The Median Isn’t the Message: Elucidating Nutrient Hot spots and Hot Moments in a Sierra Nevada Forest Soil

Abstract:

Most biogeochemical studies in forests have concentrated on nutrient pools and transformations occurring at relatively large spatial scales (i.e., stand or small catchment), over monthly or annual time scales. Many of these studies have also focused on the average or medial values observed across the spatial or temporal scale studied, discounting outliers. However, extremely high values found consistently (hot spot) or infrequently (hot moment) at a given soil microsite may be critical for nutrient acquisition by organisms and nutrient retention by terrestrial ecosystems. We have been evaluating soil nutrient hot-spot and hot-moment phenomena vertically (to a 60-cm depth) and horizontally (2-m sampling interval within a 6 m x 6 m grid) in two areas within a mixed-conifer, Sierran forest experiencing a Mediterranean-type climate. Nutrient fluxes in space and time were measured using ion exchange resin capsules placed at various depths and collected at two times (first significant precipitation in fall and post-snowmelt in spring) per year. Our previous work over a single year showed that fluxes of Ca²⁺ and Mg²⁺ in mineral soil were substantially greater in the spring than in the fall, suggesting that soil water was a major factor in controlling these nutrient fluxes. The opposite pattern was found for NH₄⁺ and Na⁺, where greater fluxes occurred following the first precipitation event in fall. Here, we report new data over two additional years at these same sites. Over the entire 3-year study, nutrient fluxes were greater in the fall for all mineral soil nutrients except Ca²⁺ and Mg²⁺. Calcium fluxes were consistent with previous results; however, Mg²⁺ demonstrated no statistical significance between fall and spring sampling dates. Generally, the number of high statistical outliers persisted through time for Ca²⁺ and Mg²⁺, suggesting hot spots for these nutrients. In contrast, large seasonal and annual changes in the number of high statistical outliers occurred for NH₄⁺, NO₃^-, and PO₄^3-, nutrients whose availabilities are more mediated by microbial activity than base cations. Further elucidation of the mechanisms responsible for nutrient hot spot-hot moment phenomena within soil should be invaluable for improving the predictive capacity of biogeochemical models and for scaling these models across space and time.