Nutrient pump (temperate lake turnover)

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

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

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

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 p567)  x20-30 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. logging Calcium Potassium Nitrate-N

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

POLARTROPICS DecompositionSlowRapid Proportion nutrients in living biomass Low (mostly in dead organic matter) High CyclingSlowRapid 5. Gradient from poles to tropics

“laterites”

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

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  pH 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)  o ptimum is 25:1  Douglas fir wood548:1  Douglas fir needles58:1  alfalfa hay18:1  activities of soil fauna e.g. earthworms

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

RATE OF DECOMPOSITION humid tropical forests about weeks temperate hardwood forests years temperate / boreal forests yr arctic/alpine / dryland forests >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)

Decomposition Rates influenced by: temperature moisture pH, O 2 quality of litter soil type (influences bugs) soil animals type of fauna / flora rapid if bacterial slow if fungal (mineral content, C/N ratio)

Litter accumulation in forest floor

Plant species % weight loss in 1 year C/N ratio # bacterial colonies # fungal colonies Bact / Fungi ratio Mulberry9025 Redbud7026 White Oak5534 Loblolly pine 4043 Relationship between rate of litter decomposition and litter quality (C/N ratio) Faster decomposition at lower C/N ratios

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

(J) J A S O N D J F M A % leaf litter remaining 0.5 mm mesh bags 7.0 mm mesh bags

Litter decomposers micromesomacro

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

Plant species % weight loss in 1 year C/N ratio # bacterial colonies # fungal colonies Bact / Fungi ratio Mulberry Redbud White Oak Loblolly pine Relationship between rate of litter decomposition and the balance between bacteria and fungi Faster decomposition at higher bact/fungi ratios x10 2