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.