Nutrient Cycling: Abiotic Factors & Volcanism

Factors Affecting Nutrient Cycling

• Nutrient cycling is affected by both abiotic and biotic factors.

Abiotic Factors: Volcanism

• Volcanoes release essential soil nutrients for plant growth
• Lava and ash contain phosphorus, sulfur, calcium, magnesium, potassium, micronutrients, and trace elements
• Soil from volcanoes are "andisols"
• Andisols have dark colour, a porous structure allowing water retention, and good drainage.
• Volcanic soils cover over 1.5 million square kilometers
• They form a surface area component in countries around the Pacific Rim of Fire.
• Many densely populated areas are near volcanoes
• Java, Indonesia has nearly 150 million people, is the world's most populous island
• 10% of the world's population lives within 100km of an active volcano.
• An example of volcanism is Mt. Paricutin, Mexico, which erupted from 1943-1952 with lava containing 7.4% calcium oxide
• Mt. Paricutin began in a farmer's cornfield and created a 424m cone after 8 years
• The eruption obliterated two nearby towns and destroyed all surrounding vegetation, but forest grew rapidly on ash deposits, with eight conifer species established by 1970
• Volcanic ash can also be toxic to plant life in the short term, because it is acidic and may contain fluoride
• Even a few millimeters of volcanic ash may kill plants
• The toxicity is short-lived and the subsequent plant regrowth benefits from increased soil fertility due to secondary succession
• Recovery in lava-covered areas may take hundreds or thousands of years
• Solid rock and weathering speed depends on rainfall
• Primary succession occurs after volcanic eruptions

Biotic Factors

• Biotic factors also affect nutrient cycling.
• Litter accumulation: accumulation rate = litter production rate - litter decomposition rate

Litter Accumulation in Different Ecosystems

• Unburned tallgrass prairie: Accumulation rate is 970-1800 kg/ha/yr while the decomposition rate is 240-450 kg/ha/yr
• Fire prevents litter accumulation and about 50% of root production decays every year, making the soil more organic over time.
• Tropical rainforest: Production is 16,000 to 45,000 kg / ha / yr, but almost 100% decomposition results in little to no accumulation due to temperature and humidity increasing microbial growth
• High latitudes: Low productivity but slow decomposition rates due to cold and dry conditions; increases organic matter over time.

Nutrient Uptake by Plants

• Soluble nutrients in contact with the root surface can be taken up
• Nutrients in soil solution or adsorbed to particles touching the root are available for uptake

Nutrient Supply Maintenance

• Nutrient supply is maintained by:
  • Root Interception: As root grows, it enters new undepleted areas
  • Mass Flow: Roots draw in dissolved ions with water
  • Diffusion: Ions move through soil solution from area of higher concentration
• Diffusion is the random movement of individual ions/molecules causing net movement towards zone of depletion
• Some nutrients diffuse more readily than others, resulting in bigger depletion zones with slower diffusion
• Plant root morphology may change (=phenotypic plasticity) to maximize uptake of the most limiting nutrient
• Fast diffusion rates occur with NO3, slow occurs with PO4 3-
• There is no depletion zone with NO3, but there is one with PO4 3-
• The root Morphology with NO3 is Long, infrequent branching, but short, frequent branching with PO4 3-
• Absorption by roots involves ions that are impermeable to cell membranes by passive diffusion
• Requires a carrier protein in the cell membrane
• Uptake works against a concentration gradient so it requires metabolic energy through ATP

Microbial Decomposition

• Microbial biomass is mainly fungi and bacteria
• Microbes can act as either a source or sink for nutrients
  • Microbes may immobilize nutrients, converting inorganic to organic forms
  • Microbes may mineralize nutrients, releasing inorganic ions from organic compounds
• For nitrogen, the C:N ratio determines if immobilization or mineralization predominates; above 20-30:1, immobilization is greater than mineralization.

Mineralization and Plant Growth Synchronization

• Mineralization and plant growth aren't always synchronized.
• Early spring soil warms, nitrogen mineralization starts before plant growth resumes causing a high leaching rate

Litter Decomposition

• Litter decomposition is inversely related to the ratio of lignin / (lignin + cellulose)
• Lignin, a polyphenol, is more recalcitrant (resistant to decomposition) than cellulose
• As litter decomposes, cellulose breaks down faster than lignin; this slows rate of decomposition

Microbial Fixation of Nitrogen

• Fixation is an important source of nitrogen
• Legumes have nitrogen-fixing bacteria
• Plants need large quantities of nitrogen in the form of NH4+ or NO3 however cannot use atmospheric nitrogen (N2)
• Some bacteria can convert atmospheric nitrogen to NH4+ via nitrogen fixation, which requires an anaerobic environment and is energy-intensive
• Legumes form nodules housing N-fixing Rhizobium bacteria which has a low O2 environment, where the host shares photosynthate and receives usable N in return
• Legumes are one of the largest plant families (over 16,000 species) and include herbaceous plants and trees

Herbivores

• Herbivores enhance nutrient cycling via grazing and fecal matter deposition
• Nutrients returned to soil increase productivity

Definitions

• Primary productivity: rate of plant (or other autotroph) biomass production per unit area, measured in dry weight (e.g. kg/ha/d) or energy (j/m²/d).
• Gross primary productivity (GPP): total amount matter/energy captured by photosynthesis
  • A portion of GPP is respired by plants and lost as CO2 / heat
• Net primary productivity (NPP) = GPP - plant respiration
• NPP represents biomass and energy available for consumption by heterotrophs.
• Secondary productivity: rate of biomass production by heterotrophs

Plant Compensation After Herbivory

• Plant compensation is the tendency for remaining plant tissue to increase relative growth rate (RGR) after herbivore attack
• RGR is calculated as (g new growth) / (g existing biomass / time)
• Mechanisms of plant compensation:
  • Accelerated nutrient cycling
  • Reduction in self-shading
  • Delayed senescence or rejuvenation of senescing leaves
  • Redistribution of nutrients from roots, flowers, fruits, or storage to remaining leaves

Herbivore Optimization Model

• Plant relative growth rate (RGR) and productivity increase at low to intermediate herbivory levels.
• Low herbivory levels: Grazing leads to the plant compensation (via reduced self-shading, enhanced nutrient availability, etc.)
• Overcompensation: RGR is strongly enhanced, and productivity increases along with it
• Partial compensation: RGR is somewhat enhanced, but overall productivity is still reduced
• High herbivory levels: Plants cannot compensate for tissue loss because of defoliation, so RGR and productivity are reduced

Other Notes on Herbivores and Plant Health

• Partial compensation is the more common outcome of herbivory.
• Goose grazing in La Perouse Bay shows evidence of accelerated nutrient cycling to enhance productivity
• Grazing enhances N-fixation by cyanobacteria
• Human activity has changed the grazing impacts by geese
• Goose populations in North America have increased steadily since 1950
• Increased agricultural fertilizer use has caused a goose population increase
• Allochthonous inputs from fertilizer increase numbers in the wintering ground which increases pressure on vegetation in breeding areas.
• Changes in La Perouse Bay were evident from 1980 to 2000
• There is a positive grazing/grubbing feedback on the salt-marsh
• Salinization is the effect of lacking vegetation cover --> evaporation --> salt deposits on surface
• Pastor & Naiman (1992) studied selective foraging
• The simulated boreal forest ecosystem had:
• Herbivory
• Moose browsing of woody plants, by removing leaves and twigs, lets regrowth occur
• Beaver cut trees at the base of their trunk, killing it with no regrowth
• Light moose herbivory regime increased short-term primary productivity and heavy moose regime reduced available N the most.
• The Hubbard Brook Experimental Forest in New Hampshire was established in 1963, featuring impervious bedrock with runoff via streams, and contains several watersheds.
• Nutrient input from precipitation and loss from stream runoff were measured in the forest.
• Ca, Mg, K, Na, Al, and SO4 showed small net losses without logging.
• These losses are potentially made up by weathering, whereas NO3, NH4, and Cl- showed a net gain
• In 1966, one watershed was logged, and logs/branches were left on the ground.
• Runoff increased by 32% for 3 years after logging. NO3 in stream water increased 40-60 times due to mineralization of nutrients in decomposing plant tissues
• Calcium in stream water increased by 417%; Mg by 408%; K by 1,558%; Na by 177% due to logging.
• Logging stimulates primary production in the short term through an "irrigation / fertilization" effect.
• Early-successional transients capture some nutrients, reducing losses through immobilization.

Lab Announcements

• The lab will be approximately 30 minutes long
• Go directly to the greenhouse (not 106 LUM) at 2:30 (all groups)
• Measure and record leaf growth for your group's and another lab section bulbs (same-numbered group)
• Monday Group 4 will also measure Wednesday Group 4

Explore how volcanism, an abiotic factor, affects nutrient cycling. Volcanoes release essential soil nutrients like phosphorus and sulfur, benefiting plant growth. Volcanic soils, known as andisols, are porous, retain water well, and provide good drainage.