UC Merced

Global climate models demonstrate that temperate ecosystems are likely to serve as carbon (C) sinks in the coming decades, however any realized increase in C storage on land will also require increases in the availabilities of plant essential nutrients such as nitrogen (N) and phosphorus (P). Additionally, nutrient limitation to plant productivity remains one of the most uncertain factors to global climate projections. Our understanding of how nutrient biogeochemistry is altered under different climatic conditions is unclear. Using a combination of field, laboratory, and advanced analytical techniques, I show P biogeochemistry is intrinsically related to soil development in a semiarid (White Mountains Elevational Transect) to Mediterranean (Southern Sierra Nevada Critical Zone Observatory) climatic gradient. Specifically, P stock increases, inorganic P transitions from calcium to iron and aluminum association, and organic P proportionally decreases with increasing climate-driven weathering. With increasing precipitation, soil and aboveground foliage became progressively more P-limited, whereas the microbial biomass and fine roots are more N-limited and enzyme activity is largely unaffected. Overall, the transition from a semiarid to Mediterranean climate creates more chemically weathered soil that is able to retain P that may not be readily bioavailable. These relatively more weathered soils appear to be relying on faster cycling organic P species to support ecosystem development. Furthermore, using three-dimensional resin capsule pots in a Mediterranean forest site, I characterized how nutrient fluxes under more biological (PO43-, NH4+, NO3-) or geochemical (Ca2+, Mg2+, and Na+) control are impacted by water year, seasonality, and depth to form hot spots (HS) or hot moments (HM). A multi-year drought occurred during the study, causing geochemically controlled nutrient fluxes to be transient over time and characterizing them as HM. Alternatively, biologically controlled outliers formed HS and HM. Macronutrient HS and HM are found at higher concentrations relative to the surrounding soil matrix, and co-occur in discrete spatial locations where belowground biomass can detect, proliferate, and exploit these resources. Overall, water flux controls nutrient biogeochemical cycles in these drylands. Therefore, nutrient HS and HM will likely play a disproportionate role in supporting ecosystem development in these temperature drylands under a changing climate.