The effects of 10 years of soil warming on the weathering degree of deep mineral soil profiles and the role of spatial heterogeneity

Soil warming has the potential to accelerate mineral soil weathering. Higher soil temperatures influence geochemical processes such as mineral dissolution and transformation, which can impact nutrient cycles, soil fertility and carbon storage. While most studies have focused on short-term warming effects in surface soils, the effects of soil warming on the soil weathering degree in whole soil profiles have not been analysed before and have become more relevant with climate change. This study analysed the effects of 10 years of +4°C soil warming on the weathering degree of 1 m deep mineral soil profiles of granitic origin in the Blodgett Forest in California. The analysis included how warming influenced pH, the elemental composition, oxalate- and dithionite-extracted aluminium, iron and manganese and the soil organic matter composition. Additionally, weathering indices such as the CIA, PIA were calculated. Furthermore, the role of spatial heterogeneity has been assessed by comparing replicate control soil profiles, which were initially assumed to be equivalent. The results showed that soil warming accelerated subsoil primary weathering which may alter nutrient redistribution and long-term ecosystem processes, but soil warming had no significant impact in the topsoil. Significantly higher iron, potassium, magnesium, sulphur and oxalate-extracted iron concentrations were found in the subsoil whereas the CIA showed lower values with soil warming. Significant spatial heterogeneity was found and was most prominent in the subsoil. It was also found that the soil profiles furthest apart from each other showed the most differences. Several variables including CIA, potassium, magnesium, sulphur and iron, which showed differences with soil warming in the subsoil were also the variables that exhibited high spatial variation in the subsoil. These findings highlight that soil warming can accelerate subsoil weathering and that it is essential to include subsoils in soil warming studies. The results also point out the need to consider spatial heterogeneity when assessing long-term ecosystem responses to climate change.