Diurnal Variations of the Carbon Fluxes of Semiarid Meadow Steppe and Typical Steppe in China

Temperate grasslands are important components of terrestrial ecosystems. Investigating the grassland carbon exchange processes and their impact factors is essential to assess the variations in the carbon source–sink of terrestrial ecosystems and their responses to future climate change. On the basis of the eddy covariance measurements of carbon fluxes of meadow steppe at Tongyu during 2011–2017 and typical steppe at Maodeng during 2013–2017, the diurnal variation of carbon fluxes and its responses to environmental factors were analyzed. Results showed that both grasslands had the strongest carbon uptake in July. The monthly peaks of gross primary production (GPP), ecosystem respiration (R_e), and net ecosystem exchange (NEE) of meadow steppe were greater than those of typical steppe. The diurnal variation of NEE was dominated by a unimodal pattern. However, when the saturated vapor pressure difference was high in July and August, GPP decreased around noon, leading to a bimodal pattern of NEE. Photosynthetically active radiation was the key factor in the diurnal variation of NEE of meadow steppe. Meanwhile, the diurnal variation of NEE of typical steppe was susceptible to shallow soil water content (5 cm). Water deficit led to a significant decrease in NEE at both grasslands. However, the meadow steppe carbon sequestration rate was more sensitive to water deficit than the typical steppe carbon sequestration rate. Meanwhile, water deficit modified the responses of GPP, R_e, and NEE to temperature and photosynthetically active radiation.