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Chapter 4 Decomposition and Nitrogen Cycling Spring 2017
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Nitrogen Cycle Simple Organic N NH4+ NO3- 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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Nitrogen Loading is already damaging the biosphere wet/dry N Deposition rates ( 0 – 60kg/ha/yr )
Deposition from ammonium (NH4) and NO2/3 Galloway et al Science 2008
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Ecosystem Impacts from Nitrogen Loading
Nutrient enrichment impact on native grassland Effects of N addition on productivity and plant diversity in natural grasslands Loss of Biodiversity and productivity decline Alteration of soil/plant community structure Eutrophication Soil Acidification Variable impacts depending on climate, geology, plant community type
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Future Phosphorus Limitations ?
Cordell et al 2009. Global Env Change 19:
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Flathead Lake is both N and P limited; increased N loading has had minimal effect on water quality.
Ellis et al PeerJ 3:e841.
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Gulf of Mexico Dead Zone
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PLANETARY BOUNDARIES Rockstrom et al. 2009 Nature 461
Related to NPP The inner green shading represents the proposed safe operating space for nine planetary systems. The red wedges represent an estimate of the current position for each variable. The boundaries in three systems (rate of biodiversity loss, climate change and human interference with the nitrogen cycle), have already been exceeded
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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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Larch Fir Growth N Demand: Stem 2 Needles 100 (100% turnover) 20 (20%) 102 22 N Supply: Retranslocation 50 10 Decomposition 51 11 Atmosphere ANNUAL N Req. 1
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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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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 Fig. 14.2 p. 255 Leaching GROUNDWATER
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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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Climate Controls on Decomposition
1 2 3 4 5 100 50 25 75 Time (years) % Dry Weight Tropics Seattle Boreal Missoula
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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 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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Remaining Mass Time Fig. 13.1 p. 229 N, P, S, “HLQ” solubles “HLQ”
Nitrogen N, P, S, “HLQ” solubles Remaining Mass Phase regulated by nutrient level and readily available carbon lignin decomposition rate Fig. 13.1 p. 229
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Fig. 13.2 p. 231 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 Fig. 13.2 p. 231
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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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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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Fig. 12.13 p. 222 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 Fig p. 222
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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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Example Model Representation of Soil Decomposition & Respiration (BIOME-BGC)
Multiple SOC pools with cascading litter quality from small, labile litter pool (rapid decomp) to large, recalcitrant (slow decomp) SOC pool. Decomp and respiration rates influenced by C:N ratio, soil moisture and temperature conditions.
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Model Parameterization using Global Observations
Respiration Sites from TRY Database Autotrophic Heterotrophic Reco = fautGPP + f(Tsoil) f(SM) SOC Soil Respiration (Rh) Soil Moisture (%Sat) Hursh et el Global Change Biol.
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