Reforestation drastically reduces CO2 release from sulfide oxidation and its climatic sensitivity – Insight from a paired catchment approach at the Draix-Bleone Observatory

Deforestation of steep mountain forests can result in a large increase in physical erosion rates. A growing body of work has highlighted that erosion may set the pace of oxidative weathering of sulfide minerals, which results in the production of sulfuric acid and can release CO₂ from carbonate minerals in rocks to the atmosphere. However, the role of land use change on this CO₂ release pathway has not been assessed, primarily due to the lack of measurements to isolate this driver over other potential environmental controls (e.g. temperature, hydrology). Here we study the stream water chemistry of two marl-dominated catchments, the Laval (0.86 km²) and the Brusquet (1.07 km²), in the Draix-Bléone Critical Zone Observatory, Provence, France. The Laval has very high rates of physical erosion (sediment yields of 8,700 t km^-2yr^-1 and 15,800 t km^-2yr^-1 in 2016-2017 and 2017-2018, respectively) that result from the combination of deforestation, bare rock surfaces, weak rocks and the hydroclimatic setting. In contrast, the Brusquet catchment was reforested at the end of the 19^th Century and has much lower present day sediment yields (45 t km^-2 yr^-1 and 492 t km^-2 yr^-1 in 2016-2017 and 2017-2018, respectively).

We collected samples from every storm event and during flow over two water years in each catchment. We measure the major ions and the sulfur and oxygen isotopic composition of dissolved sulfate (SO₄). In both catchments cation partitioning shows a dominance of carbonate (>70%) over silicate weathering. The stable sulfur isotopic signature suggests sulfide oxidation is the dominant source of sulfate in these catchments. Examination of dissolved ion rations (HCO₃/∑Cat⁺, SO₄//∑Cat⁺) shows that sulfuric acid governs mineral dissolution, rather than carbonic acid, accounting for 90±6% and 63±9% in the Laval and Brusquet, respectively.

In the highly erosive Laval catchment, the estimated CO₂ release from sulfide oxidation coupled to carbonate weathering was very high, at 22.1±7.1 tC km^-2 yr^-1. We also find evidence for seasonal changes in sulfate flux which suggest that the rates are moderated by changes in air temperature, with elevated sulfide oxidation rates in summer. These observations support independent measurements in the shallow weathering zone of the Laval catchment, that shows an increase in CO₂ release from sulfide oxidation with temperature. In marked contrast, the CO₂ release estimated in the reforested Brusquet catchment is 4 to 5 x lower (at 4.6±0.8 tC km^-2 yr^-1) and the fluxes are not seasonally moderated (i.e. not temperature controlled). We suggest this relates to changes in the supply of mineral surfaces to the shallow, oxygenated weathering zone. Reforestation could result in a marked decrease in the release of carbon from rock to the atmosphere in areas where sulfide and carbonate minerals outrcop, and make the resultant fluxes less sensitive to changing climate.