Annual cycle of CO 2 exchange over a reed ( Phragmites australis) wetland in Northeast China

Abstract

Net ecosystem exchange of CO 2 (NEE) was measured during 2005 using the eddy covariance (EC) technique over a reed ( Phragmites australis (Cav.) Trin. ex Steud.) wetland in Northeast China (121°54′E, 41°08′N). Diurnal NEE patterns varied markedly among months. Outside the growing season, NEE lacked a diurnal pattern and it fluctuated above zero with an average value of 0.07 mg CO 2 m −2 s −1 resulting from soil microbial activity. During the growing season, NEE showed a distinct V-like diel course, and the mean daily NEE was −7.48 ± 2.74 g CO 2 m −2 day −1, ranging from −13.58 g CO 2 m −2 day −1 (July) to −0.10 g CO 2 m −2 day −1 (October). An annual cycle was also apparent, with CO 2 uptake increasing rapidly in May, peaking in July, and decreasing from August. Monthly cumulative NEE ranged from −115 ± 24 g C m −2 month −1 (the reed wetland was a CO 2 sink) in July to 75 ± 16 g C m −2 month −1 (CO 2 source) in November. The annual CO 2 balance suggests a net uptake of −65 ± 14 g C m −2 year −1, mainly due to the gains in June and July. Cumulative CO 2 emission during the non-growing season was 327 g C m −2, much greater than the absolute value of the annual CO 2 balance, which proves the importance of the wintertime CO 2 efflux at the study site. The ratio of ecosystem respiration ( R eco) to gross primary productivity (GPP) for this reed ecosystem was 0.95, indicating that 95% of plant assimilation was consumed by the reed plant or supported the activities of heterotrophs in the soil. Daytime NEE values during the growing season were closely related to photosynthetically active radiation (PAR) ( r 2 > 0.63, p < 0.01). Both maximum ecosystem photosynthesis rate ( A max) and apparent quantum yield ( α) were season-dependent, and reached their peak values in July (1.28 ± 0.11 mg CO 2 m −2 s −1, 0.098 ± 0.027 μmol CO 2 μmol −1 photon, respectively), corresponding to the observed maximum NEE in July. Ecosystem respiration ( R eco) relied on temperature and soil water content, and the mean value of Q 10 was about 2.4 with monthly variation ranging from 1.8 to 4.1 during 2005. Annual methane emission from this reed ecosystem was estimated to be about 3 g C m −2 year −1, and about 5% of the net carbon fixed by the reed wetland was released to the atmosphere as CH 4.