Decreased precipitation in the late growing season weakens an ecosystem carbon sink in a semiarid grassland

1. Net ecosystem gas exchange (NEE), a balance between gross ecosystem primary productivity (GPP) and ecosystem respiration (ER), is an important indicator of terrestrial ecosystem CO2 sink or source. Increasing frequency of droughts during different periods of the growing season may affect terrestrial ecosystem carbon balance. However, detecting how drought timing controls ecosystem carbon processes is insufficiently explored because it is a challenge to accurately monitor and forecast drought dynamics.

2. In a five-year (2015-2019) precipitation manipulation experiment in a temperate steppe in northern China, we imposed a 60% decrease in precipitation in the early (April-June, DEP) and late (July-September, DLP) growing seasons in plots under rainout shelters to simulate drought-occurrence timing. Responses of GPP, ER, and NEE to DEP and DLP were examined to determine how the timing of decreased precipitation affects CO2 fluxes.

3. Both DEP and DLP reduced GPP and ER. Decreased ...