Hot Spots and Hot Moments: Another Look at Nutrient Variability in Sierra Nevada Forest Soils.

Because of the extremely dry summers, rooting is entirely absent in the O horizons in many forest ecosystems in the Sierra Nevada Mountains of Nevada and California. Thus, decomposition/N mineralization and vegetation uptake processes are spatially discoupled, and the intense competition for nutrients between roots and decomposers in the O horizon which characterizes more humid forest ecosystems is absent. The lack of plant uptake from O horizons allows inorganic N and P released during decomposition to enrich waters flowing through O horizons on top of mineral soils (a common phenomenon in these systems). We hypothesize that this O horizon interflow creates biogeochemical “hot spots” and “hot moments” where it infiltrates into preferential flowpaths in the mineral soil. In re-examining data taken with various traditional soil analyses as well as resin-based techniques in the Sierra Nevada Mountains, we consistently find positively skewed distributions and extreme outliers in inorganic N and P data indicative of hot spots. We also find considerable inter-annual variation in the exact location of such hot spots, suggesting that they are hot moments rather than consistent features of the landscape.