Discussion on the Ecological Effects of Carbon Source/Sink Conversion in Wetlands

Houye XI Rende SHI Yuanyuan LI Xue YU

Abstract This study explored the ecological effects of the transformation of carbon source and carbon sink in wetlands， that were discussed from four aspects： atmosphere， ocean， plants and soil. The results showed that the wetland changed from carbon sink to carbon source， which led to the increase of CO2 emissions in the atmosphere and the intensification of greenhouse effect， which made the earth face the threat of global warming. At the same time， the content of CO2 in seawater is increased; the pH value of seawater decreases; the balance of seawater acidbase is destroyed; and the ocean acidification is caused.

Key words Wetlands; Carbon source; Carbon sink; Ecological effect

The fourth comprehensive assessment report of the IPCC points out that the global CO2 concentration has increased from 280 mg/L before the industrial revolution to 379 mg/L in 2005. In the next 50-100 years， the CO2 concentration on the earths surface will increase by about one time， and the average temperature may rise by 1.1-6.4. The consequent global warming will lead to a series of serious ecological disasters， which will have a tremendous impact on the structure and function of wetland ecosystems， and the large amount of carbon stored in wetlands will also have a certain degree of feedback on climate change[1]. With the deepening of global change research， it is increasingly recognized that this feedback process will not only cause tremendous changes in the atmosphere， but also directly affect the regional soil， ocean， plant and biogeochemical cycle process， thus changing the structure and composition of the entire ecosystem. Therefore， understanding the process of carbon sourcesink conversion and its ecological effects in wetlands is of great significance to exploring regional sustainable ecosystem management and maintaining ecological security.

Atmospheric Impacts of Carbon Source/Sink Conversion in Wetlands

The transformation of wetlands from "carbon sinks" to "carbon sources" has great impacts on the ecosystem. In terms of the atmosphere， this will result in the increasing emissions of CO2 and other greenhouse gases， the rising of the global temperature and the intensification of the "greenhouse effect"[2]. The transformation of wetland sources and sinks has an important impact on human living environment， and has become a research hotspot of governments and scientific circles all over the world. Research shows that natural wetlands are disturbed by manmade or natural factors， which will lead to the reduction of wetland area and degradation of wetland function and then lead to the increase of CO2 emissions of wetlands， endanger the carbon sink function of wetlands， and make them become the source of atmospheric CO2. As the largest greenhouse gas in the world， CO2 contributes about 55%-60%. Its concentration in the atmosphere will inevitably lead to the change of global temperature. A doubling of atmospheric CO2 concentration will result in an average increase of 1.5-4.5 in the earths atmosphere temperature.

Wetlands also have significant impacts on the atmosphere when they change from "carbon source" to "carbon sink"， which can be proved by the related studies on the ecological service function of healthy wetlands to the atmosphere. It is pointed out that wetlands change from "carbon source" to "carbon sink"， the concentration of greenhouse gases in the atmosphere decreases， and the ability of wetlands to regulate climate is enhanced. It is pointed out that wetlands change from "carbon source" to "carbon sink"， the concentration of greenhouse gases in the atmosphere decreases， and the ability of wetlands to regulate climate is enhanced.

Oceanic Impacts of Carbon Source/sink Conversion in Wetlands

In regard to the ocean， this will lead to the increase of the CO2 concentration， the rising of the sea level and the acidification of the ocean[3]. On the one hand， the wetland is changed from "carbon sink" to "carbon source". The increase of emissions of CO2 and the global warming caused by the greenhouse effect will inevitably lead to the thermal expansion of the ocean and the melting of glaciers and polar ice and snow， which will cause sea level rise. According to the records of 21 most representative tidal stations in the world， the average global sea level rise in the past 100 years has been 0.18 cm/a. The assessment report issued by the IPCC in 1995 pointed out that if effective measures were not taken to control greenhouse gas emissions， global sea level rise would be further accelerated， with a total rise of 18 cm by 2050.

On the other hand， when the wetland is changed from "carbon sink" to "carbon source"， the emission of CO2 increases， and the atmospheric CO2 volume fraction continues to rise. In that way， excessive CO2 will dissolve into the seawater， thereby destroying the original acidbase balance of the seawater. Balanced with carbonate dissolution， the amount of CO2 absorbed by the ocean increases continuously， and the pH of seawater decreases， causing ocean acidification. According to IPCC prediction， by 2100， the average value of sea water pH will decrease by about 0.3-0.4 to 7.9 or 7.8， and the acidity of sea water will be about 100%-150% higher than that at the beginning of the Industrial Revolution. If we continue to emit CO2 at the current rate， it will decrease by 0.7-0.8 units by 2300.

Plants Impacts of Carbon Source/Sink Conversion in Wetlands

As for the plants， this will cause the increased release of greenhouse gases and the rising of the atmospheric CO2 concentration. The wetland has a "fertilization effect" on plants， which can affect the plant photosynthesis and the C/N ratio of plant tissues[6]. The effect on photosynthesis of most C3 plants is significant， but the effect on C4 plants is not. Increased CO2 concentration promoted the development of chloroplast thylakoid membranes in C3 plants， while negative effects were observed in C4 plants. A large number of studies have shown that elevated CO2 concentration can improve photosynthetic rate， and C3 crops such as soybean， sweet potato， peanut and rice have a greater increase than C4 crops such as cotton， maize， sorghum and millet.

Otherwise， this leads to an increase of CO2 and an increase in photosynthesis in plants. On the other hand， leads to an increase of CO2 and the C/N ratio in plant tissues. When CO2 concentration is doubled， the C/N ratio in litter will be increased by 20%-40%， or even doubled.

Edaphic Impacts of Carbon Source/Sink Conversion in Wetlands

In terms of the soil， this can lead to the increased concentrations of the gas carbon dioxide and affect the soil respiration[7]， which shows three different trends of increasing and decreasing without obvious changes. Soil respiration mainly includes three biological processes， namely root respiration， soil microbial respiration and soil animal respiration， as well as a nonbiological process. Among them， the amount of CO2 produced by soil animal respiration and nonbiological processes in soil is only a small proportion， which is often neglected in actual measurement. Usually， what we call soil respiration mainly refers to root respiration and microbial respiration. Generally， CO2 concentration increases and soil respiration increases. However， it was found that the effects of elevated CO2 concentration on soil respiration varied with crop growth stages. Increasing atmospheric CO2 concentration had little effect on root respiration rate at the early stage of crop growth， but increased significantly at the peak stage of crop growth， and decreased at the later stage of crop growth.

Conclusion and Prospects

The wetland is not only the source of CO2， but also the sink of this gas， which can be transformed into each other under certain conditions. The transformation of carbon source and carbon sink of wetlands has great influences on the ecosystem. Therefore， under the influence of the human activities of exploitation and utilization as well as the climate changes， how to maintain the existing CO2 absorption capacity of wetlands and improve the CO2 absorption capacity of degraded wetlands has become a scientific problem which urgently needs to be solved.

References

[1]ZHANG L， GUO ZH， LI ZY. Carbon storage and carbon sink of mangrove wetland： Research progress[J]. Chinese Journal of Appliced Ecology， 2013， 24（4）： 1153-1159.

[2]LIU CY， ZHOU WB. Progress of research on carbon cycle of wetlands in China[J]. Chinese Journal of Soil Science， 2012， 43（5）： 1264-1270.

[3]MA RY， TIAN SZ. The rise or fall trends of several sea surface elements in the pacific ocean and their impacts on sea level rise[J]. Tropic Oceanology， 1995（4）： 1-8.

[4]WANG SR， YIN KD， CAI WJ， et al. Advances in studies of ecological effects of ocean acidification[J]. Acta Ecologica Sinica， 2012， 32（18）： 5859-5869.

[5]ZHENG LP. Another source of grenhouse gas CO2： The earths interior[J]. Research of environmental sciences， 1998， 11（2）： 22-24.

[6]COTRUFO MF， INESON P， SCOTT A. Elevated CO2 reduces the nitrogen concentration of plant tissues[J]. Global Change Biology， 2010， 4（1）： 43-54.

[7]LUO Y. Response of soil microorganism to elevated atmospheric CO2 concentration[J]. Ecology and Environment， 2003， 12（3）： 108-111.