土壤学基本知识-土壤有机质性质及其分解.ppt

1、,Properties of Soil Organic matter and Its Decay,General pattern of element cycle in ecosystems,Properties 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 Nutrient content. 1 and 2

2、determine the Carbon Quality of litter/OM, while 3 determines Nutrient Quality of litter/OM,Properties of Soil OM -2: Decomposition of OM to inorganic molecules go through a long chain of biochemical processes to gradually degrade complex OM, and many soil organisms and microbes involved (recall det

3、ritus 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 to request energy and nutrients for themselves. Therefore, the higher the energy, nutrient content, the simpler the mole

4、cule structure the OM is, the easier (faster) it can be decomposed. Both OM quality (n in 2 thousands, n in 3 7)Lignin: large, amorphous, very complex compounds, and makes wood woody. From simple sugars to lignin, the structures of compounds are more complex more energy required to break them; the l

5、ess 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: Suberin: molecules composed hydrocarbons and phenolics low in energy and hydrophobic Proteins: macromolecular com

6、pound 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 these carbon compounds different decomposition rats. In litter decay, simple and soluble compound

7、s 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 amorphous form of OM in ecosystems. What is humus? It is a series of high molecular weight

8、 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 form, but presented as chitin (common in the exoskeleton of bugs and fungal hyphae)

9、, 2) humus molecules and clay particles can be bound together by metal cations (chelation), water, sugars and others disrupting spatial alignment of enzymes 2) below ground, decaying soil OM; 3) free living microbes (particularly for N fixation) usually low in most terrestrial ecosystems. N and P te

10、nd to be immobilized mostly.,Decomposition of Litter and Soil OM-5: About Nitrogen Dynamics (from 15N isotope studies): N in litter is initially used by microbes N in lignin increase during immobilization Net release of N undetectable 1-3 indicate: a. Immobilization occurs in the early stage of deca

11、y; b. High quality OM decay high N demand in growing microbial populations; c. net mineralization occurs only when low-quality OM remain microbial growth limited by C Requirement for extracting N from humus to meet fungal N demands does not exist. (experiment shown that fungal shut down the producti

12、on 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 rate. Climatic conditions (summa

13、rized 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 important roles in decompo

14、sition (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:cellulose ratio of 1

15、: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 b

16、e 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 time 2. field soil incubation for net mineralization rate of N 3. 15N pool dilution analysis very h

17、igh N immobilization 4. correlating soil respiration and gross N transformation 5. loss of total soil OM used in broad scale measurement,Decomposition of Litter and Soil OM-11: Humus production and decomposition: The methods give rather rapid turnover rates for humus, in fact, certain portion of soi

18、l OM is very inert and slow Decaying diff. fractions of humus with diff. turnover rates. Many envir. factors affect humus decay rates as expected.,Relative rate of respiration (CO2) or nitrogen release,Synthesis of the information of nutrient cycling in terrestrial ecosystems: The processes dominati

19、ng 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 ill

20、ustrate biogeochemical cycles of six most important elements in ecosystems,C,N,S,K,Ca,P,Carbon Biogeochemical Cycle (review):,Nitrogen biogeochemical Cycle: N Mineralization from decomposing materials begins with ammonification that NH4 is released by heterotrophic microbes. Soil NH4 has 5 sinks: up

21、take by plants; ammonia volatilization; immobilization by microbes; adsorption by soil particles; and nitrification, that NH4 is oxidized to NO3 by chemoautotrophic bacteria, Nitrobacter and denitrification (no O2 needed): 5CH2O + 4H+ + 4NO3- 5CO2 + 2N2 + 7H2O Extractable NH4 and NO3 in soil at any

22、time represent the net results of all these processes.,Nitrogen biogeochemical Cycle: How are these 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., al

23、l other conditions are altered that may cause over or under estimation. Total N mineralization: 15N isotope 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

24、 can be easily detected. -advantage: direct measurement in field conditions -disadvantages: complex techniques, especially 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 high NO

25、and N2O are the byproducts of nitrification & denitrification Nitrification: NH4+ NH2OH HNO NO2- NO3- | Nitrosomonas | Nitrobacter | Denitrification: 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 a

26、ffecting loss of N2O and N2 by denitrification are still not fully understood. Denitrification is usually measured w/ acetylene reduction approach that C2H2 is used to block the reaction at N2Oexamined with gas chromatography.,Nitrogen biogeochemical Cycle: Some points on N mineralization/nitrificat

27、ion/denitrification: -Net N mineralization directly relate to content of organic N in soil and C OM of high C/N low mineralization rate. -Nitrification: 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 li

28、ttle effect. -Nitrification rates are high when vegetation is disturbed because of high soil moisture, & soil temp., rapid ammonification, low vegetation uptake, low microbial immobilization (in temperate forests). In the SE pine forest ecosystems, strong microbial immobilization hold up 80% N. -Nit

29、rification generate acidity, and loss of NO3- usually accompanied by increases of cations removed from soil particle surface by H+. -Conditions stimulate nitrification also increases NO emission.,Nitrogen biogeochemical Cycle: Some points on N mineralization/nitrification/denitrification: -High atmo

30、spheric NO plants & soil may take up NO, but in most cases, atmospheric NO is below the compensation point:10 ppbv-N lose through NO is limited. -Denitrification = dissimilatory reduction as N is not incorporatedinto microbial tissue as NO3 is reduced. -Denitrification is widespread in terrestrial e

31、cosystems, even in well-drained soils: in soil aggregates & anoxic microsites. -Increase NO3 in high OM content soil stimulates denitrification, while increase organic carbon in mineral soil stimulates denitrification. -Soil pH, soil moisture, soil temperature affect nitrification and denitrificatio

32、n, as well as the amount of NO, N2O and N2 emission may be highly spatially heterogeneous. -Fire ,Nitrogen biogeochemical Cycle: -Animal,Conceptual illustration of Nitrogen Biogeochemical Cycle in ecosystems anything wrong?,N fixation,Biogeochemical Cycle of Sulfur Sulfur cycle is similar to nitroge

33、n in many ways, but: S cycles mainly among plants, litter and soil microbes S is usually unlimited element, retranslocation and immorbilazation are less important to its biogeochemical cycle. Rock weathering is one important source. S can be strongly fixed in soil by Fe and Al. S gaseous exchange wi

34、th atmosphere is small. Sulfate is an important source of acid rain-human made,Comparison of N and S cycles,Phosphorus Biogeochemical Cycle -Internal cycle of P is similar to N cycle w/ major transfer among plant uptake, litter fall, retranslocation and decomposition -major active pools: vegetation

35、and soil OM -P mineralization is hard to measure quick complexation -input from wet/dry deposition is very small, new input is from soil mineral weathering, and decreases as soil profile develops -soil Fe and Al oxides has very high potential to fix P in unavai- lable forms very little leaching low in aquatic systems.,Potassium