Nitrogen cycling beneath the soil: bioavailable nitrogen dynamics in a bedrock rhizosphere

Abstract:

Deep rooting into weathered bedrock beneath soils is common across a variety of ecosystems. While nitrogen (N) cycling in soils is well understood, it has not been established to what extent active N cycling takes place in the rarely measured weathered bedrock layer. To investigate this, we characterize N species in waters transiting through a bedrock rhizosphere underlying a mixed conifer-hardwood forest in the Northern California Coast Ranges. Previous studies at the site established that trees withdraw water from the unsaturated weathered and fractured bedrock to 12 m depth and considerable CO2 production via root respiration extends to 6-8 m depth. These findings support the reasoning that N cycling may extend deeper than the classically studied soil layer. Using a novel Vadose Zone Monitoring System, we collected water and gas samples throughout the 16 m deep weathering profile. Water samples were analyzed for a suite of solutes including TN, NH4+ and NO3-, and gasses were analyzed for CO2, O2 , and N2O. The primary form of N in water samples at all depths is organic N. Total nitrogen, as well as NH4+ and NO3-, show seasonal variations in concentration, and concentrations of both NH4+ and NO3- in the bedrock rhizosphere are comparable to those reported for some temperate forest soils. Total nitrogen and NH4+ increase with depth indicating that N transformations occur in the weathered bedrock below the soil layer. Together, our findings suggest that deep N cycling may accompany the seasonal water and carbon cycling that occurs within the bedrock rhizosphere.