Assessing the effects of oxidation of pyrite and reducing nitrogen fertilizer on chemical weathering and the carbon cycles in a karst river system by using multiple isotopes

Pyrite is the most common sulfide mineral occurring in sedimentary and igneous rocks and globally contributes a greater flux of sulfate. Large quantity of reactive nitrogen as fertilizers for agricultural production has been released into the environment in China over recent decades. Sulfuric acid formed by oxidative weathering of pyrite (OWP) and nitric acid formed by oxidation of reducing nitrogen fertilizer (ONF) through neutralization with carbonate minerals can counteract CO₂ drawdown from chemical weathering. Here, we use the multiple isotopes (¹³C-DIC, ³⁴S and ¹⁸O-SO₄^2–, ¹⁵N and ¹⁸O-NO₃^–, and ¹⁸O and D-H₂O) and water chemistry, as well as historical hydrochemical data to assess the roles of strong acids in chemical weathering and the carbon cycle in a karst river system (Chishui River, southwestern China). The variations in alkalinity and the δ¹³C-DIC along with theoretical mixing models demonstrate the involvement of strong acids in carbonate weathering. However, the strong acid weathering flux determined by δ¹³C-DIC and mixing models is considered to be overestimated due to the effects of photosynthesis and degassing of CO₂ on δ¹³C-DIC signal. The protons liberated from OWP and ONF can be constrained by water chemistry and isotope techniques with the use of a Bayesian isotope mixing model. The strong acid weathering flux determined using proton information is higher that determined by δ¹³C-DIC and mixing models. This suggests that the additional protons derived from OWP and ONF might be consumed in other ways without affecting the δ¹³C-DIC signals, such as the neutralization of acidic waters. These results indicate that OWP and ONF coupled with carbonate dissolution significantly enhanced the coupling cycles of carbon, nitrogen and sulfur in this river system.