Experimental drought increased the forest’s belowground sink strength towards temporarily increased topsoil carbon stocks

Reduced carbon assimilation by plants and increased net ecosystem exchange are often considered to reduce the overall carbon sink function of drought-stressed ecosystems. However, plants and soil may respond differently under drought, leading to imprecise predictions of carbon sequestration in soil. We determined the net carbon assimilation and related it to soil organic carbon (SOC) stocks as well as to root exudate production to measure belowground carbon investment in mature trees (F. sylvatica and P. abies) exposed to experimental drought for five growing seasons. Despite more than 50 % reduction in net carbon assimilation under drought, SOC stocks increased on average by more than 30 %. The proportion of carbon allocated as root exudates increased two- to threefold under drought. Increasing amounts of carbon in organo-mineral associations suggest increased carbon stability under water-limited P. abies but not under F. sylvatica. Our data indicate that the belowground sink strength increased rapidly for the ecological and economic most relevant tree species in Europe. However, evaluating the ecosystem´s carbon sink strength by using the net ecosystem exchange alone neglects belowground SOC accumulation under drought. Although belowground-invested carbon could contribute to reducing the soil carbon-climate feedback temporarily and may support ecosystem resilience, SOC accumulated primarily in dry mineral topsoil may be vulnerable upon exposure to rewetting events.