Nutrient cycling Ecosystem Health READINGS for this lecture

Nutrient cycling & Ecosystem Health READINGS for this lecture series: • KREBS chap 27. Ecosystem Metabolism III: Nutrient Cycles • KREBS chap 28. Ecosystem Health: Human Impacts; Pp 590 – 600 • WEB Downloads

NUTRIENT CYCLING • Energy – 1 -way flow - eventually gets “lost” • Nutrients – cycle mineralization Organic Inorganic (living organisms) (rocks, air, water) assimilation

Human activity

Three main types of cycles: 1. Biochemical cycles: • Redistribution within/between organisms • This really is r- and K-selection from first term 2. Biogeochemical cycles: • Exchange within an ecosystem • N, P - rapid exchange • Ca - long if stored in long-lived tree tissue 3. Geochemical cycles: • Exchange of chemicals between ecosystems • Nutrients and dust • CO 2, SO 2, NOx

Three main types of cycles: 1. Biochemical cycles: • Redistribution within/between organisms • This really is r- and K-selection from first term 2. Biogeochemical cycles: • Exchange within an ecosystem • N, P - rapid exchange • Ca - long if stored in long-lived tree tissue 3. Geochemical cycles: • Exchange of chemicals between ecosystems • Nutrients and dust • CO 2, SO 2, NOx

Three main types of cycles: 1. Biochemical cycles: • Redistribution within/between organisms • This really is r- and K-selection from first term 2. Biogeochemical cycles: • Exchange within an ecosystem • N, P - rapid exchange • Ca - long if stored in long-lived tree tissue 3. Geochemical cycles: • Exchange of chemicals between ecosystems • Nutrients and dust • CO 2, SO 2, NOx

Krebs Fig. 27. 12; p 573 SULPHUR CYCLE

Krebs Fig. 28. 8; p 591 photosynthesis respiration CARBON CYCLE

Krebs Fig. 27. 17; p 579 NITROGEN CYCLE

78% of air is N 2

These figures have: • All sorts of rates of transfer • We can compare between systems More interesting: • What influences the rates? • What are the impacts of altering the rates?

These figures have: • All sorts of rates of transfer • We can compare between systems More interesting: • What influences the rates? e. g. forms of nutrients, types of organisms • What are the impacts of altering the rates? e. g. disturbance, pollution, etc.

Compartment Models Quantitative descriptions of storage and movement of nutrients among different compartments of an ecosystem • “Coarse” – few broad compartments e. g. plants, herbivores • “Fine” – many detailed compartments e. g. separate species

Compartment Models POOL – “the quantity of a particular nutrient in a compartment” FLUX – “the quantity moving from one pool to another per unit time” TURNOVER TIME – “the time required for movement of an amount of nutrient equal to the quantity in the pool” (POOL/FLUX)

Krebs Fig. 27. 2 p 562 Phosphorus cycle in a lake (simplified) Turnover time (water): 9. 5 (pool) /152 (flux) = 0. 06 day

NUTRIENT PUMP • Any biotic or abiotic mechanism responsible for continuous flux of nutrients through an ecosystem • Biotic – tree roots, sea birds, Pacific salmon • Abiotic – lake overturn, ocean upwelling

Nutrient pump (Terrestrial) Mycorrhizae

Mycorrhizae

Mycorrhizae

Nutrient pump (temperate lake turnover)

Marine ecosystem

Nutrient pumps (Marine) Microbial loop Upwelling

BIOGEOCHEMICAL CYCLES: A few general points (terrestrial systems): 1. Nutrient cycling is never perfect i. e. always losses from system Inputs • Precipitation • Particle fallout from atmosphere • Weathering of substrate • Fertilizer & pollution Outputs • Runoff & stream flow • Wind loss • Leaching • Harvesting

terrestrial systems cont’d… 2. Inputs and outputs are small in comparison to amounts held in biomass and recycled (i. e. relatively “tight” cycling is the norm) 3. Disturbances (e. g. deforestation) often “uncouple” cycling 4. Gradient in rates of decomposition and nutrient cycling from poles to tropics

HUBBARD BROOK FOREST catchments Experiments done to: 1. Describe nutrient budget of intact forest 2. Assess effects of logging on nutrient cycles

Soil micelles “CEC” Cation Exchange Capacity

Annual Nitrogen budget for the undisturbed Hubbard Brook Experimental Forest. Values are Kg, or Kg/ha/yr

Deforestation is a major change in community structure, with a consequent: § § § loss of nutrients (Krebs Fig 27. 7 p 567) § x 20 -30 normal loss of NO 3 in Hubbard Brook reduction in leaf area § § § 40% more runoff (would have transpired) more leaching more erosion and soil loss decouples within-system cycling of decomposition and plant uptake processes § all the activities (and products) of spring decomposition get washed away

Logging causes decoupling of nutrient cycles and losses of nitrogen as nitrates and nitrites Nitrate losses after logging

Concentrations of ions in streamwater from experimentally deforested, and control, catchments at Hubbard Brook. Calcium logging Potassium Nitrate-N

Uncoupling of N-cycle 1) Logging causes increased nitrification: NH 3, NH 4+ NO 2 - NO 3 H+ H+ 2) H+ displace nutrient cations from soil micelles H+ >Ca++>Mg++>K+>Na+

4. Gradient from poles to tropics Decomposition POLAR TROPICS Slow Rapid Proportion Low (mostly nutrients in living in dead biomass organic matter) Cycling Slow High Rapid

“laterites”

Non-forest Polar Relative proportion of Nitrogen in organic matter components ROOTS Tropics Forest

Relative proportion of Nitrogen in organic matter components SHOOTS

DECOMPOSITION IF TOO SLOW: • Nutrients removed from circulation for long periods • Productivity reduced • Excessive accumulations of organic matter (e. g. bogs) IF TOO FAST: • Nutrient depletion • Poor chemistry and physics of soil (e. g. decreased soil fertility, soil moisture and resistance to erosion) (e. g. tropical laterites)

WHAT DETERMINES DECOMPOSITION RATES IN FORESTS? § moisture and temperature § p. H of litter and the forest floor § more acid promotes fungi, less bacteria §species of plant producing the litter § chemical composition of the litter C/N ratio - high gives poor decomposition microbes need N to use C N often complexed with nasties (e. g. tannin) optimum is 25: 1 Douglas fir wood 548: 1 Douglas fir needles 58: 1 alfalfa hay 18: 1 §activities of soil fauna e. g. earthworms § § § §

Decomposition Rates influenced by: • temperature • moisture • p. H, O 2 • quality of litter • soil type • soil animals (size) • type of fauna / flora • rapid if bacterial • slow if fungal

RATE OF DECOMPOSITION • humid tropical forests about • temperate hardwood forests • temperate / boreal forests • arctic/alpine / dryland forests 2 - 3 weeks 1 - 3 years 4 - 30 yr >40 years • generally, rate of decomposition increases with increased amount of litterfall Residence time … the time required for the complete breakdown of one year’s litter fall

Residence times (years)

Residence times (years)

Decomposition Rates influenced by: • temperature • moisture • p. H, O 2 • quality of litter (mineral content, C/N ratio) • soil type • soil animals (size) • type of fauna / flora • rapid if bacterial • slow if fungal

Litter accumulation in forest floor

Relationship between rate of litter decomposition and litter quality (C/N ratio) # # C/N bacterial fungal ratio colonies Plant species % weight loss in 1 year Mulberry 90 25 Redbud 70 26 White Oak 55 34 Loblolly pine 40 43 Bact / Fungi ratio Faster decomposition at lower C/N ratios

Decomposition Rates influenced by: • temperature • moisture • p. H, O 2 • quality of litter • soil type (influences bugs) • soil animals (size) • type of fauna / flora • rapid if bacterial • slow if fungal

100 90 % leaf litter remaining 0. 5 mm mesh bags 80 70 60 50 40 7. 0 mm mesh 30 bags 20 10 0 (J) J A S O N D J F M A

micro meso macro Litter decomposers 0. 5 mm 7. 0 mm

Decomposition Rates influenced by: • temperature • moisture • p. H, O 2 • quality of litter • soil type (influences bugs) • soil animals (size) • type of fauna / flora • rapid if bacterial • slow if fungal

Relationship between rate of litter decomposition and the balance between bacteria and fungi x 102 # # C/N bacterial fungal ratio colonies Plant species % weight loss in 1 year Bact / Fungi ratio Mulberry 90 25 698 2650 264 Redbud 70 26 286 1870 148 White Oak 55 34 32 1880 17 Loblolly pine 40 43 15 360 42 Faster decomposition at higher bact/fungi ratios