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).

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 (Pinus lambertiana Douglas), white fir (Abies concolor (Gordon & Glend.) Lindl. ex Hildebr.), ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson), and incense cedar (Calocedrus deucrrens (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 cm² for WECSA® deployed capsules and 11.4 cm² 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 > 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 dplyr package in R (R Core Team 2008; Hadley and others 2018). Precipitation (mm d^-1) 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. 

Data is saved as Excel files with mutiple tabs to indicate the different datasets.