Lecture 5

• Nutrient Cycles are being discussed

Nutrient Use Efficiency (NUE)

• Major plant nutrients like Nitrogen and Phosphorus are key components
• Nutrient cycling can be affected by abiotic factors such as fire and volcanism
• Biotic factors like microbial decomposition and herbivores also affect cycling
• In the lab this week: ethylene is being measured with groups in the greenhouse at allocated times
• Presentations: an article should to be chosen that was published in a peer-reviewed journal in the year of your birth, on a plant ecology topic

Key Concepts:

• Bioelements are elements that cycle through living organisms
• Nutrients move from one compartment to another within ecosystems
• Biogeochemical cycles involve nutrient exchanges between biological and non-biological compartments
• Cycles are closed when viewed on a global scale, and are open when viewed on a local scale
• Compartments are arbitrarily defined, containing a pool of nutrients
• Nutrients are exchanged with other compartments at a flux = the rate of movement into / out of a compartment

Plant Nutrition

• Plants take up essential elements in mineral (inorganic) form
• Macronutrients include N, K, Ca, Mg, P, and S, which come in various forms
• Micronutrients include Cl, Fe, Mn, B, Zn, Cu, Ni, and Mo
• Essential elements for some plants are Na, Co, and Si

Definitions

• Immobilization is the uptake of mineral nutrients from soil solution by microbes or plants, converting them into organic form, making the nutrient unavailable to other plants
• Mineralization is the release of mineral nutrients into soil solution from organic molecules via respiration/decomposition, making the nutrient available for use by plants

Soil Types

• Eutrophic soils are high in nutrients and can derive from recent glacial till
• They compose Large areas of the northern hemisphere, including volcanic soils
• Oligotrophic soils are old, weathered and infertile, like the former Gondwana continents
• Vegetation in these areas is adapted with efficient recycling within the plant, and leaf fall and reabsorption

Nutrient Use Efficiency (NUE) details

• Nutrient Use Efficiency (NUE) = A / L
• A = nutrient productivity, using dry weight produced / unit nutrient in plant
• L = nutrient requirement for maintenance of one unit plant biomass
• Retention Time in a plant = 1/Ln
• Ln = relative nutrient requirement for maintenance of same nutrient
• Retention Time is inversely related to turnover rate

Oligotrophic Forests

• Have a dense layer of fine roots in upper humus to maintain high NUE and productivity
• Clearing forest for agriculture and removing the humus/root layer makes soils unproductive
• Eutrophic soils maintain productivity even when humus/root layer is removed

The Nitrogen Cycle

• Human addition of N to the global N cycle has resulted in: -Increase in N-based trace gases in the atmosphere -Increased atmospheric NH3 (mostly from fertilizers.) -Increased deposition of N on land / in oceans from fertilizers, often coupled with P additions -Eutrophication of soils, lakes, and coastal areas
• Forest production in Sweden is 30% higher than in the 1950s
• Critical load = the amount of N that can be added and absorbed by plants

Consequences of exceeding Critical Load

• If Critical Load is exceeded, excess N goes into groundwater or back to the atmosphere
• Nitrate ions remove calcium, magnesium, and potassium ions from solution
• Excess nitrate can lead to limitation of growth by other nutrients
• The Netherlands have the highest rate of N deposition in world due to intensive livestock operations
• Species-rich heathlands are being converted to species-poor grassland/forest
• Plant and animal species adapted to sandy, infertile soils are being lost because of N enrichment

Hydrologic Cycle and Rainfall

• P = E+T + R + I (Precipitation = Evapotranspiration + Runoff + Infiltration)
• Precipitation carries nutrients in solution into an ecosystem
• Runoff and infiltration remove nutrients from a system or move them down a soil column
• Evapotranspiration concentrates or conserves nutrients
• Human activity elevating concentrations of Ca, K, Mg, N, Na, and S in precipitation in Europe and N. America
• Rainfall combines with carbon dioxide to produce carbonic acid (H2CO3), making it acidic
• "Pure" water in equilibrium with the atmosphere has a pH of 5.65
• "Acid" rain has a pH lower than 5.65
• Hydrogen ions in acid rain displace Ca2+, Mg2+, and K+ in soil, often leading to nutrient deficiency
• Evapotranspiration is calculated with Impermeable and Permeable bedrock

Wildfires & Prescribed Burns

• Occur in most terrestrial ecosystems
• Canada: destroys ~2 million ha (0.6%) of coniferous forest/yr
• Decreased precipitation and increased wind increase fire risk
• Natural fire ignition sources include lightning, which increases in frequency with decreasing latitude
• Ignition probability increases with precipitation, humidity, temperature, and wind
• Eastern US has moist conditions all year
• Southwestern US has dry conditions in late winter/early spring
• Altered nutrient availability with a Release from dead vegetation
• Non-volatile nutrients are mobilized for plant use
• Nitrogen availability may increase or decrease
• Fire clears leaf litter, allowing light to reach soil
• Shade-intolerant plants can regenerate

Boreal Forests & Grasslands

• Low temperature and low pH lead to slow decomposition and acid soils
• Burning raises soil pH, increasing growth of N-fixing bacteria (Azotobacter, Rhizobium)
• Fires eliminate litter, allow sunlight to reach soil, and makes non-volatile nutrients available
• May reduce N through volatilization
• The effect is similar to grazing, where defoliation-tolerant grasses are maintained and trees/shrubs are eliminated

Chaparral & Fire-adaptations

• Chaparral accumulates litter that traps nutrients and shades the soil
• Is a Mediterranean shrubland, occurring around the coast of California
• Summers are warm and winters are mild
• Fire and drought are common in summer
• Fires burn everything creating an ashen landscape.
• It is adapted through drought deciduous/dormant root systems and Evergreen root systems
• Fire-resistant bark protects living layers
• Fir and larch pine in western North America exhibits 10cm thick bark
• Epicormic sprouts grow from latent buds beneath bark
• Pitch Pine, redwoods, oaks and eucalyptus display epicormic sprouts
• Lignotubers are swellings @ root/shoot interface
• Contain buds, food reserves, and are protected underground
• Moderate heating stimulates bud development
• Suckers allow regrowth from roots
• Requires scarification of seeds or by heating from fire for Seedlings to grow in post-fire conditions with high light/nutrients
• Seeds stay in serotinous cones
• Land use practices decreased fire frequency in North America
• Prescribed burns are a solution, but are not always safe or effective