Co-metabolism Of Organic Carbon In Sediments Of Typical Eutrophication Lakes And Its Influencing Mechanism

The accumulation of organic carbon from two or more sources may result in co-metabolism effect.It has been intensively studied in terrestrial ecosystems such as soil and forest,and makes an important theory contribution to the improvement of global carbon cycle.The sedimentary carbon pool is the main site of organic carbon sequestration,migration and transformation,and plays an important role in the global carbon balance.Under the dual effects of climate warming and eutrophication,algal organic carbon accumulation can stimulate the co-metabolism effect of sediments,which changes the relationship between source and sink of sedimentary carbon pool.Therefore,exploring the intensity and mechanism of co-metabolism effect in lakes will be benefit for better understanding the dynamics of sedimentary carbon pool,which is of great significance to further understand the carbon cycle process of lake.In this study,Taihu Lake was selected as the research area.The spatio-temporal variation of the intensity and scale of co-metabolism effect and its influencing mechanism on sedimentary carbon pool were analyzed by using stable carbon isotopes,fatty acids monomolecular biomarkers and decomposition bags.In addition,the influence of climate warming,limiting nutrient elements and stoichiometric ratio on co-metabolism effect was studied.Combined with dynamics of microbial community,the relationship among environment factor,end-members,microbial community,co-metabolism effect and carbon pool was built in order to reveal spatio-temporal variation pattern and the regulation mechanism of co-metabolism effect in lake sediments.The main conclusions are as follows:(1)The concept of co-metabolism effect in eutrophic lake ecosystems was proposed,that is,the labile organic carbon(mainly algal source)will promote or inhibit the biodegradation of refractory organic carbon(aquatic macrophyte organic detritus and sedimentary organic matter,etc.).The intensity of co-metabolism effect in eutrophic lakes were characterized by"two-component kinetic model"and"isotopic endmember mixed model".The environmental effects caused by co-metabolism effect in eutrophic lakes were preliminarily revealed,which mainly manifested the formation of anaerobic water environment,the release of nitrogen and phosphorus nutrients,the emission of odor substances and greenhouse gases,etc.(2)The co-metabolism of alga-aquatic plant residues and sediment-alga-aquatic plant residues accelerated the decomposition rate of aquatic plant residues in the algal aggregation area of Taihu Lake.The decomposition rate of treatment with Phragmites australis,treatment with Potamogeton malaianus and treatment with Nymphoides peltatum in water body of algal aggregation area increased by42.52%,16.02%and71.38%than that in control area,respectively.The decomposition rate of treatment with Phragmites australis,treatment with Potamogeton malaianus and treatment with Nymphoides peltatum in sediment of algal aggregation area increased by 78.79%,51.58%and 88.88%than that in control area.When two or three plant residues were mixed and then placed in the algal aggregation area,the decomposition rate was also higher than that in control zone.The mean co-metabolism intensity of mixing plant residues treatment with Potamogeton malaianus and Phragmites australis,Nymphoides peltatum and Phragmites australis,Nymphoides peltatum,Potamogeton malaianus and mixture of 3 plant residues were 59.01%,41.59%,130.09%and 38.14%,respectively.It is confirmed that the mixed of algae detritus and aquatic macrophyte residues caused co-metabolic in eutrophic lakes.(3)There was a significant difference in co-metabolism intensity of each treatment.The co-metabolism intensity of aquatic macrophyte residues in sediments was significantly higher than that in water.Except for the treatment with Phragmites australis and Potamogeton malaianus in sediments,the co-metabolism intensity of treatment with Phragmites australis,treatment with Potamogeton malaianus,treatment with Nymphoides peltatum,treatment with Phragmites australis and Nymphoides peltatum,treatment with Potamogeton malaianus and Nymphoides peltatum and treatment with mixture of 3 plant residues in sediment increased by 85.3%,221.97%,24.52%,30.67%,529.79%,237.18%than that in water.In addition,the composition of plant residues also affected the co-metabolism intensity.The higher initial lipid and crude protein content,the higher the co-metabolism intensity in each treatment,while the higher cellulose content,the weaker the co-metabolism intensity.(4)The co-metabolism intensity of sediment organic matte was changed by the addition of nitrogen and phosphorus.In low carbon system(C=0.23mg/g DW),the co-metabolism intensity of sediment organic matter was increased 33.67%and 21.64%by adding different nutrient concentrations(N=33?g/g DW,P=3.3?g/g DW and N=167?g/g DW,P=3.3?g/g DW).However,under high carbon system(C=0.93mg/g DW),the co-metabolism intensity of sediment organic matter was increased 87.54%and 169.20%when the same addition amount of nitrogen and phosphorus.It is suggested that algal source(high carbon)eutrophication lakes(high nitrogen and phosphorus),the co-metabolism intensity of algal-induced sediment organic matter is higher.(5)Warming increased the co-metabolism intensity of sediments organic matter.At the beginning of the experiment(0-9d),the co-metabolism intensity increased rapidly at both 25?and 35?compared with 15?.However,at the end of the experiment(35d),the cumulative co-metabolism intensity at 25?was the highest,which was 33.75%and 153.74%higher than that at 15?and 35?,respectively.Overtemperature may increased the intrinsic activation energy of sedimentary organic matter and promoted the sediment decomposition rate,thus reduce the decomposition of organic matter in sediments result of the co-metabolism effect.(6)The co-metabolism intensity of sediment organic matter was intensified by carbon sources multiple addition.The co-metabolism intensity were 325.95?g CO₂·g^-1DW,467.18?g CO₂·g^-1DW and 527.65?g CO₂·g^-1DW in single addition treatment(add all carbon sources at once),interval addition treatments(add carbon sources every6 days and add in 6 times and add carbon sources every 3 days and add in 12 times)respectively.Considering that the outbreak frequency and range of algae may further increase in eutrophic lakes,the co-metabolism effect triggered by the continuous input of algae organic detritus cannot be ignored,which weakens the function of carbon sink of lake sediments.