Effect of droughts on future weathering rates in Sweden

Abstract. In a future warmer climate, extremely dry, warm summers might become more common. In Scandinavia, the extreme summer of 2018 was such an event. Soil weathering is affected by temperature and precipitation, and climate change as well as droughts can therefore affect soil chemistry and plant nutrition. In this study, climate change effects on weathering were studied on seven forest sites across widely different climate zones in Sweden, using the dynamical model ForSAFE. Two climate scenarios were run, one climate change base scenario and one drought scenario. The model results show a large geographical variation of weathering rates for the sites. There is, however, no geographical gradient, despite the strong dependence of temperature on weathering, as also soil texture and mineralogy have strong effects on weathering. There is a pronounced seasonal dynamic, with much lower weathering rates during winters than during summers, and with more variable summer weathering rates depending on more variable soil moisture and temperature. According to the climate change base scenario, the weathering rates will increase by 5–17 % per degree of warming. The relative increase is largest in the two south-eastern sites, with low total weathering rates caused by relatively coarse soils and often dry summers. Changes in seasonal dynamics due to climate change differ between regions. At sites in southern Sweden, future weathering increase occurs throughout the year, though generally most in spring and summer. In the north the increase in weathering during winters is almost negligible, even though the temperature increase during winter is high, as the winter temperatures still will mostly be below zero. The drought scenario has the strongest effect in southern Sweden and here weathering can temporally become as low as winter weathering during drought summers. Soil texture also has an effect on how fast the weathering decrease during drought occurs, as well as how fast the soil rewets and resume normal weathering rates after the drought, where coarse soils respond quicker. Yearly weathering during the drought years in the most affected site is only 78 % of the weathering of the base scenario. In the north, the soils do not dry out as much despite the low precipitation, and in the northernmost site weathering is not much affected. The study shows that it is crucial to take seasonal climate variations and soil texture into account when assessing the effects of a changed climate on weathering rates and plant nutrient availability.