Biodiversity Gradient Experiment: Baseline Soil Sampling

Sample Collection and Processing

The LTER Biodiversity plots are 9.1 × 27.4 m (30 × 90 feet). Soil cores are collected at each of two sampling stations along a transect that is centered between the east and west sides the plot. One sampling station is 9 meters from the south border of the plot on the west side of the transect, the second station is 18 meters from the south border of the plot on the east side of the transect. The cores are taken in an area of approximately 0.5 × 0.5 meters. Half of the cores are taken within the crop row and half are taken between the rows. Distances to sampling stations are estimated by pacing.

A total of six 0-25 cm cores (three at each station) are collected and composited. A second set cores, from 0-10 cm in depth, are collected at the two sampling stations, two cores at each station, and composited. Before sampling each plot, a core is taken at the edge of the plot and discarded to prevent carryover between plots. Latex gloves are worn when collecting soil cores in the field and while sieving. Soil samples are sieved using a 4 mm mesh sieve.

Soil samples are refrigerated at 4C until processing is complete. Ideally, samples for inorganic nitrogen and soil moisture should be processed within 24 hours of collection. The maximum time between collection and processing is 48 hours. Archive samples to be used for microbial (e.g. Phospholipid Fatty Acids) or DNA analysis should be frozen at -80C within 48 hours of collection.

The 0-25 cm soil sample is analyzed for inorganic nitrogen (NO3 and NH4 pools) and soil moisture. The remainder of the sample is air dried and archived. The 0-10 cm soil cores are archived. Four whirlpaks containing 25-30 g each are frozen at -80C and the remainder is air dried and archived.

Inorganic Nitrogen

Weigh 10 ±0.01 g of sieved soil into a urinalysis cup (u-cup). Add 100 ml of 1 M KCl, shake for one minute and allow to stand overnight. Shake again for one minute the next day and let stand for about one hour or until the supernatant partially clears. Filter the supernatant with a syringe filter using a type A/E 1 µm pore size glass fiber filter. Refrigerate filtered supernatant if analysis will be done within one week, otherwise freeze extract at -10C to hold for analysis. Analysis for inorganic nitrogen is performed on an Alpkem segmented flow analyzer.

Moisture

All weights for soil moisture are measured to nearest 0.01 g. Label a soil tin with sample ID. Record weight of “Tin, Lid, and Label”. Place 20-25 g of sieved, field moist soil in the tin. Weigh and record the weight of the fresh soil. Dry open tins in an oven at 105C for at least 24 hours. Remove from the oven and place lid on tin. When cooled to room temperature, record weight of “Tin, Lid, Label, and Dry Soil”. The weight of the dry soil is calculated by subtracting the “Tin, Lid, and Label” weight from the “Tin, Lid, Label, and Dry Soil” weight. Soil moisture is expressed on a % dry weight basis by the formula:

% Soil Moisture = 100 * [(Fresh weight – dry weight)/dry weight]