| Carbon transform and sequestration inputted from crop root system are the main origins of soil organic matter,however,the contribution for that could still not be quantitatively estimated now.Root biomarkers,a kind of alkane fatty acids which contained specificity in crop/species,might be used to indicate soil carbon input through plant for they usually have different length chain and structure of carbon molecules.In this paper,a qualitative and quantitative analysis method of crop root biomarkers was exploited.The bulk and rhizosphere soil sampled from typical rice and rape rotation system in the middle and lower reaches of Yangtze River,were employed to investigate root biomarkers.Content of root biomarkers in different crop species and growing stages as well as distribution of which in different particle-size fraction of soil was determined in order to analyze the contribution rate of soil organic carbon from crop root system,as well as discuss renewal of soil organic matter.The main results were as following:1.Qualitative and quantitative analysis was exploited to separate root biomarkers(a,ω-DCAs)of rice and rape.Root biomarkers of rice which included Hexadecanedioic acid(C16:0 DA),Octadecenedioic acid(C18:1 DA)and Octadecanedioic acid(C18:0 DA)were obitaned through GC-MS analysis,they could be as the biomarkers of rice root.Besides the three kinds of a,ω-DCAs molecules above,low level of Eicosanedioic acid(C20:0 DA)and Docosanedioic acid(C22:0 DA)was also detected in rape root and soil samples.The C20 and C22 could be considered as rape specific root biomarkers.The contents ofα,ω-DCAs were more than 10-10g,and also sensitivity and detection limit of GC corresponded to the quantitative demand of the object(α,ω-DCAs).The recovery(>80%)and coefficient of variation(CV<20%)of experimental method could be accepted,which could be used to treat and analyze rice and raperoot biomarkers.2.A pot experiment with rice growth was conducted.Sampled across the growth stages,both rhizosphere and bulk soil were anylyzed for organic carbon and root biomarker(hydrolysableα,ω-DCAs)content.Root biomarker contents were always much higher in rhizosphere samples than in bulk samples,indicating organic carbon input via plant photosynthesis and root-deposition,which was preserved and stabilized by soil.There was a weak correlation of root biomarker(C16:0 DA)contents with root biomass(y=1.71x-3.34,p=0.072).Before rice cultivation,content of a,ω-DCAs was 4.56±0.59 mg/kg in bulk soil,and then increased 60.5%at the tillering stage and decreased 35.9%to the filling stage in the rhizosphere.Comparing to hybrid rice,super rice showed greater decrease in root biomass and root biomarker contents from tillering to filling stage,the decrease was 33.1%and 41.3%,respectively.3.A field experiment was conducted on variation of root biomarker contents,soil organic carbon(SOC),total nitrogen(TN)with cultivars and growing stages.Contents both of major nutrients and SOC were higher under super rice than under hybrid rice.Before rice transplantation,content of a,ω-DCAs in super rice plot decreased a little compared with that in hybrid rice plot.They were 9.57±1.60 mg/kg and 8.36±1.36 mg/kg,respectively.Till the tillering stage,content of a,ω-DCAs in bulk soil under hybrid rice and super rice decreased 4.1%and 9.9%;while which in the rhizosphere was 6.46±1.17 mg/kg and 8.64±1.47 mg/kg,respectively.During tillering to filling stage,the biomarkers contents in rhizosphere decreased more sharply under super rice than under hybrid rice about 3.26mg/kg.Comparing to hybrid rice-rape rotation system,the original SOC in super rice-rape rotation system had decreased before super rice was cultivated.However,at the harvest of corresponding rape,SOC in super rice-rape rotation system increased,which corresponded to the variation of root biomarker contents.4.A comparative analysis was conducted on content and composition of rice and rape derived root biomarkers.Root biomarker contents both of rice and rape were higher in the vegetative growing stages than in harvest stage,they were 217.97±40.38 mg/kg and126.15±36.35 mg/kg in rice root,820.96±173.82 mg/kg and 189.84±21.94 mg/kg in rape root.Rape-derived root biomarker contents were higher than rice-derived in the soil.The mean residence time(MRT)of rape root in soil was longer than rice root,the ratio of that was 76.17～79.65/100(rice/rape).Comparing to hybrid rice-rape rotation system,root biomarkers in the soil of super rice-rape rotation system originated more from rice,they were 23.15～21.89%and 29.26～27.72%from 5-year to 8-year continuous cultivation of super rice,which meant contribution of super rice root to the SOC was higher than hybrid rice.5.Through analyzing different particle-size fractions in the soil under eight-year continuous cultivation of super rice,distribution of SOC,TN and root biomarkers,to value effect of super rice cultivation to soil carbon capacity compared with three years before.After five-year continuous super rice cultivation,the macro-aggregates(2-0.2 mm)in bulk soil increased significantly then mean weight diameter(MWD)of fractions also increased significantly;SOC in 2-0.2 mm and<0.002 mm decreased significantly without variation coupled by TN;root biomarkers mainly decreased significantly from 9.7±2.8mg/kg to 4.7±0.5 mg/kg in 0.02-0.002 mm fraction.After eight-year continuous cultivation of super rice,the micro-aggregates(0.2-0.02 mm)increased significantly in the rhizosphere,SOC and TN contents in<0.002 mm increased significantly;meanwhile root biomarker contents mainly increased from 6.1±1.0 mg/kg to 7.8±1.0 mg/kg in 0.02-0.002 mm.As a conclusion,comparing with hybrid rice cultivation,more residual root carbon was found in the soil under super rice and inputted into relative stable finer size-particle fractions.Conclusion was drawn as following:applicable analysis method of root biomarkers in soil and crop roots was exploited.Results showed that root biomarkers could indicate derivation of SOC from different crops and molecular progress of carbon distribution.Meanwhile root biomarkers could also react sensitively to the variation of SOC with crop cultivars in short time,i.e.crop growing,especially distribution of aggregates under different cultivars.Therefore,relationship of root biomarkers with SOC renews and new,while old carbon need to be researched furthermore. |
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