Nitrogen production from geochemical weathering of rocks in southwest Montana, USA

Abstract

[1] A 30 day time course laboratory weathering experiment was conducted using rock samples collected from the West Fork of the Gallatin River watershed (WFW) in southwestern Montana, USA. The goal of these experiments was to quantify the amount of labile nitrogen in rock samples collected from the watershed and determine if chemical weathering is a source of dissolved nitrogen in stream water. Several rock samples investigated produced nitrate in significantly higher concentrations than the silica bead control (p < 0.05), and the data were consistent with elevated NO 3 À concentrations measured in associated WFW streams. Isotopic analyses of 15 N-NO 3 in 22 stream water samples from the WFW and four rock:water extracts from the laboratory experiments indicated that the isotopic composition of NO 3 was comparable with rocks and stream water samples in the same watershed and differed strongly from waters downstream of development. We suggest that the NO 3 À measured in WFW streams includes nitrogen derived from mineral dissolution products from soils and rock. The results presented herein further indicate that rock weathering is a source of stream water N in the West Fork watershed and inform water quality assessment, total maximum daily load development, and the relative influences of natural and anthropocentrically derived N sources across this developing mountain watershed.

Temporal Variability in Nitrate‐Discharge Relationships in Large Rivers as Revealed by High‐Frequency Data

“…Researchers have extensively studied the potential physical and hydrologic process controls on the export behavior of solutes from watersheds. Potential drivers of chemostatic behavior of solute export in watersheds can include large and spatially homogenous legacy solute stores (Basu et al, ), high production rates of weathering products (Godsey et al, ; Montross et al, ), extent of point versus nonpoint source contributions (Gardner et al, ), and external sources of solutes (Gall et al, ). Potential processes that drive dilution responses can include snowmelt pulses of relatively dilute meltwater (Oczkowski et al, ), while enrichment responses can occur due to contributions from high concentration source areas, such as the connection between a river network and proximal source areas.…”

Direct quantification of long‐term rock nitrogen inputs to temperate forest ecosystems

“…Nitrogen is common among sedimentary and metasedimentary rocks (Bebout et al 2013 ), but questions remain over the extent to which rock weathering contributes to net N inputs in ecosystems (Schuur 2011, Ciais et al 2013. Rocks have been implicated in high N accumulation rates in soils (Dahlgren 1994, Dixon et al 2012, streams (Holloway et al 1998, Montross et al 2013, groundwater (Strathouse et al 1980, Hendry et al 1984, and plant pools (Cornwell andStone 1968 , Morford et al 2011 ); however, these conclusions have been more correlative than direct, relying on comparisons of N-rich bedrock to measured N pools in biota, soils, and natural waters as opposed to directly quantifying the depletion of rock N pools in the soil (Houlton and Morford 2015 ). Here we present a new method to directly quantify in situ weathering of rock N that can be used to improve estimates of rock N inputs to ecosystems.…”

Geochemical and tectonic uplift controls on rock nitrogen inputs across terrestrial ecosystems

“…Crustal rock could contribute additional amounts of N to key ecosystem pools, however, given the widespread nature of N-rich parent materials and the massive size of this N reservoir in the Earth system (supporting information Table S1). Indeed, early biogeochemical studies pointed to the potential importance of bedrock N in global N budgets [Hutchinson, 1944;Rayleigh, 1939] and a number of recent studies have supported the idea that rocks are an overlooked source of N to terrestrial and aquatic ecosystems [Cornwell and Stone, 1968;Dixon et al, 2012;Hendry et al, 1984;Holloway et al, 1998;Montross et al, 2013;Morford et al, 2011;Strathouse et al, 1980]. However, incomplete knowledge of controls on the chemistry, denudation, and chemical weathering of rock N sources across terrestrial landscapes has limited our understanding of this N input pathway [e.g., Ciais et al, 2013;Houlton and Morford, 2015].…”