Plant life form determines net ecosystem CO2 exchange in a salt marsh under precipitation changes

Abstract

Plant life form (annuals and perennials) largely determines community response to the environment. Due to variations in photosynthesis-respiration rates, changes in life form can affect ecosystem respiration (ER) and gross primary productivity (GPP), thus altering net ecosystem CO₂ exchange (NEE). Precipitation changes can alter soil moisture-salinity conditions in salt marshes, thus causing significant changes in life form and CO₂ sink functions. However, the directional trend and magnitude of such changes along the precipitation gradient are unclear. To fill this knowledge gap, we conducted a field manipulation experiment in a salt marsh in the Yellow River Delta, China since 2014. After six years of simulated precipitation changes (-60 %, -40 %, +0 %, +40 % and +60 % of ambient precipitation), we implemented field monitoring of plant life form, biomass, NEE, ER, GPP and soil moisture-salinity content for three years (2020–2022). Average NEE values under different treatments ranged from -4.8 ± 0.8 to -2.0 ± 0.1 μmol m^-2 s^-1. We observed that precipitation changes altered plant life form by regulating soil salinity. Soil salinity increased under the precipitation reduction treatments, and annuals with lower productivity replaced perennials as the dominant species, which reduced NEE by suppressing GPP. Conversely, increased precipitation decreases salinity and favored the growth of perennials with high productivity, thus stimulating NEE. NEE showed an increasing trend along the precipitation gradient in this experiment, and the presence of perennials was the key to maintaining high NEE levels. However, species simplification resulting from ongoing increases in precipitation, coupled with a greater aboveground to belowground biomass ratio indicating greater nutrient consumption to support plant growth, may reduce ecosystem resistance and negatively impact future CO₂ uptake. Our results highlight that exploring how precipitation changes affect NEE based on life form can simplify the study of ecosystems. These findings can serve as a scientific reference for ecological management.