Nutrient Cycling in Ecosystems

• Nutrient cycles involve the storage of chemical elements in nutrient pools, or compartments, and the flux, or transfer of nutrients between pools.
• Decomposition rate is influenced by temperature, moisture, and the chemical composition of litter and the environment.
• Plants and animals can modify the distribution and cycling of nutrients in ecosystems.
• Disturbance generally increases nutrient loss from ecosystems.
• Exchange of nutrients between organisms and the environment is essential to ecosystem function.
• Energy makes a one-way trip through ecosystems, while elements are recycled.
• Nutrients are elements required for development, maintenance, and reproduction.
• Nutrient cycling involves the use, transformation, movement, and reuse of nutrients.

Nutrient Cycles

• Nutrient cycles include storage in nutrient pools or compartments and movement/nutrient flux between pools in an ecosystem.
• Ecologists are interested in factors affecting distribution and rates of flux.
• A nutrient sink absorbs a nutrient faster than it releases it.
• A nutrient source releases nutrients faster than it is absorbed.

Phosphorus Cycle

• Phosphorus is not abundant in the biosphere and has no substantial atmospheric pool.
• Largest quantities of phosphorus are found in mineral deposits and marine sediments.
• It's slowly released in terrestrial and aquatic ecosystems via weathering of rocks.
• Phosphorus in soils aren't in chemical form directly available to plants.
• Mycorrhizae play a role in uptake by plants.

Nitrogen Cycle

• Includes major atmospheric pool (N₂).
• Only nitrogen fixers can use atmospheric supply directly.
• Nitrogen-fixing is an energy-demanding process.
• N₂ can be reduced to ammonia (NH3) under anaerobic conditions by nitrogen fixers., and also occurs with high pressures/energy of lightning.
• Humans industrially convert N₂ to NH4.
• Once fixed, nitrogen is available to other organisms in the ecosystem.

Nitrogen Release

• Nitrogen released from dead organisms by fungi and bacteria through decomposition.
• Nitrogen is released as ammonium, NH₄⁺.
• Bacteria convert NH₄⁺ to nitrate, NO3¯.
• NH4+ and NO3 can be can be used directly by plants, bacteria, and fungi.
• Nitrogen exits the organic matter pool through denitrification by bacteria.
• Denitrification is an energy-yielding process, with NO3 converting to N2.
• It moves nitrogen back to the atmosphere pool.

Carbon Cycle

• Moves between organisms and atmosphere(photosynthesis and respiration).
• In aquatic ecosystems, CO₂ must first dissolve into water before being used by primary producers.
• Some C cycles rapidly and some remains sequestered(unavailable forms) for long periods of time: soils, peat, fossil fuels, and carbonate rock.

Fossil Fuels and Carbon Cycle

• Burning of fossil fuels increases atmospheric CO2 concentrations.
• Increase has been slower than predicted based on the rate of emissions and known carbon sinks
• Oceans and northern/tropical forests are known carbon sinks.

Rates of Decomposition

• Rate at which nutrients are made available to primary producers largely depends on the rate of mineralization.
• Conversion from organic to inorganic form occurs primarily during decomposition.
• Decomposition is the breakdown of organic matter, with release of CO2, and involves chemical and physical processes.
• It's influenced by temperature, moisture, and chemical composition of litter.

Decomposition and Leaf Litter

• Gallardo and Merino studied the impact of chemical and physical factors on the decomposition of leaf litter.
• There was an increased rate of decomposition at a wetter study site.
• Differences in mass loss based on physical and chemical leaf characteristics.
• Tougher leaves with lower nitrogen concentrations decomposed slower.

Lignin/Nitrogen Ratios

• Melillo et al. used litter bags to study leaf decomposition in temperate forests.
• Leaves with higher lignin:nitrogen ratios lost less mass.
• Higher soil N availability might lead to higher decomposition rates.
• Decomposition rates seem positively correlated with temperature, moisture, and AET. Rates of decomposition are generally higher in tropical forests.
• Higher soil concentrations of soluble phosphorus correlated with higher rates of aboveground net primary production, litter fall, and decomposition.
• Leaf decomposition in aquatic ecosystems is affected by species, temperature, and nutrient concentrations.
• Leaves with a higher lignin content decomposed at a slower rate, because higher lignin inhibits the fungal colonization of leaves.
• Leaves decayed faster in streams with higher nitrate and phosphorus concentrations (up to 20 μg/L phosphorus).

Organisms and Nutrients

• Stream nutrients subject to downstream transport, with little nutrient cycling in one place.
• Webster described stream nutrient dynamics as nutrient spiraling.
• Spiraling length is the length of the stream required for a nutrient atom to complete a cycle and is related to the rate of nutrient cycling and the velocity of downstream movement.
• Nutrient retentiveness is the tendency to retain nutrients.

Stream Invertebrates

• Grimm showed aquatic macroinvertebrates significantly increase the rate of N cycling.
• Macroinvertebrates in Arizona creek had daily N ingestion rates of 131% of the creek's rate of N retention.
• Rapid recycling of N may increase primary production as macroinvertebrates excreted/recycled 15-70% of the ammonia pool.
• This reduces T, reduces spiral length, and increases nutrient retentiveness.

Vertebrate Species

• Organisms allocate nutrients in different ways.
• Herbivores and detritivores must overcome low food nutrient content to meet needs.
• Variations in the N:P ratio for species can influence the excreted N:P ratio.
• Vanni et al. (2002) found a negative correlation between excretion ratios of N:P and the N:P of fish and amphibians. Animal transport of nutrients across ecosystem boundaries affects nutrient cycling

Animals and Nutrient Cycling

• Huntly and Inouye (1988) found pocket gophers altered the N cycle by bringing N-poor subsoil to the surface.
• Whicker and Detling (1988) found prairie dog feeding activities affect the nitrogen content of the remaining grass.
• McNaughton proposes the grazer pathway in the ecosystem speeds up the rate of nutrient cycling,. Serengeti herbivores are functionally similar to collector-gatherer stream invertebrates.

Plants and the Nutrient Dynamics

• Plant species influence ecosystem dynamics as variations in nutrient uptake, allocation, and loss affect nutrient cycling.
• Plants in low-nutrient ecosystems tend to grow more slowly, have decreased nutrient demand, allocate more resources to roots, and produce litter with high lignin and low nutrient content that decomposes slowly.
• Vitousek/Walker found the invading N-fixing tree Myrica faya is altering Hawaiian ecosystem N dynamics.
• Introduced late 1800's as ornamental/medicinal, Nitrogen fixation by Myrica is the largest N input because leaves contain high N content and are associated with a high decomposition rate, which increases the N content of soils.

Disturbance and Nutrient Loss

• Deforested and undisturbed stream valleys effects on nitrogen were compared.
• Before deforestation, over 90% of ecosystem nutrients were in soil organic matter.
• Nitrate losses were 40 to 50x higher after deforestation as rapidly growing vegetation after a fire can be a nitrogen sink (Turner et al. 2003).

Nutrients in Streams

• Meyer and Likens (1979) studied long-term phosphorus dynamics in Bear Brook.
• Phosphorus exports were highly episodic and associated with periods of high flow with annual peaks in P input and export occur during autumn leaf fall and spring snowmelt.
• Most export was irregular, driven by periodic flooding.

Explore nutrient cycles focusing on nitrogen, phosphorus, and carbon. Understand the roles of bacteria, fungi, and mycorrhizae. Learn about nutrient sinks, sources, decomposition, and the impact of human activities.