DOI: 10.6071/m3qq3z
title: Soil nutrient fluxes and hot spots in a Mediterranean mixed-conifer forest

description: We evaluated the spatial and temporal patterns in nutrient HSs in two
 mixed-conifer forest soils at the Southern Sierra Critical Zone
 Observatory. The study area is located on the western slope of the
 southern Sierra Nevada, California, USA. This region experiences a
 Mediterranean-type climate where mean annual air temperature is 9.8 °C,
 and mean annual precipitation is 1325 mm y-1, with 35-60% falling as
 winter snow (Yang et al. 2021). The vegetation is a mixed-conifer forest
 and soils are derived from granitic parent material. Ion exchange resin
 capsules were installed in three-dimensional plots over multiple seasons
 and years. Data include soil nutrient (phosphate, ammonium, nitrate,
 sodium, calcium, and magnesium) concentrations (micromole per squared
 centimeter) and fluxes (micromole per squared centimeter per day),
 designation of resin locations as soil nutrient hot spots (binary; 0 not
 identified as a hot spot, 1 identified as a hot spot), volumetric water
 content (average from 10, 30, 60, and 90 centimeter depths; meter cubed
 per meter cubed), daily precipitation (millimeters), and penetrometer
 measurements of soil resistivity (0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20,
 22.5, 27.5, 30, 32.5, 35, 37.5, 40, 42.5, and 45 centimeters; kilopascal).
description: The study area is located on the western slope of the southern
 Sierra Nevada, California, USA. The vegetation is a mixed-conifer forest
 with canopy-dominant tree species of sugar pine
 (<em>Pinus</em> <em>lambertiana</em> Douglas),
 white fir (<em>Abies concolor</em> (Gordon & Glend.)
 Lindl. ex Hildebr.), ponderosa pine (<em>Pinus</em>
 <em>ponderosa</em> P. Lawson & C. Lawson), and incense
 cedar (<em>Calocedrus</em> <em>deucrrens</em>
 (Torr.) Florin). Soils are derived from granitic parent material and
 classified in the Shaver soil series, a coarse-loamy, mixed, superactive,
 mesic Humic Dystroxerept (Johnson and others 2014). Johnson and others
 (2014) established a 6- x 6-m grid of resin capsules (UNIBEST PST-1,
 https://www.unibestinc.com/technology; UNIBEST Corporation, Washington,
 USA; Yang and Skogley 1992; Dobermann and others 1994) at two
 representative plots within a Sierran mixed-conifer forest. At each plot,
 16 resin capsules were placed carefully at 2-m intervals horizontally
 within this grid immediately below the O horizon (i.e., 0-cm mineral
 depth) using a hand trowel to minimize disturbance. Adjacent to these
 resin capsules, WECSA® Access System units (WECSA® LLC; Montana, USA) were
 installed at 20-, 40-, and 60-cm depths at an angle that varied according
 to the microtopography, such that the deeper capsules would all be
 vertically coincident with their respective 0-cm capsule. After the
 capsules were collected and upon returning to the laboratory at the
 University of California, Merced, USA, adhering soil was removed with
 deionized water. The resin capsules were processed following the procedure
 described in Johnson and others (2014).   Nutrient
 fluxes were calculated by converting nutrient concentrations in the
 extracts to a nmol cm-2 d-1 basis; this was achieved by transforming mass
 to moles for that particular nutrient, multiplying the concentrations by
 the total extractant volume (60 mL), and then dividing those values by the
 cross-sectional area of the capsule exposed to the soil (5.7
 cm<sup>2</sup> for WECSA® deployed capsules and 11.4
 cm<sup>2</sup> for 0-cm depth capsules; Johnson and others
 2014) and the incubation period. Hot spots (HS) were identified as
 positive moderate outliers using the following equation (Johnson and
 others 2014): X &gt; Q3 + 1.5(IQR) where X is the HS value (i.e.,
 outlier), Q3 is the third quartile value (75th percentile) and IQR is the
 interquartile range (25th–75th percentiles). The 25th percentile, 75th
 percentile, and IQR for a given nutrient flux were determined using all
 grid locations, depths, seasons, and years, but data from each plot were
 analyzed separately Precipitation and soil moisture
 data were acquired over the entire study period to evaluate the hydrologic
 environment experienced by the resin capsule during incubation. Volumetric
 water content (m3 m-3) was collected from the north aspect Upper
 Providence Creek station (Long. 37.075°, Lat. -119.182°; 1920 m elevation)
 located ~2 km from the resin plots (Critical Zone Observatory; https://doi.org/10.6071/Z7WC73). Values were recorded hourly using Decagon Devices ECHO-TM at 10-, 30-, 60-, and 90-cm depths and then averaged by day using the <em>dplyr</em> package in R (R Core Team 2008; Hadley and others 2018). Precipitation (mm d<sup>-1</sup>) was also collected at the Upper Providence station using a Belfort-TM 50780 shielded weighing rain gauge (Kings River Experimental Watershed; Hunsaker and Safeeq 2018). Soil penetration resistance (kPa; i.e., soil strength) was measured at each plot to identify bulk density differences that may impact water fluxes using a Field Scout SC900 Soil Compaction Meter (Spectrum Technologies, Inc., Aurora, IL, USA) with a 1.27-cm cone size. Compaction measurements were made after the last set of resin was collected and taken throughout both plots in undisturbed soil between the locations where WECSA® access systems were installed. Penetration resistance was logged at 106 and 87 points in Plot 1 and Plot 2, respectively from 0- to 45-cm (maximum instrument depth) and averaged across all recorded points within a plot at every 2.5-cm depth increment. Missing penetration resistance measurements indicate the penetrometer could not be pushed further into the soil, indicating a root or rock was reached. 
description: Data is saved as Excel files with mutiple tabs to indicate the
 different datasets.