中国半干旱区草甸草原和典型草原碳通量日变化特征

半干旱草原碳收支对陆地生态系统碳源汇功能变化具有重要影响。本文基于通榆草甸草原站2011～2017年和毛登典型草原站2013~2017年涡动相关法观测数据，分析了生长季碳通量日变化特征，研究了碳通量日变化过程对主要环境因子的响应。结果表明:两处草原7月碳吸收活动最强，草甸草原生长季各月总初级生产力（gross primary production, GPP）、生态系统呼吸（ecosystem respiration, R_e）和净碳交换量（net ecosystem exchange, NEE）的峰值均高于典型草原。NEE的日变化以单峰型为主，但7月、8月饱和水汽压差较高时，GPP在正午前后降低，引起NEE的双峰型日变化。光合有效辐射是草甸草原NEE日变化的主导因子，而在典型草原，浅层土壤含水量（5 cm）也主导了NEE日变化。水分亏缺使草原碳交换速率显著降低，草甸草原固碳速率对水分亏缺的敏感性强于典型草原。同时，水分亏缺也改变了GPP、R_e和NEE对温度和光合有效辐射的响应关系。

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.