Characterization of CO2 flux in three Kobresia meadows differing in dominant species

Abstract

Aims Kobresia meadows, the dominant species of which differ in different habitats, cover a large area of alpine grassland on the QinghaiTibetan Plateau and act as potential CO2 sinks. Kobresia meadows with different dominant species may differ in carbon sink strength. We aimed to test the hypothesis and to clarify the differences in CO2 sink strength among three major Kobresia meadows on the plateau and the mechanisms underlying these differences. Methods We measured the net ecosystem exchange flux (NEE), ecosystem respiration flux (ER), aboveground biomass (AGB) and environmental variables in three Kobresia meadows, dominated by K. pygmaea, K. humilis, or K. tibetica, respectively, in Haibei, Qinghai. NEE and ER were measured by a closed-chamber method. Environmental variables, including photosynthetic photon flux density (PPFD), air and soil temperature and air and soil moisture, were monitored during the above flux measurements. Important findings The measured peak AGB increased with soil water content and was 365, 402 and 434 g dry weight m � 2 for K. pygmaea, K. humilis and K. tibetica meadow, respectively. From the maximum ecosystem photosynthetic rate in relation to PPFD measured during the growing season, we estimated gross ecosystem photosynthetic potential (GEPmax) as 22.2, 29.9 and 37.8 lmol CO2 m � 2 s � 1 for K. pygmaea, K. humilis and K. tibetica meadow, respectively. We estimated the respective gross primary production (GPP) values as 799, 1 063 and 1 158 g C m � 2 year � 1 and ER as 722, 914 and 1 011 gCm � 2 year � 1 . Average net ecosystem production (NEP) was estimated to be 76.9, 149.4 and 147.6 g C m � 2 year � 1 in K. pygmaea, K. humilis andK. tibetica meadows, respectively. The results indicate that (i) the three meadows were CO2 sinks during the study period and (ii) Kobresia meadows dominated by different species can differ considerably in carbon sink strength even under the same climatic conditions, which suggests the importance of characterizing spatial heterogeneity of carbon dynamics in the future.