Site selection and sampling
Samples were collected from the network of 10 Critical Zone Observatories (CZOs, http://criticalzone.org) across the USA, which represents a wide range of hydrogeological provinces, soil orders, and vegetation types as described in Brewer et al. (2018). Soils were collected at peak greenness (as estimated from NASA’s MODIS: MODerate-resolution Imaging Spectroradiometer) between April 2016 and November 2016, with the exception of the Eel River CZO samples, which were collected in May 2017 (also at peak-greenness). At each CZO, we excavated two separate soil profiles (“sites”) selected to represent distinct soil types and landscape positions (Table 1). Any organic horizon was first removed, and then mineral soils were sampled in 10-cm increments with a sterile hand trowel dug into the face of each soil pit to a depth of at least 100 cm or to refusal.
All soil samples were shipped overnight at 4 °C to the University of California, Riverside for processing. A portion of each field sample was sieved (< 2 mm), homogenized, divided into subsamples for further analyses, and frozen (−20 °C). For some soils (particularly some wet, finely textured depth intervals), sieving was impractical. These samples were homogenized and larger root and rock fragments were removed by hand. In addition, as samples from SHAL (70-100 cm depth; see Table 1 for site abbreviations) consisted almost entirely of medium-sized weathered bedrock (Cr material), soil was collected by manually crushing weathered bedrock with a ceramic mortar and pestle with this material then passed through a 2-mm sieve.