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Decomposition and Nitrogen Cycling
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People, despite their artistic pretensions, their sophistication, and their many accomplishments, owe their existence to a 6” layer of topsoil – and the fact that it rains. Source Unknown
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How do trees get nutrients?
Roots Storage Internal cycling growth and mycorrhizae mass flow diffusion
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Decomposition The process by which complex organic matter is structurally disintegrated into CO2 Water Mineral components It constitutes the major ecosystem process by which nutrients are recycled
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Climate Controls on Decomposition
100 Boreal 75 Missoula % Dry Weight 50 Seattle 25 Tropics 1 2 3 4 5 Time (years)
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Leaf Decomposition Rates
1 2 3 4 5 100 50 25 75 Time (years) % Dry Weight Pine (20% Lignin) Maple (10% Lignin) Tomato (0% Lignin)
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Decomposition of an abscised leaf with (a) high (25%) and (b) low (5%) lignin content
The decomposition of an abscised leaf in relationship to initial concentrations of protein, simple carbohydrates, hemicellulose and lignin. The amount of leaf C remaining is controlled by the concentration and decomposition rate constant of protein and simple sugars, cellulose and hemicellulose, and lignin. Note that differences in initial concentration have a profound influence on leaf decomposition. High lignin concentrations and low concentrations of other constituents result in relatively slow decomposition rates. Fig p. 555
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Litter Particulate matter Dissolved organic matter Bacteria Fungi
Weathering Leaching Inorganic Nutrients* Particulate matter Dissolved organic matter Bacteria Fungi Decomposer animals render litter more susceptible to microbial degradation alter litter chemically – more attractive to microbes some decomposition mix OM with soil burrowing increases water/O2 flow in soil nematodes, termites*, fly larvae, beetles (larvae/adults), pill bugs (isopods), earthworms*, snails, mites Decomposer Animals Predators Faeces
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Decomposition is the Result of 3 Simultaneous Processes
Leaching Weathering Biological activity Leaching – rapid loss of soluble material from detritus by action of rain or water flow most important in fresh litter even complex OM may be leached certain cations are susceptible to leaching Na+, K+, Ca++, Mg++ Weathering mechanical breakdown of detritus due to physical factors such as wind abrasion, freezing/thawing Biological Action – 2 components 1 – mechanical fragmentation by decomposer fauna 2 – microbial degradation – bacteria/fungi
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Litter Quality & Decomposition
Type of chemical bonds and amount of energy released by decay (carbon quality) Size and 3-D complexity of molecule Nutrient concentration (nutrient quality) Decomposition can take days to years
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Decomposition Glucose, simple sugars Cellulose* Tannins & Lignins
*Cellulose – most common molecule in plant component of terrestrial ecosystems source of fiber for paper and paper products (nearly pure cellulose) extracellular enzymes are required to cleave the bonds Tannins – polyphenols thought to be a defence mechanism against animal consumption Lignins among the most complex and variable in nature class of compounds with variable structure 2nd only to cellulose in quantitative importance in most plant tissues – makes the plant “woody” encrusted on and around cellulose in cells walls to provide rigidity and strength – causes cellulose to decay more slowly than it otherwise would. can’t actually perform tests to determine how much is lignin it is what is left after a series of treatments “proximate” analysis
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Decomposition Glucose, simple sugars
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Decomposition Cellulose
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Decomposition Tannins & Lignins
Tannin (lhs) - Lignin (rhs) -
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Humus Humus is a complex & amorphous
form of organic matter in ecosystems. It is high in nitrogen and large polyphenolic molecules, but low in cellulose. It can take thousands of years to decompose. Humus is the stable, slowly decomposing form of soil organic matter – it can take thousands of years to decompose
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Model of a Soil Aggregate
Clay – organic matter complex Open pore Bacteria Pore opening Fungal hyphae Organic matter- Sesqui oxides Clay domain Closed pore Quartz
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Precipitation Deposition 1, 10, 50 kg/yr 100 kg 100 Litter Fall kg
Fixation 30 kg/yr Uptake Litter Fall Soil Leaching Weathering Rock Chemical Reactions Deposition 1, 10, 50 kg/yr Decomposition 50 kg/yr 3000 kg 500 kg 100 kg 100 kg 50 kg
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Sand and loamy sand Sandy loam Loam Silt Silty clay loam Clay Percentage of clay 0.24 0.20 0.16 0.12 0.08 0.04 0.096 0.080 0.064 0.048 0.032 0.016 Percentage of nitrogen Percentage of phosphorus
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Fig. 4.9, p. 127 of Waring & Running
The rate at which different components in litter decompose is not identical. Nevertheless, a general index of decomposition is often obtained by assuming comparable rates for all materials in broad categories. The MRT decreases exponentially with increasing annual litterfall across biomes, varying from more than 40 years in some boreal needle-leaved evergreen forests to less than 1 year in some tropical forests. Average forest floor mass plotted against litterfall for Boreal needle-leaved evergreen (n = 16), Boreal broad-leaved deciduous (n = 7), Temperate needle-leaved evergreen (n = 0.73), Temperate broad-leaved evergreen (n = 11), Temperate broad-leaved deciduous (n = 40), Tropical broad-leaved evergreen (n = 31), and Tropical broad0leaved deciduous forests (n = 2). Assuming the forest floor is in steady-state, the average mean residence time (MRT, years) can be calculated as forest floor mass / litterfall mass. The straight lines indicate these values. The dashed curve is fit to the mean values and is the basis for deriving a related equation [Eq. (4.2)] that predicts MRT from equilibrium annual litterfall. (From Landsberg and Gower, 1996.) Equation 4.2: MRT (year) = 55.4 e-0.443x r2 = 0.93
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Larch Fir Growth N Demand: Stem 2 Needles 100 (100% turnover) 20 (20%) 102 22 N Supply from: Retranslocation 50 10 Decomposition 51 11 Atmosphere ANNUAL N Req. 1
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Nitrogen Cycle NH4+ NO3- Simple Organic N Mineralization
Different species take up NH4+, NO3- convert to amino groups (-NH2) in plant Both occur simultaneously net result depends on substrate Immobilization
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Remaining Mass Time N, P, S, “HLQ” solubles “HLQ” Nitrogen
Phase regulated by nutrient level and readily available carbon lignin decomposition rate
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Atmosphere Plants and Soil microorganisms Nitrification
Denitrification NO3- NH4+ N2 NO N2O Biological assimilation Abiological reactions Aqueous phase Gaseous phase of soil This slide shows the role of nitrification in the emissions of N trace gases Results in a loss of N from the soil Similar to Fig , p. 565
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Relative N cycling rate
Total N cycling Nitrate (NO3-) Relative Uptake Ammonium (NH4+) 3 points: Direct uptake of organic N appears to be an important process in systems with very low rates of N cycling (2) As N availability increases, organic N uptake may decrease, while net nitrification remains minimal, and NH4+ is the predominant form of N utilized by plants (3) Further increases in net mineralization lead to the induction of net nitrification, and both NO3- uptake and leaching could become increasingly important Organic Relative N cycling rate
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Dashed line is loss of OM
Dark line is nutrient dynamic increasing amount of N in decomposing litter is called net immobilization highest in early stages of decay when the most easily decomposed compounds are degraded as C and energy yield from OM declines, so does demand for nutrients -> mineralization -> increased availability to plants 2 sources of imobilized nutrients: throughfall (above ground) mineralized N from older litter & soil OM (below-ground) ?fixation by free-living microbes – generally low
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end of slide show
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NITROGEN Litter Microbes Humus Vegetation Soil Solution ATMOSPHERE
Gaseous losses (e.g., respiration, denitrification) ATMOSPHERE Precipitation Fixation SOIL ORGANIC Litter Humus Microbes Vegetation Decomposition Retranslocation Mineralization Immobilization Throughfall, exudation SOIL INORGANIC Uptake Soil Solution Similar to C cycle in some respects, differs in significant ways Atmosphere is 79% nitrogen gas, substantial energy requirement limits amount of fixation that can occur. -microbes free-living symbiotic N fixation, denitrification relatively low in most natural ecosystems Precipitation – NH4+, NO3- increasing due to human activities Internal cycle dominated by NH4+, NO3- uptake “closed” system Translocation is a big difference from C Decomposition can lead to mineralization or immobilization (microbes competing with plants) Only 3 forms of N that plants take up: NH4+, NO3-, organic N N is not a significant component of primary or secondary minerals Exchange Sites PLANTS Exchange Leaching GROUNDWATER
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