Geologically-derived nitrogen and phosphorus as a source of riverine nutrients

Abstract

The role of geological-derived nitrogen (N) and phosphorus (P) as sources of riverine nutrients was investigated in the Long Valley caldera of eastern California. Stream and spring waters were collected during the biologically-critical summer dry season when spring water/ground water inputs comprised >90% of streamflow. Samples were collected above and below major spring water inputs and irrigated pastures grazed by beef cattle to isolate the effects of spring waters versus cattle grazing as potential sources of riverine N and P. Inputs of N and P from geologic materials were identified as the primary source of elevated nutrient concentrations throughout the study area. Nitrogen, in the form of NO3–N (0.11–0.33 ​mg/L), originated from the slightly-thermal, shallow aquifer subsystem that intersected lacustrine/fluvial materials incorporating nitrogen-bearing geologic materials. Phosphorus, as PO4–P (0.132–0.255 ​mg/L), originated from both the shallow and deeper (thermal) groundwater systems and was attributed to its rapid release from weathering of the volcanic deposits. Grazing activities generated no discernible increases in mineral N or TP loads when comparing streamflow above and below grazed locations. There was a substantial loss of NO3–N during flood irrigation of pastures that was attributed to denitrification and/or vegetation uptake, with a corresponding increase of dissolved organic nitrogen ascribed to leaching of plant and/or fecal materials. This study demonstrates that geologically-derived nutrients (N and P) from volcanic terrains can be an important source of stream water nutrients. Results of this study are consistent with our previous results in volcanic areas of northern California and highlight that geologically-derived nutrients are often overlooked as a source of riverine nutrients and can have a profound effect on aquatic ecosystems.