The Median Isn’t the Message: Elucidating Soil Nutrient Hot Spots

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Abstract
Soil nutrient distribution is heterogeneous in space and time, potentially altering nutrient acquisition by trees and microorganisms. Ecologists have distinguished “hot spots” (HSs) as areas with enhanced and sustained rates of nutrient fluxes relative to the surrounding soil matrix. We evaluated the spatial and temporal patterns in nutrient flux HSs in two mixed-conifer forest soils by repeatedly sampling the soil solution at the same spatial locations (horizontally and vertically) over multiple seasons and years using ion exchange resins incubated in situ. The climate of these forests is Mediterranean, with intense fall rains occurring following summers with little precipitation, and highly variable winter snowfall. Hot spots formed most often for NO3- and Na+. Although nutrient HSs often occurred in the same spatial location multiple times, HSs persisted more often for NO3-, NH4+, and PO43- and were more transient for Na+, Ca2+, and Mg2+. Sampling year (annual precipitation ranged from 701 to 1216 mm) impacted the occurrence of HSs for most nutrients, but season was only significant for PO43-, NH4+, NO3-, and Na+, with HSs forming more often after fall rains than spring snowmelt. The frequency of HSs significantly decreased with soil depth for all nutrients, forming most commonly immediately below the surficial organic horizon. Although HSs accounted for less than 17% of the sampling volume, they were responsible for 56-88% of PO43-, NH4+, and NO3- fluxes. Our results suggest that macronutrient HSs have a disproportional contribution to soil biogeochemical structure, with implications for vegetation nutrient acquisition strategies and biogeochemical models.

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