Different Responses of Growing Season Ecosystem CO2 Fluxes to Rain Addition in a Desert Ecosystem

Abstract

Desert ecosystem CO₂ exchange may play an important role in global carbon cycling. However, it is still not clear how the CO₂ fluxes of shrub-dominated desert ecosystems respond to precipitation changes. We performed a 10-year long-term rain addition experiment in a Nitraria tangutorum desert ecosystem in northwestern China. In the growing seasons of 2016 and 2017, with three rain addition treatments (natural precipitation +0%, +50%, and +100% of annual average precipitation), gross ecosystem photosynthesis (GEP), ecosystem respiration (ER), and net ecosystem CO₂ exchange (NEE) were measured. The GEP responded nonlinearly and the ER linearly to rain addition. The NEE presented a nonlinear response along the rain addition gradient, with a saturation threshold by rain addition between +50% and +100%. The growing season mean NEE ranged from -2.25 to -5.38 μmol CO₂ m^-2 s^-1, showing net CO₂ uptake effect, with significant enhancement (more negative) under the rain addition treatments. Although natural rainfall fluctuated greatly in the growing seasons of 2016 and 2017, reaching 134.8% and 44.0% of the historical average, the NEE values remained stable. Our findings highlight that growing season CO₂ sequestration in desert ecosystems will increase against the background of increasing precipitation levels. The different responses of GEP and ER of desert ecosystems under changing precipitation regimes should be considered in global change models.

Keywords: CO2 fluxes; CO2 sink; desert ecosystem; nonlinear response; rain addition.

Figure 1

Daily precipitation (a) and soil water content (SWC) at depths of 10, 20, and 50 cm (b) in the two growing seasons in 2016 and 2017 on the N. tangutorum-dominated nabkhas. Black arrows represent rain addition treatments. C = control, A + 50% = rain addition by 50%, and A + 100% = rain addition by 100%. SWC at depth of 20 cm in August and September of 2017 was missing due to instrument failure.

Figure 2

Changes in the CO₂ fluxes ((a), gross ecosystem photosynthesis, GEP; (b), ecosystem respiration, ER; and (c), net ecosystem CO₂ exchange, NEE) on the N. tangutorum-dominated nabkhas in two growing seasons under rain addition treatments. Error bars represent standard errors. C = control, A + 50% = rain addition by 50%, and A + 100% = rain addition by 100%. The monthly, interannual, and overall effects can be found to Table S1, Table 1 and Table 2, respectively.

Figure 3

Regression relationships between total rainfall amounts (TRAs) and mean growing season CO₂ fluxes ((a), gross ecosystem photosynthesis, GEP; (b), ecosystem respiration, ER; and (c), net ecosystem CO₂ exchange, NEE) in 2016 and 2017. For GEP and NEE, nonlinear regressions are shown, while for ER, linear regressions are shown according to smaller p values during regression. The regression equations with statistical significance were GEP in 2017 (y = −1.339 + 0.119x − 3.03 × 10⁻⁴x²), ER in 2016 (y = 0.027x − 0.448) and 2017 (y = 0.024x + 0.853), and NEE in 2017 (y = 2.769 − 0.107x + 3.94 × 10⁻⁴x²).

Figure 4

Relationships between the CO₂ fluxes ((a,d), gross ecosystem photosynthesis, GEP; (b,e), ecosystem respiration, ER; and (c,f), net ecosystem CO₂ exchange, NEE) and soil nutrient factors (STN = soil total nitrogen, SOC = soil organic carbon).