Nitrogen cycling “hotspots”: An approach for watershed scale assessments

The high level of spatial and temporal heterogeneity in nitrogen cycling processes hinders our ability to develop an ecosystem-wide understanding of this cycle. This study examines how the incorporation of spatial “hotspots” of soil moisture, carbon, nutrients, and soil texture can better explain ecosystem nitrogen cycling? Sites (80 m x 80 m) distributed over an elevation gradient (788 m-1389 m) and regionally representative vegetation gradient. Starting in November 2010, surveys assessing soil moisture (bimonthly) using electromagnetic induction (EMI) and soil nutrient and carbon content (extractable NH₄⁺, acid extractable PO₄³⁺, total C, and total N) (quarterly) using near infrared reflectance spectroscopy (NIRS). Soil texture analysis was conducted in June of 2011. As indicators of potential nitrogen cycling rates, quarterly laboratory assays were conducted for potential denitrification and nitrification from March 2011 through March 2012. Stepwise multivariate regression analysis with minimum corrected Akaike Information Criterion (AICc) was used to determine the most appropriate model of predictors for nitrogen cycling processes. Overall, NIRS data was able to explain 38-44% of the potential denitrification rates (p<0.01) and 88-98% of potential nitrification variability (P<0.01). EMI data was significantly related to soil moisture, explaining 20%-52% of the variability. Multivariate regression combined with AICc analysis, revealed that conductivity and clay content explained 45% (0-5 cm) and, when also including P, 74% (10-20 cm) of the potential denitrification variability (p<0.05). This study showed the potential of geophysical tools in developing an ecosystem level understanding of the nitrogen cycle.