Properties of Soil Organic matter and Its Decay

1. Properties of Soil Organic matter and Its Decay

2. General pattern of element cycle in ecosystems

3. 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 & be a critical link in nutrient cycles. • Decomposition 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: • The type of chemical bonds & energy released as they decay; • The size and spatial complexity of the molecules; • Nutrient content. • 1 and 2 determine the Carbon Quality of litter/OM, while 3 • determines Nutrient Quality of litter/OM

4. 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 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 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 oxidation process requiring participation of soil organisms/ microbes factors affect soil organisms/microbes and general aeration condition of soil will influence rates of decomposition.

6. Properties of Soil OM-3: • Major Biochemical Constituents of Litter and Their Decay: • Simple sugars: mono- and disaccharides: glucose, sucrose.. • Starch: polymer of n glucoses linked by 1-4 glucoside bond. • Cellulose: polymer of n glucoses linked by 1-4 glucoside bond, but slightly different 3-D structure from starch. • Hemicellulose: polymer of n different simple sugars to form chain and branched structures; • (n in 2 ≈ thousands, n in 3 & 4 ≈ thousands to tens of thousands)

7. Properties of Soil OM-4: • 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 to • their sizes, and water • repellent feature, they • are more decay resistant • than carbohydrates

8. Properties 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 simple 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.

9. Properties of Soil OM-6: • Due to complexity of lignin in chemistry, its determination is rather proximate: • Suberin: molecules composed ~½ hydrocarbons and ~½ phenolics low in energy and hydrophobic • Proteins: macromolecular compound consists of various amino acids  not only high carbon quality, but also high nutrient quality due to high N contents.

10. Properties of Soil OM-6: Different plant tissues have different composition of these carbon compounds  different decomposition rats. In litter decay, simple and soluble compounds decay fast, while complex, large compound accumulate (Fahey et al. 1984).

11. 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 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), & heterocyclic compounds (i.e., phenolic combined with amino groups).

12. Properties of Soil OM-8: Two larger groups of compounds can be separated from humus: fulvic acids and humic acids -organic colloids.

13. Properties of Soil OM-9: Formation 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 into 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.

14. Properties of Soil OM-10: Decomposition and Stabilization of Humus: 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 spatial alignment of enzymes & humus molecules, deactivate enzymes.

15. Properties 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 density Light soil fractions have more OM and low bulk density In 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.

16. Decomposition of Litter and Soil OM-1: • Decomposition of litter and soil OM are determined by various • internal and external variables: litter quality, soil conditions, and • environmental conditions. • Decomposition rates are often measured as a percentage weight loss per unit time, and this yields: • % original remaining = e-kt • where t = time, k = litter-specific decay rate constant assume • litter decay rate is constant through time, but it is not true, because • Water soluble compounds can be lost purely by leaching. • Water insoluble compounds do not decompose much at same time: substances easier to decay decay first. • Complex high molecular compounds: lignin, suberin can shield cellulose from microbial attack. • Byproducts of decomposition (humus-mineral complex) are very resistant to decay.

17. 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-kt

18. Decomposition 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 presented in litter in excess of microbial requirements, nutrients are released as decomposition proceeds.

19. 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 immobilization Net increase of absolute amount of nutrients in decomposing litter is called net immobilization As 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  mineralization Sources 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.

20. 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 decay; • 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 & E.

21. 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 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 N NH4 DON

22. Decomposition of Litter and Soil OM-7: In many terrestrial ecosystems, a potentially 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?

23. Decomposition of Litter and Soil OM-8: • N content in decaying OM plays diff. roles at diff. stages of decay: • -Short-term study: high initial N content facilitate decay • -Long-term study: not quite so • Two hypotheses: • Adding N to OM randomizes the bond • structure in old litter/humus make existing • enzymes less effective; • Requirement for extracting • N from humus to meet • fungal N demands does not • exist. (experiment shown that • fungal shut down the production • of lignin/humus degrading • enzymes when large amount of • N added) • the system is messed up!

24. 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 (summarized as AET) has a high correlations with litter decay.

25. 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 decomposition (earthworms for example), but the details are less known.

26. 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 ~1:1. 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.

27. Decomposition of Litter and Soil OM-10: Humus production and decomposition: Turnover rate is used to evaluate humus decomposition. Turnover rate can 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 time 2. field soil incubation for net mineralization rate of N 3. 15N pool dilution analysis  very high N immobilization 4. correlating soil respiration and gross N transformation 5. loss of total soil OM  used in broad scale measurement

28. Flowchart of CENTURY Turnover rate 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 soil 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

29. Synthesis of the information of nutrient cycling in terrestrial ecosystems: The processes dominating element cycling differ widely for different nutrient elements. However, they can be presented as part of a larger, generalized framework of nutrient cycle.

30. As mentioned before, biogeochemical cycles of nutrient elements can be grouped into gaseous cycle and sediment cycles. The diagrams below illustrate biogeochemical cycles of six most important elements in ecosystems S C N P K Ca

31. Atmospheric CO2 Photosynthesis Respiration Primary production Secondary Production (primary consumption) 2ndary consumption Tertiary Consumption Detritus Organisms Litter, soil organic matter Carbon Biogeochemical Cycle (review):

32. Nitrogen biogeochemical Cycle: N Mineralization from decomposing materials begins withammonificationthat NH4 is released by heterotrophic microbes. Soil NH4 has5 sinks: uptakeby plants;ammonia volatilization;immobilizationby microbes;adsorptionby soil particles; andnitrification, that NH4 is oxidized to NO3 by chemoautotrophic bacteria, Nitrobacter & Nitrosomonas, coupled with C fixation: 2NH4+ + 3O2 2NO2- + 2H2O + 4H+ + hv and 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 has5 sinks:uptakeby plants;immodilization by microbes;assimilatory reductionback to NH4; lost torunoff; anddenitrification(no O2 needed): 5CH2O + 4H+ + 4NO3- 5CO2 + 2N2 + 7H2O Extractable NH4 and NO3 in soil at any time represent the net results of all these processes.

33. 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., all 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 can be easily detected. -advantage: direct measurement in field conditions -disadvantages: complex techniques, especially when gaseous products produced.

34. Nitrogen biogeochemical Cycle: Emission of Nitrogen Gases from soil (NH3, NO, N2O & N2): Ammonia Volatilization: NH4++OH-NH3+H2O as pH is high NO 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 affecting 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 N2Oexamined with gas chromatography.

35. 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. -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 little 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. -Nitrification 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.

37. Nitrogen biogeochemical Cycle: Some points on N mineralization/nitrification/denitrification: -High atmospheric 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 incorporated into microbial tissue as NO3 is reduced. -Denitrification is widespread in terrestrial ecosystems, 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 denitrification, as well as the amount of NO, N2O and N2 emission  may be highly spatially heterogeneous. -Fire …

38. Nitrogen biogeochemical Cycle: -Animal…

39. Dry/wet deposition Nitrous Oxides & N2 Retrans- location uptake leaching Conceptual illustration of Nitrogen Biogeochemical Cycle in ecosystems anything wrong? N fixation

40. Biogeochemical Cycle of Sulfur • Sulfur cycle is similar to nitrogen 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 with atmosphere is small. • Sulfate is an important source of acid rain-human made Comparison of N and S cycles

41. 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 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.

42. Potassium Biogeochemical Cycle -unlike C, N, S & P, metal-like cation K cycles in ecosystem in a unique way: it is not bound into organic compounds but moves in the plant in ionic form  very easy to be leached  K enriched in stem flow and throughfall important in K cycle. -only loosely held in the litter and is leached quickly. -in soil, K is held by soil surface negative charges, not involved in reactions leading to unavailable forms. -major new inputs are from precipitation and rock weathering. -tight cycling within ecosystems, but excess K can lead to leaching to groundwater

43. Calcium & Magnesium Biogeochemical Cycles -organic cycling, weathering and cation exchange are important processes in Ca and Mg biogeochemical cycles. -both are important constituent elements in organic compounds. -exchangeable cations of Ca & Mg are the major available pool in ecosystems. -stem flow and throughfall are less important input source. -litter and decomposition are important in Ca & Mg cycles. -immobilization is unimportant for both Ca & Mg. -they both can leached to ground water. -Mg concentration in biological materials is much less than Ca, mostly in leaves (chlorophyll).