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.