The influence of drought intensity on soil respiration during and after multiple drying-rewetting cycles

Global climate change is projected to intensify soil drying-rewetting (DRW) events with extended drought, especially in arid and semiarid ecosystems. However, the extent to which the soil DRW with intensified drought can alter soil respiration (Rs) in forests is still under debate, and subsequent legacy effects on Rs are not well understood. Here, we conducted a 180-d soil incubation experiment to investigate how soil DRW with different drought intensities alter the Rs in poplar (Populus simonii) and Mongolian pine (Pinus sylvestris var. mongolica) plantations. The incubation experiment included four 30-d cycles of 1) constant moisture treatment (control), 2) DRW with 10-d drying and 20-d rewetting (DRW10-20) or 3) DRW with 20-d drying and 10-d rewetting (DRW20-10), and then an extended 60-d incubation under constant moisture. During the four DRW cycles, the direct C release with respiration of Mongolian pine soils (27 g C·m−2 in DRW10-20 and 140 g C·m−2 in DRW20-10, respectively) decreased to a much lower extent than that of poplar soils (228 g C·m−2 in DRW10-20 and 498 g C·m−2 in DRW20-10, respectively). Rs did not significantly change during the extended 60-d incubation in the DRW10-20 treatment compared to control treatment. However, the respired CO2 were increased by 68 g C·m−2 in the poplar soils and 19 g C·m−2 in the Mongolian pine soils in the DRW20-10 treatment, which approximately compensated for 14% of the decreases of total respiration during four DRW cycles. This legacy effect induced by the DRW with intensified drought was attributed to the higher amount of remaining substrates and soil microbial biomass. Our study highlights that DRW can cause both direct and legacy effects on Rs, but the effects vary with drought intensity and forest type.