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Nutrient hot spots in a Sierra Nevada soil: Physical sssessments and contributing factors

Woodward, C. 2012. M.S. thesis. University Of Nevada, Reno, Nevada


An understanding of soil chemistry is important for biogeochemical cycling not only belowground but above ground as well. Soil nutrients play a key role in the function of an ecosystem. Nutrient hot spots in the mineral soil are believed to be vital components of a soil system influencing biological uptake and biogeochemical cycling. Three studies were done each looking into different aspects of soil nutrient hot spots at two sites in the Kings River Experimental Watershed (KREW). The first study consisted of using three different resin based sampling methods (UniBest resin capsules, PRS™ probes, and resin lysimeters) to investigate the presence and temporal effects of soil nutrient hot spots. All three sampling methods showed nutrient hot spots in the soil. Individual nutrient hot spots were more prominent than a sample point that had nutrient hot spots for all the nutrients. Inorganic nitrogen was the most common hot spot found. The size of the hot spots varied from ~2 cm (the size of a single capsule) to 40 cm2. UniBest resin capsules and PRS™ probes appeared to be more influenced by interflow after a single precipitation event rather than being left in the soil for the entire season. Calcium on the probes appeared to be displacing potassium and ammonium on the membrane when the probes were left in the soil for the entire season. The resin lysimeters showed that hot spots can be ephemeral (hot moments). The other two studies were focused on potential influences on the formation of hot spots. One study looked at correlations between microbial biomass and water extractable nutrient hot spots in the mineral soil. The results were inconclusive. At one site the data showed that water extractable nutrient hot spots were co-located with microbial biomass, however at the other site it did not. This suggests that microbial biomass is not always associated with nutrient hot spots. The study also showed that nitrate was the most frequent hot spot found and the hot spots were commonly located in locations of lower concentrations for other nutrients. The last study investigated the influences of O-horizon interflow on soil nutrient concentrations and hot spot formation. The study found that at one site inter flow can increase decomposition rates and therefore possibly increase nutrient availability into the soil. A 17% increase in soil moisture content did not increase the formation of hot spots in the soil or average nutrient concentrations for resin capsules or water extractable nutrients; however, truncating interflow with PRS™ probes caused an increase in the number of individual hot spots, but did not cause the formation of hot spots for multiple nutrients in one location.