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

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-SO42-, 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.