Surface Soil Sampling

Biofuel Cropping System Experiment

Since 2016, surface soil cores are taken annually in the fall in the main plots of all BCSE treatments to monitor for fertilizer recommendations and/or changes in inorganic nitrogen. Three soil cores are taken at each of the 3 established sampling stations per plot (see plot maps with sampling station locations) within a 2 m radius of each station flag by sampling (push) probes (2 cm dia., 25 cm depth) or by bucket auger (5 cm dia., 25 cm depth) when the soil is too dry to use push probes. All cores from one plot are composited in the field into one sampling bag. For plots with row crops, an equal number of cores are taken within rows and between rows, avoiding wheel tracks between rows. For poplar plots, an equal number of cores are taken within the tree row and between tree rows. Every effort is made to sample all plots on the same day. Microplots are also included in the sampling and analyses every other year or so; nine cores are composited from each microplot.

Changes in BCSE protocol:

• Note that the BCSE in Arlington WI (ARL) was retired at the end of the 2017 growing season.
• From 2013-2015, soil cores were taken 3 times per year using the above procedure: once prior to planting of annuals (May), once after fertilizer application (June), and once just before harvest (October). And from 2010-2012, soil cores were taken roughly every two weeks from April through October at KBS or March through December at Arlington to 25 cm depth (KBS) or 15 cm depth (ARL) following the above procedure.
• In 2009, soils were not sampled but resin strips were used to monitor nitrogen availability. Resin strips (2.5 × 10 cm) were placed in the field on a monthly basis from March to November at each of the three sample stations and left for 4 weeks.

Scale-up Experiment

Surface soils are sampled in the late fall on a near annual basis to monitor for fertilizer recommendations and/or inorganic nitrogen. Ten soil cores are collected to a depth of 25 cm with a push probe (2 cm dia.) at each of the ten sample stations per field; samples are composited by sampling station. For plots with row crops, an equal number of cores are taken within rows and between rows, avoiding wheel tracks between rows.

Changes in Scale-up protocol:

• Inorganic nitrogen was also measured in the spring from 2009-2013.

Processing of Surface Soil Samples

Soil samples are brought to the lab and refrigerated until processing. Processing of composite samples follows the procedure outlined in Composite Soil Sampling—MCSE protocol. Sieved samples are subsampled for Gravimetric Soil Moisture and Soil Inorganic N determinations. On occasion, subsamples may also be taken for soil pH and total carbon and nitrogen analysis. Samples collected in the fall are then air dried and subsamples are sent to an analytical laboratory for a standard suite of agronomic soil analyses (pH, lime index, phosphorus, potassium, calcium, magnesium, and cation exchange capacity). The remaining dried sample, or a portion thereof, is placed in labeled glass canning jars and archived.

Deep Core Soil Sampling

Soil cores to 1 m depth are taken every 5 to 10 years to primarily follow long-term changes in organic carbon and nitrogen. Intact cores (7.6 cm dia.) are taken with a hydraulic probe (at KBS: Geoprobe model 540MT, Geoprobe Systems; Salina, Kansas; at Arlington: Giddings probe, Giddings Machinery Co, Windsor, Colorado) at each of the three sampling stations (one core/station) in the BCSE and at each of the 10 sampling stations in the Scale-Up fields. In 2008, BCSE deep cores were taken in late spring/summer at KBS and in the fall at ARL. From 2013 onward, deep cores are taken only in the fall. In 2013, additional surface cores were taken at KBS, to assess the effect of season on total carbon and nitrogen. Similarly for the Scale-Up fields, cores were taken in spring 2009 and in the fall of 2014, with additional surface samples taken in the spring 2015, to assess the effect of season. For treatments with row crops, deep cores are taken between the rows.

Cores are refrigerated until analyzed. Deep-core samples are processed following the procedure outlined in the Soil Bulk Density -Deep Cores protocol with the exception that cores are cut into depth intervals of 0-10, 10-25, 25-50, and 50-100 cm. Both soil bulk density and soil water content of fresh deep-core samples are determined for each interval. For the BCSE, depth intervals are then composited by plot replicate and subsampled for total carbon and nitrogen and for agronomic soil chemistry. Analysis of the 2008 deep cores also included depth to major soil horizon and soil texture for each depth interval. For the Scale-up Experiment, depth intervals are analyzed individually (one per sampling station).

The dried soil samples, composited by plot for each depth interval, are stored in glass canning jars for archival; Jars are labeled with sample date and a treatment code as T-R-S where T = Treatment or Field, R = Replicate # for BSCE, S = Station # and depth interval (0-10 cm, 10-25 cm, 25-50 cm and 50-100 cm).