A new open-path methane analyzer (LI-7700) was adopted to measure methane (CH4) fluxes using eddy covariance method over irrigated rice fields in Yancheng, Jiangsu Province, China, throughout the 2016 growing season. A clear seasonal variation in the daily CH4 flux was observed. The CH4 flux started to increase 3 days after the fields were flooded. The peak CH4 flux was 0.37 gC m−2 d−1 and was reached during late vegetative stage (August 2). A distinct single-peak diurnal pattern of the CH4 flux was observed during the vegetative stages. The CH4 flux started to increase after sunrise and reached the peak at approximately 14:30 in the afternoon. Similar results were not observed during the reproductive and ripening stages. The diurnal patterns of soil temperature (Tsoil) and gross ecosystem photosynthesis (GEP) were consistent with that of the CH4 flux. The partial F tests showed that soil temperature and volumetric water content (VWC) were the most important factors controlling CH4 emissions from rice fields on seasonal timescale. The friction velocity (u*) was also found related to the CH4 emissions. Good agreements between the measured and modeled CH4 fluxes were obtained (R2 = 0.82, 0.86 and 0.86) using the models with different factors over the whole season. Including ambient pressure (P) and GEP in the model did not significantly improve the performance of the model. The best agreement between the measured and modeled CH4 fluxes was achieved by running the regression separately for each growth stage (R2 = 0.90). After the daily CH4 series was gap-filled, the total amount of CH4 emitted over the whole season was 19.20 ± 3.20 gC m−2.
To investigate the defensive strategies of clam Cyclina sinensis in response to environmental ammonia exposure, we investigate the 96 h median lethal concentration (LC50-96 h) and the 96 h safe concentration (SC) of total ammonia nitrogen (TAN) for C. sinensis, and on the basis we examined glutamine synthetase (GS) activity, glutamine content, urea content and the antioxidant enzyme activities of super oxide dismutase (SOD) and catalase (CAT) in 96 h at three different levels of TAN as 0 (control), 73.94 (T1) and 227.04 mg/L (T2). Results showed that LC50-96 h and SC for C. sinensis were 65.79 and 6.58 mg/L, respectively. The LC50-96 h and SC of NH3 were 1.70 and 0.17 mg/L, respectively. Ammonia exposure had significantly effects on SOD and CAT activities in the hepatopancreas tissue. Both the level of SOD activity and CAT activity increased with increasing concentration of TAN. No significant differences between T1 and T2 were found in GS activity from 3 h to 96 h after exposed to ammonia, whereas they were significantly higher than those in the control. Both the level of glutamine content in T1 and T2 increased significantly from 6 h to 24 h after exposed to ammonia and they were significantly higher than those in the control. There were no significantly differences were found in the level of urea concentration between T1 and T2 from 6 h to 96 h, while they were significantly higher those in the control. In conclusion, enhancing hepatopancreas antioxidant responses as well as converting ammonia into glutamine and urea worked in combination to allow C. sinensi to defend against acute ammonia exposure.
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