Effects Of Elevated Atmospheric CO₂ On The Aquatic Environment In Paddy Field

As a result of carbon source increasing and carbon sink decreasing resulted from industrial revolution, the concentration of carbon dioxide (CO₂) in the atmosphere is rising rapidly. Thus it affected the structure, function and process of ecosystem. But in the many reports on that there were few reports about the paddy field and even more few reports on the aquatic system in paddy field. So, by the aid of the internationally exclusive rice-wheat rotation FACE (Free Air CO₂ Enrichment) system (started in 2004, located in Jiangdu county, Jiangsu province), this research did dynamic study on the mineral elements, organic matter and microorganisms in paddy field water in 2006 in order to perfect our knowledge about the response of ecosystems to the elevated atmospheric CO₂. The main results were as follows:1. The elevated atmospheric CO₂ had significantly decreased the pH by 2.59% on average and significantly increased the concentrations of calcium, magnesium and silicon ions (P<0.01). Compared to the ambient plot, the concentrations of calcium, magnesium and silicon ions were respectively enhanced by 32.26%,74.16%å’Œ77.88% on average in FACE plot. Under the Free Air CO₂ Enrichment, the concentration of calcium ion in paddy water was heightened greatest at rice booming periods with 33.23% enhanced on average in contrast with the ambient plot. And it was heightened slightest with 28.66% at the early stage of rice growth and with 32.94% enhanced at the latter stage. The concentration of magnesium ion was elevated greatest with 112.96% increased at the early stage of rice growth while it was elevated slightest with 62.89% increased on average at rice booming periods and at the latter stage it was elevated 69.14%. The concentration of silicon ion was also advanced greatest with 163.19% improved at the early stage of rice growth. And it was slightest advanced 31.43% at the latter stage of rice and with 64.92% improved on average at rice booming periods. The concentrations of the three ions in paddy field water had significant difference between different rice growing stages (P<0.01). 2. The elevated atmospheric CO₂ had significantly increased total organic carbon (TOC), total nitrogen (TN), dissolved organic carbon (DOC) and dissolved nitrogen (DN) (P<0.01). The TOC, TN, DOC, DN had increased by 31.2%, 25.9%, 28.3% and 25.6% on average, respectively, than those in ambient plots (P<0.01). During rice growing stages, elevated CO₂ showed similar effects on TOC and DOC. It presented the greatest effects on them in the early stage of rice with 35.4% and 35.2% enhanced respectively, the middle effects in the booming stage of rice with 32.2% and 26.9% elevated and the least effects in the latter stage with 24.2% and 25.7% increased. However, elevated CO₂ showed different effects on TN and DN. The greatest effects on TN was in the booming stage of rice with 27.9% increased and on DN was in the latter stage of rice with 29.1% increased. The middle effects on TN was in the latter stage of rice with 25.8% enhanced and on DN was in the booming stage of rice with 26.5% enhanced. And the least effects on both were all in the early stage of rice with each 19.9% and 19.4% elevated. There were significantly differences on all of them between different rice growing stages (P<0.01).3. The elevated atmospheric CO₂ had significantly increased the population sizes of the bacteria and the coliform group (P<0.01). Compared to the ambient plot, bacteria and coliform group were respectively enhanced by 45.9%,68.8% on average in FACE plot. At the early stage of rice growth, elevated atmospheric CO₂ had the least effect on the population sizes of bacteria and coliform group with 21.8% and 40.4% enhanced respectively compared to that of the ambient. At the booming stage of rice elevated atmospheric CO₂ had the mean effect on the population sizes of bacteria and coliform group with 47.7% and 70.2% enhanced on average. At the latter stage elevated atmospheric CO₂ had the greatest effect on the population sizes of bacteria and coliform group with 64.6% and 93.0% enhanced. There were significantly differences in bacteria population size and coliform group population size between different rice growing stages (P<0.01).In conclusion, the elevated atmospheric CO₂ significantly decreased pH and significantly increased mineral elements, organic matter and microorganisms in the paddy field water. The results demonstrated that the elevated atmospheric CO₂ will influence the rice production and the environment in paddy field. Besides, the elevated atmospheric CO₂ may aggravate the export of mineral elements, organic matter and microorganisms in paddy ecosystem to the surrounding ecosystems through field drainage and rainstorms at the early stage of rice. As a result, that will affect the paddy ecosystem carbon and nitrogen cycling, soil productivity, the biogeochemical cycle and interchange of these mineral elements between land ecosystem and aquatic ecosystem, the surrounding water quality and human health.