Nutrient Cycles: Abiotic & Biotic Factors

• Lecture 6 is on nutrient cycles

Factors Affecting Nutrient Cycling

• Cycling is affected by abiotic and biotic factors
• Abiotic factors include volcanism
• Biotic factors include:
  • Plant uptake
  • Decomposition
  • Herbivores

Lab Announcement

• In this week's lab, visit the greenhouse (not 106 LUM) at 2:30 for approx. 30 minutes
• Students measure and record leaf growth for their bulb group, and the same numbered group from the other lab section
• E.g. Monday Group 4 will measure Wednesday Group 4

Recall: Hubbard Brook Experimental Forest, New Hampshire

• The study began in 1963
• The bedrock is impervious, so all runoff occurs via streams
• Several watersheds were studied
• Nutrient input (precipitation) and loss (stream runoff) were measured
• Without logging, Ca, Mg, K, Na, Al, SO4, and silica showed small net losses
  • Losses were made up for by weathering
• Without logging, NO3, NH4, and Cl- showed a net gain in the area
• One watershed was logged in 1966 with logs and branches left on the ground

Effect of Logging in Hubbard Brook

• Runoff increased by 32% for 3 years following logging
• NO3 in stream water increased 40-60 times as a result of increased mineralization of nutrients in decomposing plant tissues
• Calcium increased by 417%, magnesium by 408%, potassium by 1,558%, and sodium by 177% in stream water
• Primary production is stimulated in the short term due to the "irrigation / fertilization" effect
• Early-successional transients capture some nutrients, which reduces losses, called immobilization

Volcanism

• Mt. Paricutin in Mexico erupted from 1943-1952
• The volcano emitted lava flows containing 7.4% calcium oxide
• The volcano began in a farmer's cornfield
• The cone grew to 424m after 8 years
• Two nearby towns were obliterated
• All surrounding vegetation was destroyed, mostly by falling ash
• Forests grew rapidly on the ash deposits
• Eight conifer species were established by 1970

Volcanoes and Soil Nutrients

• Volcanoes provide essential soil nutrients for plant growth
• Lava and ash contain P, S, Ca, Mg, K, micronutrients, and trace elements
  • The nutrient content varies from one eruption to another
• Volcanic soils, called andisols, are dark colored with a porous structure for water retention and good drainage
• Volcanic soils cover over 1.5 million sq km
• Volcanic soils form a component of the surface area in countries along the Pacific Rim of Fire
• Some of the most densely populated areas on Earth are close to volcanoes
• Java, Indonesia is the world’s most populous island with nearly 150,000,000 people
• 10% of the world's population lives within 100 kilometers of an active volcano

Volcanic Ash Toxicity

• Volcanic ash is toxic to plant life in the short term
• Volcanic ash is acidic and contains fluoride
• A few mm of ash may kill plants
• Toxicity is short-lived, and subsequent plant re-growth benefits from increased soil fertility and secondary succession

Recovery in Lava-Covered Areas

• Recovery in lava-covered areas takes hundreds or thousands of years due to solid rock
• The speed of weathering depends on rainfall
• Recovery undergoes primary succession

Biotic Factors: Litter Accumulation

• Litter accumulation rate = (litter production rate) - (litter decomposition rate)
• In unburned tallgrass prairie, the accumulation rate is 970-1800 kg/ha/yr
• The decomposition rate is 240-450 kg/ha/yr
• Fire is important in preventing litter accumulation
• About 50% of root production decays every year, making the soil deeper and more organic over time

Litter Accumulation in Different Climates

• In the tropical rainforest, production = 16,000 to 45,000 kg / ha / yr
• The rainforest has almost 100% decomposition, resulting in little or no accumulation
• The rainforest sees high temperature and humidity, which increases microbial growth
• High latitudes have low productivity but slow decomposition rates because they are cold and dry
• Organic matter accumulates over time in high latitudes

Nutrient Uptake by Plants

• Only soluble nutrients in contact with root surface can be taken up
• Nutrients may be present in soil solution or adsorbed to particles touching the root
• Uptake causes depletion around the root surface
• Nutrient supply is maintained by:
  • Root interception: roots grow into new, undepleted areas
  • Mass flow: roots draw in dissolved ions with water
  • Diffusion: ions diffuse through soil solution from areas of higher concentration

Nutrient Uptake: Diffusion

• Diffusion is the random movement of individual ions/molecules causing net movement towards the zone of depletion
• Some nutrients diffuse more readily than others; slower diffusion = bigger depletion zone
• Root morphology may change (=phenotypic plasticity) to maximize the uptake of the most limiting nutrient
  • NO3- is characterized by a fast diffusion rate, no depletion zone, and long, infrequent root branching
  • PO43- is characterized by a slow diffusion rate, a present depletion zone, and short frequent branching

Absorption by Roots

• Root cell membranes are impermeable to ions by passive diffusion
• Absorption requires a carrier protein in the cell membrane
• Uptake is usually against a concentration gradient, which requires metabolic energy (ATP)
• Uptake of phosphate occurs via a co-transporter that also transports H+ into the cell (=assisted diffusion) and transports H+ into the cell
• Phosphate "piggy-backs" with H+ that has been actively pumped out of the cell at an energy cost

Microbial Decomposition

• Biomass consists mainly of fungi and bacteria
• Microbes can act as either source or sink for nutrients
  • Microbes immobilize nutrients (convert inorganic to organic)
  • Microbes mineralize nutrients (release inorganic ions from organic compounds)
• For nitrogen, the C:N ratio determines immobilization versus mineralization, with > 20-30:1 favoring immobilization

Mineralization and Plant Growth: Early Spring

• Mineralization and plant growth aren't always synchronized
• During early spring, soil temperature increases, increasing N mineralization
• N mineralization begins before plant growth resumes, resulting in a high leaching rate

Decomposition

• Decomposition of litter is inversely related to the ratio of lignin / (lignin + cellulose)
• Lignin = polyphenol
• Lignin is more recalcitrant, or resistant to decomposition, than cellulose
• As litter decomposes, cellulose content decreases faster than lignin content, which slows the decomposition rate

Microbial Fixation

• Microbial fixation is an important source of nitrogen
• Legumes are an example of nitrogen-fixing bacteria
• Plants need large quantities of nitrogen in the form of NH4+ or NO3-
• Plants are not able to use atmospheric nitrogen, N2
• Some bacteria converts N₂ → NH4+, called nitrogen fixation
  • Requires anaerobic environment and is energy-intensive
• Legumes form root nodules inhabited by N-fixing Rhizobium bacteria, having low O2
  • The host shares photosynthate and receives usable N in return
• Legumes are one of the largest plant families with over 16,000 species, including herbaceous plants and trees

Herbivores

• Nutrient cycling is enhanced by grazing and deposition of fecal matter
• Nutrients returned to soil --> increased productivity

Definitions

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

Herbivory: Plant Compensation

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

Herbivore Optimization Model

• Plant RGR and productivity may be increased at low to intermediate levels of herbivory
• Low levels of herbivory lead to compensation, due to reduced self-shading and enhanced nutrient availability
• Overcompensation occurs when RGR is strongly enhanced and productivity is increased
• Partial compensation occurs when the RGR is enhanced somewhat, but overall productivity is still reduced
• High levels of herbivory lead to undercompensation, due to plants not compensating for tissue loss, causing a reduction in RGR and productivity
• Partial compensation is the most common outcome of herbivory

Goose grazing in La Perouse Bay

• Accelerated nutrient cycling enhances productivity under goose grazing
• Grazing can also enhance N fixation by cyanobacteria Human activity has altered grazing impacts by geese
• Goose populations in North America have increased steadily since 1950
• The increase in agricultural fertilizer use has caused a goose population increase by way of allochthonous inputs, like nutrient subsidy fertilizer
• Fertilizer inputs in wintering areas → increased numbers → increased pressure on vegetation in breeding areas (e.g. La Perouse Bay)
• As a result, there is salinization attributed to lack of vegetation cover, leading to evaporation and subsequent salt deposits on the surface
• Pastor & Naiman, 1992: studied selective foraging in a simulated boreal forest ecosystem
  • Moose "browse" woody plants, removing leaves and twigs. The plant remains alive and may be able to regrow
    • Beavers cut trees at the base of the trunk, which kills trees and prevents regrowth
• Light moose herbivory regime increased short-term primary productivity in Pastor & Naiman's model
• Heavy moose herbivory regime reduced available N greatly in models

Lecture on nutrient cycles, focusing on the abiotic and biotic factors affecting cycling; including volcanism, plant uptake, decomposition, and herbivores. Details from the Hubbard Brook Experimental Forest study (1963) in New Hampshire are discussed.