Nutrient hot spots in some sierra Nevada forest soils

Abstract

Re-examination of data distributions from several forested sites in the eastern Sierra Nevada Mountains shows consistent, positive outliers and skew for NH4+, NO3−, and mineral N in resin lysimeters and resin capsules, indicating that most values were low but hotspots of high N flux were present in most cases. Exact causes of these N flux hotspots is not known, but could include water flux hotspots (e.g., preferential flowpaths), microbial hotspots, and possibly the entry of N-enriched O horizon interflow. Soil and resin stake (PRS probe) data from one site (North Lake Tahoe) also showed consistent, positive outliers and skew for NH4+, NO3−, and mineral N, suggesting the presence of microbially produced hotspots. Bicarbonate-P data from soils and ortho-P data from PRS probes also showed highly positive skew and extreme outliers, but Bray (HCl/NH4F-extractable) P in soils did not. Other measured nutrients (extractable Mg2+, K+, SO42−, and Ca2+) also showed positive skew and outliers, but less so than NH4+, NO3−, and mineral N. Calcium stood out among measured nutrients as the most abundant nutrient with the least outliers and the lowest (sometimes negative) skew. The differences in distributions of NH4+, NO3−, and mineral N and those of Ca2+ may reflect relative abundance: the most abundant ion, Ca2+, shows little evidence of hotspots whereas the much less abundant ions, NH4+, NO3− consistently show evidence of hotspots. We hypothesize that the differing distributions of N and Ca reflect the relative biological competition for these nutrients and that positively-skewed distributions and hotspots will be characteristic of any other nutrient when it is in limited supply relative to biological demand.