Seasonal precipitation distribution determines ecosystem CO₂ and H₂O exchange by regulating spring soil water-salt dynamics in a brackish wetland

The intensification of the global hydrological cycle is anticipated to increase the variability of precipitation patterns. Brackish wetlands respond to changes in precipitation patterns by regulating the absorption and release of CO2 and H2O to maintain the stability of ecosystem functions. However, there is limited understanding of how the inter-seasonal precipitation distribution affects ecosystem CO2 and H2O exchange compared to annual precipitation totals. Here, we conducted four consecutive years of field experiments in a brackish wetland, manipulating the proportion of precipitation across different seasons while maintaining a constant annual precipitation total. We utilized five inter-seasonal precipitation distribution proportions (+73%, +56%, control (CK), -56%, and -73%) to examine the effects of seasonal precipitation distribution (SPD) on ecosystem CO2 and H2O exchange. Our findings revealed that the ecosystem CO2 and H2O fluxes showed a trend of decreasing with the decrease of spring precipitation distribution. Among them, the annual net ecosystem CO2 exchange (NEE), evapotranspiration (ET), carbon use efficiency (CUE), and water use efficiency (WUE) were shown to be more sensitive to decrease in spring precipitation distribution and increase in summer and autumn precipitation distribution. This negative asymmetric response pattern suggests that annual ecosystem CO2 and H2O exchange is primarily governed by seasonal precipitation variability, with spring soil water-salt dynamics identified as the key driver. Therefore, this association can be explained by the fact that drought of the early growth stage exacerbates soil salinization and inhibits vegetation colonization and growth, thereby greatly impairing the annual CO2-H2O exchange capacity of brackish wetlands. Our results emphasized that the spring's extreme precipitation-induced soil water-salt conditions will greatly influence CO2 and H2O exchange in brackish wetlands in the future. These findings are crucial for improving predictions of the carbon sequestration and water-holding capacity of brackish wetlands.

Methods
Abiotic factors measurements
From April 2022 to March 2023, a soil three-parameter sensor (HdyraProbe Lite Soil Moisture sensor, Stevens Water, USA) was used to measure surface (0-10 cm) soil volumetric moisture content (SM), soil electrical conductivity (EC), and soil temperature (ST). Data were logged once a day throughout the experimental period using a data logger (CR1000, Campbell Science, Inc., USA).

Biotic factors measurements
In May 2019, a 1 × 1 m quadrat was selected in each plot as a permanent vegetation survey area by diagonal crossing. Vegetation surveys were conducted on a clear morning of every quarter in 2022 to record indicators such as coverage. At the end of the 2022 growing season, aboveground biomass (AGB) was obtained by the cutting method, and belowground biomass (BGB) was obtained by root drilling on the 1 × 1 m quadrate. Subsequently, AGB and BGB were placed in an oven, first baked at 105 ℃ for 1 h, then converted to 70 ℃ for 48 h, and finally weighed to obtain dry weight.

Ecosystem CO2 and H2O exchange measurements
The ecosystem CO2 and H2O fluxes were quantified using a Li-6400 portable photosynthesis system (Li-Cor, Inc., Lincoln, NE, USA) in conjunction with the closed chamber method. From April 2022 to March 2023, measurements were carried out at 9:00-11:00 AM on sunny days, twice a month, before and after precipitation. A 0.25 × 0.25 × 0.03 m3 acrylic plexiglass base was permanently installed within each plot, with one end inserted into the soil at a depth of 3 cm and the other end flush with the ground. Before measurements, a 0.25 × 0.25 × 1.2 m3 acrylic plexiglass chamber was placed on the base to ensure an airtight environment throughout the entire space. Two small fans were positioned at the top corners of the chamber to ensure complete air mixing during the measurement process and reduce errors. The rate of CO2 concentration change under light conditions was denoted by net ecosystem CO2 exchange (NEE), with the ecosystem net carbon uptake and release represented by NEE of "-" and "+", respectively. After lighting conditions were measured, the chamber gas was evenly mixed with the outside air, and the chamber was covered with a black cloth. At this time, the concentration changes rate of CO2 represented ecosystem respiration (ER). Gross primary productivity (GPP), evapotranspiration (ET), water use efficiency (WUE), and carbon use efficiency (CUE) were calculated as follows: GPP=-NEE+ER; WUE=GPP/ET; CUE=1-ER/GPP=-NEE/GPP.

In addition, their sensitivities were calculated as: |∆X|=|(XT-XC)/XC|. Among them, XT represents CO2 and H2O fluxes under ±SPD treatments, and XC represents CK. ΔX absolute value (|ΔX|) represents the sensitivity of fluxes. A positive asymmetric response is defined when |ΔX| under +SPD treatments is greater than under -SPD treatments, and a negative asymmetric response is defined when |ΔX| under +SPD treatments is less than under -SPD treatments.