Oxidation of pyrite and reducing nitrogen fertilizer enhanced the carbon cycle by driving terrestrial chemical weathering

Sen Xu; Siliang Li; Jing Su; Fujun Yue; Jun Zhong; Shuai Chen

doi:10.1016/j.scitotenv.2020.144343

Oxidation of pyrite and reducing nitrogen fertilizer enhanced the carbon cycle by driving terrestrial chemical weathering

Sci Total Environ. 2021 May 10:768:144343. doi: 10.1016/j.scitotenv.2020.144343. Epub 2020 Dec 30.

Authors

Sen Xu¹, Siliang Li², Jing Su¹, Fujun Yue³, Jun Zhong¹, Shuai Chen¹

Affiliations

¹ Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China.
² Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China; State Key laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin 300072, China; Tianjin Key Laboratory of Earth Critical Zone Science and Sustainable Development in Bohai Rim, Tianjin University, Tianjin, 300072, China. Electronic address: siliang.li@tju.edu.cn.
³ Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China; Tianjin Key Laboratory of Earth Critical Zone Science and Sustainable Development in Bohai Rim, Tianjin University, Tianjin, 300072, China.

PMID: 33736336
DOI: 10.1016/j.scitotenv.2020.144343

Abstract

Sulfuric acid formed by pyrite oxidation and nitric acid formed by oxidation of reducing nitrogen fertilizer through neutralization with carbonate minerals can rapidly perturb the carbon cycle. However, these processes and corresponding mechanisms have not been well documented due to the lack of information about both the sources of acids and the processes of oxidative weathering. Here, multiple isotopes (¹³C-DIC, ³⁴S and ¹⁸O-SO₄^2-, ¹⁵N and ¹⁸O-NO₃^-, and ¹⁸O and D-H₂O), hydrochemistry and historical monitoring data were used to assess the roles of strong acids in chemical weathering and the carbon cycle in a karst river system. The variations in alkalinity and the δ¹³C-DIC signals, along with theoretical mixing models, indicated that strong acids were involved in carbonate weathering. However, the contribution of weathering driven by strong acids to the total weathering budget determined by mixing models was lower than that determined by assuming that all protons were neutralized by minerals. These protons were liberated from oxidation of pyrite and reducing nitrogen fertilizers constrained by isotope techniques and hydrochemistry with the use of a Bayesian isotope mixing model. The strong acid weathering could account for 66% of total weathering if all of the protons were neutralized by carbonate and silicate, which was not consistent with the result provided by mixing models. These results indicated that in addition to being neutralized by minerals, the protons might be largely neutralized by HCO₃^- derived from rock weathering driven by both carbonic and strong acids. The coupling cycles of carbon, nitrogen and sulfur would be boosted due to oxidation of pyrite and reducing nitrogen fertilizers. This study suggests that the CO₂ uptake by terrestrial chemical weathering should be re-evaluated after adequately considering the effects of strong acids liberated by natural processes and anthropogenic activities.

Keywords: Carbon cycle; Carbonate weathering; Coupling cycle; Nitrogen fertilizer; Pyrite oxidation.