Search

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 in droughts during different periods of the growing season may affect terrestrial ecosystem carbon (C) balance. However, detecting how drought timing controls ecosystem C processes is insufficiently explored because it is a challenge to accurately monitor and forecast drought dynamics. In a 5-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. The responses of GPP, ER and NEE to DEP and DLP were examined to determine how the timing of decreased precipitation affects CO2 fluxes. Both DEP and DLP reduced GPP and ER. Decreased precipitation in the late growing season reduced NEE due to a greater decline in GPP than ER. In contrast, the lack of an effect of DEP on NEE can be attributed to a proportional decline in GPP and ER. Reduced normalized difference vegetation index (NDVI) and GPP induced by decreased precipitation explained the decline in GPP and ER respectively. Drought in the late growing season weakened the ecosystem C sink. The result indicates a low resistance in net C uptake to DLP. However, we did not find evidence that previous precipitation decreases resulted in a lower overall rate of ecosystem C exchange, indicating a high resilience in C processes in this semi-arid grassland. Synthesis and applications. Our study provides solid evidence that drought in the late growing season weakens ecosystem C sink in the semi-arid grassland. The findings suggest that management practices aimed at enhancing water availability during the late growing season should be preferred for increasing C sequestration.

Climate models predict significant changes in precipitation magnitude and timing in semi-arid grasslands, where soil carbon release is particularly sensitive to changing precipitation. Using data from a 4-year (2015-2018) field manipulation experiment, we explored effects of changed precipitation in different periods on soil respiration (SR) and heterotrophic respiration (HR) in a semi-arid grassland in northern China. The results showed that: (1) Decreased precipitation both in the early (DEP), late (DLP), and entire growing season (DP) reduced SR, whereas DP and DLP rather than DEP reduced HR. The declines of SR in DLP and DP are larger than in DEP; (2) Increased precipitation in the early (IEP), late (ILP), and entire (IP) growing season promoted SR, but had little effect on HR. Enhancement of SR in IEP is larger than in ILP; (3) The change of SR and HR can be attributed to varied plant community cover which is related to soil water content; and (4) SR and HR are more sensitive to decreased than to increased precipitation, to DLP than DEP, and IEP than ILP. The findings suggest an asymmetric response of soil carbon process to precipitation in different periods, highlighting that future study should not neglect the role of precipitation timing in regulating ecosystem carbon fluxes.