Decomposition and Nutrient Cycling

Carbon is fixed through photosynthesis and returned to the atmosphere through cellular respiration.
Primary production also depends on mineral (inorganic) nutrients taken up by autotrophs.
The nutrients are incorporated into their tissues.
Sources of essential nutrients include the atmosphere or rocks.
When organisms lose parts or die, the nutrients are returned to soil or sediments as dead organic matter and enter the detrital food chain.

Plants uptake inorganic nitrogen (as ammonium and nitrate) from the soil through their roots.
N is needed to make proteins & nucleic acids.
Plants convert the N from inorganic to organic form.
N availability in the soil limits: Rate of NPP by decreasing the rate of photosynthesis.
N is returned to the soil as dead organic matter.
Plants absorb some of the nutrients from senescing (dying) tissues into the perennial parts of the plant through retranslocation or resorption.

Decomposition is the breakdown of the chemical bonds formed when organic molecules and tissues are built which releases energy originally fixed by photosynthesis.
Decomposition releases carbon dioxide and water by respiration.
Organic compounds are converted into mineral nutrients.
Decomposition is a KEY PROCESS of nutrient recycling in ecosystems.
Decomposers obtain energy by oxidizing carbon compounds through cellular respiration or fermentation.

All heterotrophs are decomposers to some degree by digesting food.
Decomposers/detritivores are organisms that feed on dead organic matter or detritus.
Categorized based on size and function:
- Microbial decomposers
- Fungal decomposers
- Invertebrate detritivores
Invertebrate detritivores are classified by body width.

Microbial decomposers secrete enzymes into tissues to break down organic compounds then absorb them.
- Bacteria (may be aerobic or anaerobic): dominant decomposers of dead animal matter.
- Fungi: dominant decomposers of plant material.

Fragment detritus (dead organic matter).
Four major groups classified by body width:
1. Less than 100 µm – microfauna and microflora
2. Between 100 µm and 2 mm – mesofauna
3. Between 2 and 20 mm – macrofauna
4. Over 20 mm – megafauna

Feed on bacteria and fungi.
Includes: protists (amoebas), springtails, nematodes, beetle larvae (grubs), mites.
Smaller microbivores feed on bacteria and fungal hyphae.
Larger forms feed on both microflora and detritus.

Litter is dead plant tissue.
Litterbags are mesh bags made of synthetic material that does not decompose, with 1–2 mm holes.
The holes are large enough to allow decomposers but small enough to prevent loss of plant material in the bag.
Leaf packs are used in aquatic systems.

Phytoplankton have a low lignin concentration so decompose quickly.
Vascular plants (e.g., sea grasses, marsh grasses, reeds) can have lignin concentrations similar to terrestrial plants.
Bacteria do not decompose lignin, only select fungi.
Decomposition is affected by $O_2$ concentration:
- LOW $O_2$ : most decomposition is done by anaerobic bacteria
- HIGH $O_2$ : fungi are able to decompose lignin

Temperature & moisture
- Low temperatures/dry conditions inhibit microbes
- Warm temperatures/wetter conditions allow for higher decomposition rates
This influence of temperature on decomposers can cause a seasonal and/or a diurnal pattern of microbial activity.
More carbon dioxide is released during the warmer parts of the year/day.

Mineralization: Transformation of organic compounds to mineral nutrients
Immobilization: Uptake and assimilation of mineral nutrients by decomposers. It is the reverse of mineralization.
Net mineralization rate = Mineralization rate − Immobilization rate