Material cycles and flows

Decomposition

dL/dt = I – kL , where I = input and k = decomposition rate

Decomposer food chains

Soil populations can be high

Microbial populationsbacteria 100,000,000 - 500,000,000 /g

actinomycetes 1,000,000 /gfungal myceta 5,000,000 /g

Animal populationsnematodes 1,700,00-6,300,000 /m2

springtails & mites 400,000 / m2 (conifer litter)earthworms 10,000 – 1,000,000 /acre

The Clemson experiment

Day 1: Fresh 17sp

Day 2: Bloated 48 sp

Day 3: Active decay 255 sp

Day 4: Advanced decay426 sp

Day 6: Dry 211 sp

Start with dead baby pig

Observe animal visitors

Time course of N and P in pine litter

C/N C/P

Initial 134 2630

1 year 85 1330

2 years 66 912

3 years 53 948

4 years 46 869

Fungal biomass 12 64

The critical value to measure for organism response is not total N, or even available N, but rate of supply of available N. The same applies to P

Terrestrial nitrogen cycle

Nitrogen fixation in nodules

Denitrifying Bacteria (anaero) Nitrate Bact, 17 kcal/mole

N2 NO3 NO2 Lightning, Photchemical (Nitrobacter) 65 kcal/mole Plants Nitrifying Bacteria

(Nitrosomonas) Nitrogen fixers Ammonifying bacteria

Org N

NH4 Bacteria & bluegreen algae Plants

Terrestrial nitrogen cycle

Figure 8.6

Figure 8.7

Weirs are used to gauge stream flow; nutrient loss is determined by flow times concentration.

Rain gauges and stemflow gauges are used to measure influx in regular rain, rain falling through a canopy, and rain running down tree stems.

Variation in atmospheric contributions of cations with distance from the ocean.

Nutrient concentrations in rain water is increased by flowing down stems or over leaves.

The standing crop of cations tends to show a net loss compared to atmospheric input, whereas anions and carbon tend to show a net gain.

Water loss to evapotranspiration tends to be relatively constant from year to year, but total stream flow is highly sensitive to overall rainfall.

Season pattern in stream water nitrate levels reflecting spring melt with winter decomposition, and summer drawdown owing to biomass increment.

Note year-to-year variance in stream water nitrate levels at Hubbard Brook, but generally an upward trend resulting from anthropogenic inputs.

Experimental manipulation of a watershed and comparison with a control allowed the impact of disturbance on nutrient dynamics to be assessed.

Cutting of a watershed at Hubbard Brook lead to a dramatic increase in nitrogen release in stream water.

Site Disturbance Cont Dist

Hubbard Brook, NH Kill plants 2 yrs 2.0 97.0

Gale River, NH Commercial clearcut 2.0 38.0

Fernow, WV Commercial clearcut 0.6 3.0

Coweeta, NC Complex 0.05 7.3

Andrews, OR Commercial clearcut 0.08 0.26

Loss of NO3 (mg/l) from control and experimental watersheds.

Summary Points – 1–

• Organisms depend on nutrients. Understanding of ecosystem function must include understanding of nutrient influxes, cycling, and losses.

• Decomposition is used to describe a large number of interrelated processes by which organic matter is broken down into particles and to soluble forms of nutrients that are available for plant uptake

• The standing crop of litter can be modeled as input divided by standing crop equals a system-specific decomposition constant that varies with latitude, moisture, temperature, and litter quality.

• Decomposer organisms are important for fragmenting, mixing, and mineralizing dead organic matter

Summary Points – 2–

• The nitrogen cycle has many links that depend on prokaryotic organisms, including nitrogen fixation, ammonification, nitrification, nitrate production, and denitrification.

• System-level studies of nutrient fluxes are often best studied with closed systems such as ponds and watersheds.

• Most nutrient influx to watersheds comes from the atmosphere and from weathering of rock and soil. Anions tend to come from the former and cations from the later.

• Variation in nutrient loss is related to season, recent disturbance events, and the degree to which the system is increasing in biomass.