Global patterns of nitrate isotope composition in rivers and adjacent aquifers reveal reactive nitrogen cascading

Ioannis Matiatos,Alejandro García-Moya,Widad Fadhullah,Carlos Alonso-Hernández,Asunción Romanelli,Nikolaos Kantiranis ,Christina Biasi,Anh Duc Trinh ,Lhoussaine Bouchaou,Wendell W Walters ,Joseph Richmond Fianko ,Nandana V Edirisinghe,Lucilena Rebêlo Monteiro ,Jason J Venkiteswaran,Fredrick Tamooh,Fangxin Yue ,Viviana Ré ,Diego Rivera ,Pascal Boeckx,Leonard I Wassenaar,Elisa Sacchi,Sakhila Priyadarshanee,Minh Thi Nguyet Luu ,Prasanta Sanyal,Daren C Gooddy,Greg Michalski,Nina Welti

doi:10.1038/s43247-021-00121-x

Abstract

Remediation of nitrate pollution of Earth’s rivers and aquifers is hampered by cumulative biogeochemical processes and nitrogen sources. Isotopes (δ15N, δ18O) help unravel spatiotemporal nitrogen(N)-cycling of aquatic nitrate (NO3−). We synthesized nitrate isotope data (n = ~5200) for global rivers and shallow aquifers for common patterns and processes. Rivers had lower median NO3− (0.3 ± 0.2 mg L−1, n = 2902) compared to aquifers (5.5 ± 5.1 mg L−1, n = 2291) and slightly lower δ15N values (+7.1 ± 3.8‰, n = 2902 vs +7.7 ± 4.5‰, n = 2291), but were indistinguishable in δ18O (+2.3 ± 6.2‰, n = 2790 vs +2.3 ± 5.4‰, n = 2235). The isotope composition of NO3− was correlated with water temperature revealing enhanced N-cascading in warmer climates. Seasonal analyses revealed higher δ15N and δ18O values in wintertime, suggesting waste-related N-source signals are better preserved in the cold seasons. Isotopic assays of nitrate biogeochemical transformations are key to understanding nitrate pollution and to inform beneficial agricultural and land management strategies.

Highlights

Remediation of nitrate pollution of Earth’s rivers and aquifers is hampered by cumulative biogeochemical processes and nitrogen sources
The δ18O values of nitrate in rivers and groundwaters worldwide were indistinguishable from each other (+2.3 ± 6.2‰, n = 2790, vs +2.3 ± 5.4‰, n = 2235, p-value > 0.05, respectively) (Supplementary Tables 1 and 2)
The lower median NO3− concentrations in rivers compared to adjacent shallow aquifers can be explained by the decrease of agricultural N inputs at the industrialized countries of the Northern hemisphere due to more severe fertilization regulations, whereas many aquifers do not yet exhibit decreasing nitrate contents

Summary

Introduction

Remediation of nitrate pollution of Earth’s rivers and aquifers is hampered by cumulative biogeochemical processes and nitrogen sources. Reactive nitrogen cascades through aquatic ecosystems differentially[1], since some systems accumulate N, whereas others transform it through diverse biogeochemical N-cycling processes like nitrification, denitrification, N2-fixation, dissimilatory nitrate reduction (DNRA), ammonification, and biological assimilation at rates dependent on the environmental conditions. These complexities make it difficult to unravel what point-based NO3− concentrations in rivers or their connected aquifers embody at any point in time, apart from regulatory pollutant exceedances, and quantitative knowledge of the roles of N-cycling processes in the terrestrial aquatic environments remains deficient. Despite uncertainties in the assessment of sources of nitrate pollution using δ15N and δ18O, these stable isotopes retain fundamental source and process information to help decipher the complexity of N-biogeochemistry in aquatic environments[21,23,24]

Methods

Results

Conclusion