samples:

Deep core (1 m depth) samples are taken every 5-10 years to primarily monitor for changes in soil carbon and nitrogen. One deep core is taken near each sampling station and divided into four depth intervals (0-10, 10-25, 25-50, and 50-100 cm). Each interval is sieved separately through a 4-mm mesh screen and then subsampled for soil bulk density determination. The remaining sample is air dried and a subsample is pulverized for total carbon and nitrogen analysis; the remainder, or a potion thereof, is placed in a jar and archived.

The first deep cores were taken in spring 2009; the protocol then changed to take deep core samples in the fall. To determine if samples taken at different times of the year can be compared, surface samples (0-25 cm and split into 0-10 and 10-25 intervals) were also taken in spring and fall of 2009 and analyzed for soil bulk density and total carbon and inorganic nitrogen as above.

Plant Sampling

Yield:

Since 2009, machine harvested yields of all three crops are measured in the fall. Annual grain crops (soybean in 2009; corn thereafter) are recorded in bushel per acre and reported in kilogram per hectare at standard moisture (Mg/ha). Corn grain yields are then converted to a dry mass basis using the standard 56 pounds per bushel at 15.5% moisture as a conversion. Since 2015, corn stover is mowed, windrowed, baled, and weighed. Stover yields are converted to a dry matter basis by drying weighed subsamples of harvested stover and determining moisture content. Switchgrass and restored prairie fields are mowed, baled and the bales counted and weighed. Subsamples of harvested yields are weighed then oven dried to determine moisture content and convert yields to a dry matter basis (Mg dry matter/ha).

Post-harvest residue samples were collected in 2015 and 2016 to measure amount of biomass remaining on fields after harvest. Samples were dried, weighed, and analyzed for total carbon and nitrogen.

Peak biomass:

From 2009-2016, plant biomass samples were taken at or near peak biomass of the dominant species for estimates of aboveground net primary production (ANPP) according to the ANPP protocol. One sample at each of the ten sampling stations was collected by hand-clipping plants within a 1m2 quadrat at ground level. Dead plant material remaining on the soil surface was then collected and labeled as surface litter; it is not included in ANPP estimates. Plant samples were sorted to species, oven dried and weighed. Row-crop species (soybean in 2009 and corn from 2010 onward) were first weighed whole then were threshed and the dry mass of grain* determined. ANPP sampling was discontinued in 2017.

Note that the standing dead label refers to plants that grew in the current year but had senesced during the year and no longer identifiable; when encountered those samples should be included in ANPP estimates. The standing dead designation was retired; those samples are included with the unsorted.

Plant total carbon and nitrogen:

Subsamples from peak biomass (ANPP) plant samples were finely ground and analyzed for total carbon and nitrogen and include 1) stover* and grain* for corn and soybean (L1, M1) at each sampling station; 2) switchgrass and an aggregate of the remaining plant species, combined across sampling stations, for switchgrass fields (L2, M3); and 3) each of the three top species, by biomass, and an aggregate of the remaining species when warranted, combined across sampling stations, in restored prairie (L3, M2) and the reference (M4) fields. Surface litter was also analyzed when collected. The remainder of the plant sample, or a portion thereof, was archived for future analyses. CN analysis of plant material was discontinued in 2017 along with ANPP sampling.

*Note that stover is used as the designation for all aboveground plant material minus grain (seeds) at both peak biomass (ANPP samples) and at harvest. And that grain is used as designation for seeds at both peak biomass (ANPP samples) and at harvest.

Canopy species composition and height:

Starting in 2018, the line-point intercept method is used to determine species composition and plant canopy height in the switchgrass, restored prairie, and reference fields. The first plant species intercepted by this method and its height is recorded every 5 meters along four N-S transects that run the full length of fields.

Greenhouse Gas (GHG) Sampling

GHG fluxes:

Fluxes of N2O, CO2 and CH4 were measured several times over the 2009-2016 growing seasons in each treatment field using the static chamber method. Four round static chambers were semi-permanently placed in each field and GHG fluxes sampled and determined according to the static chamber method protocol. Soil moisture and temperature around the chamber, as well as air temperature, were also measured and recorded.

Eddy covariance towers:

One eddy covariance tower is deployed in each scale-up field and continuously measures carbon dioxide flux and radiation data. These towers are deployed and maintained by Dr. Jiquan Chen and the Landscape Ecology & Ecosystem Science (LEES) Lab at the Center for Global Change and Earth Observations (CGCEO) and the Department of Geography at MSU.