Rock-Sourced Nitrogen in Semi-Arid, Shale-Derived California Soils

Nina L Bingham,Eric W Slessarev,Oliver A Chadwick,Peter M Homyak

doi:10.3389/ffgc.2021.672522

Abstract

Models suggest that rock-derived nitrogen (N) inputs are of global importance to ecosystem N budgets; however, field studies demonstrating the significance of rock N inputs are rare. We examined rock-derived N fluxes in soils derived from sedimentary rocks along a catena formed under a semi-arid climate. Our measurements demonstrate that there are distinct and traceable pools of N in the soil and bedrock and that the fraction of rock-derived N declines downslope along the catena. We used geochemical mass balance weathering flux measurements to estimate a rock-derived N flux of 0.145 to 0.896 kg ha–1 yr–1 at the ridgecrest. We also developed independent N flux estimates using a 15N-based isotope mixing model. While geochemical mass-balance-based estimates fell within the 95% confidence range derived from the isotope mixing model (−1.1 to 44.3 kg ha–1 yr–1), this range was large due to uncertainty in values for atmospheric 15N deposition. Along the catena, N isotopes suggest a diminishing effect of rock-derived N downslope. Overall, we found that despite relatively large N pools within the saprolite and bedrock, slow chemical weathering and landscape denudation limit the influence of rock-derived N, letting atmospheric N deposition (7.1 kg ha–1 yr–1) and N fixation (0.9–3.1 kg ha–1 yr–1) dominate N inputs to this grassland ecosystem.

Highlights

Rock-derived nitrogen (N) can supplement N availability in terrestrial ecosystems (Morford et al, 2011); its contribution to the global N budget has been estimated at 6–17% (Morford et al, 2016b; Houlton et al, 2018), and in some ecosystems inputs of rock N are comparable to the flux from atmospheric fixation (Morford et al, 2016a)
Analysis of rock-derived N at the pedon scale – i.e., the scale of the soil profile and underlying bedrock – has been limited to a few humid, forested montane sites underlain by shale (Wan et al, 2021) or relatively N-rich ammonia-bearing metamorphic rock (Morford et al, 2016a)
N in this type of lithology can be stored as NH4+ in interlayer crystalline clays or as organic-N trapped as micro-detritus during diagenesis (Holloway and Dahlgren, 2002)

Summary

Introduction

Rock-derived nitrogen (N) can supplement N availability in terrestrial ecosystems (Morford et al, 2011); its contribution to the global N budget has been estimated at 6–17% (Morford et al, 2016b; Houlton et al, 2018), and in some ecosystems inputs of rock N are comparable to the flux from atmospheric fixation (Morford et al, 2016a). Analysis of rock-derived N at the pedon scale – i.e., the scale of the soil profile and underlying bedrock – has been limited to a few humid, forested montane sites underlain by shale (Wan et al, 2021) or relatively N-rich ammonia-bearing metamorphic rock (Morford et al, 2016a). A multi-year, intensive pore-water and gas sampling study set in the humid and montane Upper Colorado River Basin used element proxy-to-N ratios and temporally resolved subsurface water flux rates to estimate the contribution to subsurface N from the weathering of marine shale (Wan et al, 2021). We expect weathering rates – and rock-derived N fluxes – to be lower in more arid regions than in the humid regions where rock N has been studied (Houlton and Morford, 2015)

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Conclusion