《土壤学基本知识-土壤有机质性质及其分解教学教材.ppt》由会员分享，可在线阅读，更多相关《土壤学基本知识-土壤有机质性质及其分解教学教材.ppt（44页珍藏版）》请在taowenge.com淘文阁网|工程机械CAD图纸|机械工程制图|CAD装配图下载|SolidWorks_CaTia_CAD_UG_PROE_设计图分享下载上搜索。
1、土壤学基本知识土壤学基本知识-土壤有土壤有机质性质及其分解机质性质及其分解General pattern of element cycle in ecosystemsProperties of Soil OM-1:Chemical properties of litter and soil organic matter(OM):Litter(dead leaves,twigs falling on the ground)and soil organic matter contain nutrients&be a critical link in nutrient cycles.Decompos
2、ition is the process that converts the organic forms of nutrients to inorganic forms available to plants.The nutrient content and the chemical structure of litter/OM,determine the quality of litter/OM&affect the decomposition rates.Three general characteristics determine the litter/OM quality:1)The
3、type of chemical bonds&energy released as they decay;2)The size and spatial complexity of the molecules;3)Nutrient content.1 and 2 determine the Carbon Quality of litter/OM,while 3 determines Nutrient Quality of litter/OMProperties of Soil OM-2:Decomposition of OM to inorganic molecules go through a
4、 long chain of biochemical processes to gradually degrade complex OM,and many soil organisms and microbes involved(recall detritus food web).These soil organisms and microbes break(decay)the chemical bonds in OM through the enzymes they produced(cost energy).The soil organisms and microbes decay OM
5、to request energy and nutrients for themselves.Therefore,the higher the energy,nutrient content,the simpler the molecule structure the OM is,the easier(faster)it can be decomposed.Both OM quality&environment factors affect the biochemical work of the soil organisms/microbes:decomposition is a oxidat
6、ion process requiring participation of soil organisms/microbes factors affect soil organisms/microbes and general aeration condition of soil will influence rates of decomposition.Properties of Soil OM-3:Major Biochemical Constituents of Litter and Their Decay:1)Simple sugars:mono-and disaccharides:g
7、lucose,sucrose.2)Starch:polymer of n glucoses linked by 1-4 glucoside bond.3)Cellulose:polymer of n glucoses linked by 1-4 glucoside bond,but slightly different 3-D structure from starch.4)Hemicellulose:polymer of n different simple sugars to form chain and branched structures;(n in 2 thousands,n in
8、 3&4 thousands to tens of thousands)Properties of Soil OM-4:5)Hydrocarbons(fats and waxes):partially saturated,long-chain carbon compounds in which hydroxyl(OH)groups are displaced by hydrogen ions(H+)hydrophobic.Such as cutin that is present in the cuticle,the outer waxy layer in plant leaves.Due t
9、o their sizes,and water repellent feature,they are more decay resistant than carbohydratesProperties of Soil OM-5:Major Biochemical Constituents of Litter and Their Decay:6)Polyphenols:made from diff.phenolic acids(tannins);7)Lignin:large,amorphous,very complex compounds,and makes wood woody.From si
10、mple sugars to lignin,the structures of compounds are more complex more energy required to break them;the less energy can be extracted from decaying them,the slower the decay rates are.Properties of Soil OM-6:Due to complexity of lignin in chemistry,its determination is rather proximate:8)Suberin:mo
11、lecules composed hydrocarbons and phenolics low in energy and hydrophobic9)Proteins:macromolecular compound consists of various amino acids not only high carbon quality,but also high nutrient quality due to high N contents.Properties of Soil OM-6:Different plant tissues have different composition of
12、 these carbon compounds different decomposition rats.In litter decay,simple and soluble compounds decay fast,while complex,large compound accumulate(Fahey et al.1984).Properties of Soil OM-7:Litter decay cross certain threshold and become soil organic matter,also called humus,another complex and amo
13、rphous form of OM in ecosystems.What is humus?It is a series of high molecular weight polymers with a high content of phenolic rings and quite variable side chains.It is high in N and large polyphenolic molecules,low in cellulose and hemicellulose.A large%of N is in neither protein no amino acid for
14、m,but presented as chitin(common in the exoskeleton of bugs and fungal hyphae),&heterocyclic compounds(i.e.,phenolic combined with amino groups).Properties of Soil OM-8:Two larger groups of compounds can be separated from humus:fulvic acids and humic acids-organic colloids.Properties of Soil OM-9:Fo
15、rmation of Humus:still not completely comprehended yet.Three hypotheses:Hypothesis I:formed through a slow,but continue modification of existing plant residues,particularly lignin,by microbes condensation into larger&larger molecules with proteins.Hypothesis II:microbes break all large molecules int
16、o smaller ones that then repolymerize chemically&form high molecular humic and fulvic acids.Hypothesis III:intermediate between I and II.Phenolic substances from either lignin or synthesis are converted to quinones and then polymerized.Properties of Soil OM-10:Decomposition and Stabilization of Humu
17、s:Humus decay very slowly because:1)low biochemical quality carbon compounds(lignin and lignin-like compounds);2)humus molecules and clay particles can be bound together by metal cations(chelation),water,sugars and others disrupting spatialalignment of enzymes&humus molecules,deactivate enzymes.Prop
18、erties of Soil OM-11:Minerals,water and OM have very different gravities(minerals 2.2,water 1.0 and OM 1.0.Soils combines mineral and OM with various composition.Therefore,by the gravity,soils are grouped into Heavy and Light fractions.Heavy soil fractions have more minerals and higher bulk densityL
19、ight soil fractions have more OM and low bulk densityIn general,heavy fractions have OM of more decomposed organic fragments,while light fractions,more still-recognizable litter,therefore,they have very different C/N ratios and N release rates.Decomposition of Litter and Soil OM-1:Decomposition of l
20、itter and soil OM are determined by various internal and external variables:litter quality,soil conditions,andenvironmental conditions.Decomposition rates are often measured as a percentage weight loss per unit time,and this yields:%original remaining=e-ktwhere t=time,k=litter-specific decay rate co
21、nstant assume litter decay rate is constant through time,but it is not true,because1.Water soluble compounds can be lost purely by leaching.2.Water insoluble compounds do not decompose much at same time:substances easier to decay decay first.3.Complex high molecular compounds:lignin,suberin can shie
22、ld cellulose from microbial attack.4.Byproducts of decomposition(humus-mineral complex)are very resistant to decay.Decomposition of Litter and Soil OM-2:5.Therefore,litter composition and chemical,physical properties are changing all the time.The equation is just an approximate:%original remaining=e
23、-ktDecomposition of Litter and Soil OM-3:Rate of weight loss from decomposing litter indicates amount of energy and carbon available for microbes.As the energy and C availability for microbes increase,nutrients may become limiting and microbes may uptake nutrients from soil solution.If nutrient pres
24、ented in litter in excess of microbial requirements,nutrients are released as decomposition proceeds.Decomposition of Litter and Soil OM-4:When nutrient is limiting,and microbes uptake nutrients from soil solution,total nutrient contents in litter will increase immobilizationNet increase of absolute
25、 amount of nutrients in decomposing litter is called net immobilizationAs the energy and C yields from decomposition decline,so does the microbial demand for nutrient.Nutrients released from decay increases,plus those from dead microbes excess nutrients released in mineral forms mineralizationSource
26、s of immobilized nutrients:1)above soil surface,stem flow and throughfall are the major sources;2)below ground,decaying soil OM;3)free living microbes(particularly for N fixation)usually low in most terrestrial ecosystems.N and P tend to be immobilized mostly.Decomposition of Litter and Soil OM-5:Ab
27、out Nitrogen Dynamics(from 15N isotope studies):1.N in litter is initially used by microbes2.N in lignin increase during immobilization3.Net release of N undetectable1-3 indicate:a.Immobilization occurs in the early stage of decay;b.High quality OM decay high N demand in growing microbial population
28、s; mineralization occurs only when low-quality OM remain microbial growth limited by C&E.Decomposition of Litter and Soil OM-6:Significant immobilization can occur sorely chemically between litter and soil solution.In extremely poor sites,soluble OM in litter may be important in controlling nutrient
29、 balance.It is hypothesized that:competition for little available N plants produce leaf high in tannins release N in dissolved organic form mycorrhizae can take up.A short-circuit N cycle reduce leaching loss of NDONNH4Decomposition of Litter and Soil OM-7:In many terrestrial ecosystems,a potentiall
30、y large fraction of weight loss in“decaying”litter can occur as leaching of soluble organics.The fate of this leached soluble organics is still unknown.Can we guess where these leached DOC go?Decomposition of Litter and Soil OM-8:N content in decaying OM plays diff.roles at diff.stages of decay:-Sho
31、rt-term study:high initial N content facilitate decay -Long-term study:not quite soTwo hypotheses:1.Adding N to OM randomizes the bond structure in old litter/humus make existing enzymes less effective;2.Requirement for extracting N from humus to meet fungal N demands does not exist.(experiment show
32、n that fungal shut down the production of lignin/humus degrading enzymes when large amount of N added)the system is messed up!Decomposition of Litter and Soil OM-9:Litter Decay Prediction:Litter nutrient quality determine short term decay rate,while litter carbon quality determines long term decay r
33、ate.Climatic conditions(summarized as AET)has a high correlations with litter decay.Decomposition of Litter and Soil OM-10:Litter Decay Prediction:In addition,litter type(leaf,roots,woody materials have very different chemical compositions)makes the picture more complicated.Soil animals play importa
34、nt roles in decomposition(earthworms for example),but the details are less known.Decomposition of Litter and Soil OM-10:Humus production and decomposition:Litter decay begins with a wide variety of materials of very diff.chemical quality and produces a much more homogeneous humus with lignin:cellulo
35、se ratio of 1:1.To predict production of humus,modified equation%original remaining=e-kt is used,but k is affected by initial litter N,Mg and Ca contents.Decomposition of Litter and Soil OM-10:Humus production and decomposition:Turnover rate is used to evaluate humus decomposition.Turnover rate can
36、be expressed as a changing rate per unit time or the time period a completely replacement required.Measuring methods(difficult to measure humus decay):1.mesh bag method for soil OM weight loss long time2.field soil incubation for net mineralization rate of N3.15N pool dilution analysis very high N i
37、mmobilization4.correlating soil respiration and gross N transformation5.loss of total soil OM used in broad scale measurementDecomposition of Litter and Soil OM-11:Humus production and decomposition:The methods give rather rapid turnover rates for humus,in fact,certain portion of soil OM is very ine
38、rt and slow Decaying diff.fractions of humus with diff.turnover rates.Many envir.factors affect humus decay rates as expected.Flowchart ofCENTURYTurnover rateRelative rate of respiration(CO2)or nitrogen releaseSynthesis of the information of nutrient cycling in terrestrial ecosystems:The processes d
39、ominating element cycling differ widely for different nutrient elements.However,they can be presented as part of a larger,generalized framework of nutrient cycle.As mentioned before,biogeochemical cycles of nutrient elements can be grouped into gaseous cycle and sediment cycles.The diagrams below il
40、lustrate biogeochemical cycles of six most important elements in ecosystems CNSKCaPCarbon Biogeochemical Cycle(review):Atmospheric CO2Primary productionSecondary Production(primary consumption)2ndaryconsumptionTertiaryConsumptionPhotosynthesisRespirationLitter,soil organic matterDetritusOrganismsNit
41、rogen biogeochemical Cycle:N Mineralization from decomposing materials begins with ammonification that NH4 is released by heterotrophic microbes.Soil NH4 has 5 sinks:uptake by plants;ammonia volatilization;immobilization by microbes;adsorption by soil particles;and nitrification,that NH4 is oxidized
42、 to NO3 by chemoautotrophic bacteria,Nitrobacter&Nitrosomonas,coupled with C fixation:2NH4+3O2 2NO2-+2H2O+4H+hvand 2NO2-+O2 2NO3-,CO2+4H+hv CH2O+H2O(obviously,nitrification occurs under aerobic conditions)NO3 can also be produced through oxidation of OM by hetero-trophic nitrification.Soil NO3 also
43、has 5 sinks:uptake by plants;immodilization by microbes;assimilatory reduction back to NH4;lost to runoff;and denitrification(no O2 needed):5CH2O+4H+4NO3-5CO2+2N2+7H2OExtractable NH4 and NO3 in soil at any time represent the net results of all these processes.Nitrogen biogeochemical Cycle:How are th
44、ese N transformations in soil are measured?Net N mineralization:buried bag method/tube incubation-how to do it?-advantage:direct measurement in field conditions.-disadvantages:except soil temp.,all other conditions are altered that may cause over or under estimation.Total N mineralization:15N isotop
45、e approach:microbial mineralization prefer lighter isotope-14N 15N/14N in NH4 decline comparing to the ratio in soil OM.With known initial 15N/14N in various soil N pools,N cycle in ecosystem can be easily detected.-advantage:direct measurement in field conditions-disadvantages:complex techniques,es
46、pecially when gaseous products produced.Nitrogen biogeochemical Cycle:Emission of Nitrogen Gases from soil(NH3,NO,N2O&N2):Ammonia Volatilization:NH4+OH-NH3+H2O as pH is highNO and N2O are the byproducts of nitrification&denitrificationNitrification:NH4+NH2OH HNO NO2-NO3-|Nitrosomonas|Nitrobacter|Den
47、itrification:NO3-NO2-NO N2O N2 each step is catalyzed by unique reduction enzyme.nitrification in terrestrial ecosystems,1-3%N is volatilized as NO.Factors affecting loss of N2O and N2 by denitrification are still not fully understood.Denitrification is usually measured w/acetylene reduction approac
48、hthat C2H2 is used to block the reaction at N2Oexamined with gas chromatography.Nitrogen biogeochemical Cycle:Some points on N mineralization/nitrification/denitrification:-Net N mineralization directly relate to content of organic N in soil and C OM of high C/N low mineralization rate.-Nitrificatio
49、n:high:NH4 abundant,mid range pH,high soil H2O.low:low/high pH,low O2,low soil H2O,high C/N.-Availability of other nutrients usually have little effect.-Nitrification rates are high when vegetation is disturbed because of high soil moisture,&soil temp.,rapid ammonification,low vegetation uptake,low
50、microbial immobilization(in temperate forests).In the SE pine forest ecosystems,strong microbial immobilization hold up 80%N.-Nitrification generate acidity,and loss of NO3-usually accompanied by increases of cations removed from soil particle surface by H+.-Conditions stimulate nitrification also i