DOI: 10.5061/dryad.gqnk98smv
title: Decreased precipitation in the late growing season weakens an ecosystem carbon sink in a semiarid grassland

description: 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 precipitation in the late growing season reduced net ecosystem
 gas exchange (NEE) due to a greater decline of GPP than ER. In contrast,
 the lack of an effect of DEP on NEE can be attributed to a proportional
 decline of GPP and ER. Reduced normalized difference vegetation index
 (NDVI) and GPP induced by decreased precipitation explained the decline of
 GPP and ER, respectively. Drought in the late growing season weakened the
 ecosystem carbon sink. The result indicates a low resistance in net carbon
 uptake to decreased precipitation in the late growing season. However, we
 did not find evidence that previous precipitation decreases resulted in a
 lower overall rate of ecosystem carbon exchange, indicating a high
 resilience in carbon processes in this semi-arid grassland. 4. Synthesis
 and applications. Our study provides the rare evidence for the important
 role of drought timing in regulating ecosystem carbon balance. The
 findings are significant for management practices aimed at assessing
 source-sink functional conversion in the grassland ecosystems under
 climate change scenarios.
description: MEASUREMENTS OF ECOSYSTEM C
 FLUX Gross ecosystem primary productivity
 and ER were measured with an LI-6400 Portable Photosynthesis System
 (Li-Cor, Lincoln, NE, USA) attached to a transparent chamber (0.5 × 0.5 ×
 0.5 m<sup>3</sup>). This was always conducted on a sunny
 morning (8:00 -12:00 pm) three times a month over the growing seasons. For
 the measurement of NEE and evapotranspiration (ET), the transparent
 chamber was placed on a preset iron frame (0.5 × 0.5
 m<sup>2</sup>), which had been inserted into the soil at the
 depth of 3 cm, and provided a flat surface between the soil surface and
 the sampling chamber (Niu et al. 2012). There were two fans inside the
 chambers running continuously to mix the air during measurements. GPP
 measurement were taken at 10-sec intervals during the 90-sec after
 steady-state conditions were achieved within the chamber after 2 minutes.
 After measurement of GPP, the box was lifted and ventilated for 30
 seconds, the original iron frame was re-covered, and the box was covered
 with an opaque cloth to block plant photosynthesis for ER measurement (Niu
 et al. 2012). Net Ecosystem Exchange was estimated as the difference
 between ER and GPP.
description: Each block had seven treatments, including 1) control (C), 2) 60%
 decrease in precipitation in the early growing season (April-June, DEP),
 3) 60% decrease in precipitation in the late growing season
 (July-September, DLP), 4) 60% decrease in precipitation during the entire
 growing season (April-September, DP), 5) 60% increase in precipitation in
 the early growing season (April-June, IEP), 6) 60% increase in
 precipitation in the late growing season (July-September, ILP), and 7) 60%
 increase in precipitation during the entire growing season
 (April-September, IP). Each treatment had five replicates, resulting in a
 total of 35 plots.