This collection contains belowground biomass samples collected from the megapit at each terrestrial site (NEON sample class: mpr_perrootsample_in.archiveID). Each site is sampled a single time at the initiation of the site. At each site, a soil pit is dug in the dominant soil type to a maximum depth of 2 m. On the exposed face of the soil pit, a tape measure visually divides the soil profile into 10 cm depth increments. Each soil pit has three vertically-oriented sampling profiles, roughly corresponding to the left, center, and right of the pit sampling face. These profiles are referred to as profiles number 1, 2 and 3, respectively. From each profile, a block of soil is removed from each 10 cm depth increment, starting from the surface down to 100 cm. Once a depth of 100 cm is reached, each profile is divided into 20 cm depth increments. By the end of sampling, up to 45 soil samples are collected. Roots are sieved and then divided into four categories distinguishing between status (alive or dead) and size (> 2mm and < 2 mm, unless otherwise noted). After oven-drying at 65 degrees Celsius for at least 48 hours, dry mass is recorded, then samples are ground and sent for analysis of carbon and nitrogen concentrations and stable isotopes. If there is any material leftover, sample biomass is archived in 20 mL scintillation vials (glass or HDPE) and stored at room temperature. See link below for the NEON data product that provides mass as well as carbon and nitrogen concentrations and stable isotopes of these same root samples.