Detritus Food Chains and Biogeochemical Cycles Lecture 8 Chapters 21 and 22

1. Much recycling of materials occurs within organisms Example: water/nutrients withdrawn from senescent leaf tissues of plants  roots Dead matter broken down via detritral food chain Decomposition: multistep process leading to mineralization mineralization: conversion of organic nutrients to mineral form Terms: Fixation = incorporation to organic molecule Mineralization = reduction of organic molecule returning component elements to inorganic associations

Life depends on recycling chemical elements Nutrient circuits in ecosystems involve biotic and abiotic components and are often called biogeochemical cycles Focus on: Each chemical’s biological importance Forms in which each chemical is available or used by organisms Major reservoirs for each chemical Key processes driving movement of each chemical through its cycle

Fixation Mineralization Exchange Pool

Two food chains: Grazing food chain Detritus food chain Herbivore  carnivore Detritus food chain Dead matter and waste from grazing food chain and primary production Provides input to grazing food chain

Microfauna and flora <100um Mesofauna 100um 2mm Macrofauna Detrivore food chain heterotrophs: feed on dead material Provide prey in herbivore foodchain Fragmentation: Microfauna and flora <100um Protozoans and nematodes Mesofauna 100um 2mm Mites, potworms, springtails Macrofauna Millipedes, earthworms, snails, amphipods & isoods Decomposition: Bacteria and fungi – produce extracellular enzymes

Fungi belong to a separate kingdom several groups produce long, thread-like strands (hyphae) reproductive structures may be large and visible

Bacteria: two distinct kingdoms Single celled Microscopic Various shapes Many may not be easily cultured May develop populations quickly

Study of Decomposition – Litterbag Studies Weighed sample in mesh bag placed in soil Withdrawn after time to determine remaining dry-weight Dry weight estimate distorted by biomass of decomposer Gives estimate of decomposition impacted by Species conditions

Other factors which may impact rate of decomposition? Decomposition of red maple leaves more rapid in warmer, more humid climatesdde

Decomposition of different species and materials vary Simple sugars  bacteria and fungi polymers (as cellulose)  mainly fungi, some bacteria Lignins  only certain fungi

Cellulose goes second, degraded by bacteria and fungi Lignin – the stuff of wood, slow to degrade and degredation rate levels off as it remains Cellulose goes second, degraded by bacteria and fungi Proteins, solub. Carbohydrates easily degraded

Fig. 55-15 EXPERIMENT Ecosystem type Arctic Subarctic Boreal Temperate A Grassland G Mountain M B,C D P T H,I S E,F U N L O J K Q R RESULTS 80 Figure 55.15 How does temperature affect litter decomposition in an ecosystem? 70 60 R U O 50 K Q T Percent of mass lost J 40 P D S N 30 F C I M L 20 A H B E G 10 –15 –10 –5 5 10 15 Mean annual temperature (ºC)

The Rhizoshpere and Decomposition ~40% photosynthetic dry matter fuels microbial growth in rhizosphere Fuels microbial growth which eventually releases nutrients  increased availability to plants … via mineralization Soil Microbial Loop

Case Study: Nutrient Cycling in the Hubbard Brook Experimental Forest Vegetation strongly regulates nutrient cycling Research projects monitor ecosystem dynamics over long periods The Hubbard Brook Experimental Forest has been used to study nutrient cycling in a forest ecosystem since 1963

(a) Concrete dam and weir Fig. 55-16a Figure 55.16a Nutrient cycling in the Hubbard Brook Experimental Forest: an example of long-term ecological research (a) Concrete dam and weir

Nitrate concentration in runoff Fig. 55-16c 80 Deforested 60 40 20 Nitrate concentration in runoff (mg/L) 4 Completion of tree cutting 3 Control 2 1 Figure 55.16c Nutrient cycling in the Hubbard Brook Experimental Forest: an example of long-term ecological research 1965 1966 1967 1968 (c) Nitrogen in runoff from watersheds

Decomposition in Aquatic systems Impacted by environment Photosynthetic processes at surface Detritus falls to benthic zone Low oxygen, cool temperatures Stratification of water occurs through summer spring/fall turnover events  mixing of water column

Export of resources  loss of nutrients Example: 1. logging Logs = nutrients removed from forest Increased nutrient water flow  nutrient loss via streams Stream salmon fishery Salmon represent nutrient transfer mechanism from sea to terrestrial ecosystem Fire Alters mineralization rate/processes Subsequent leaching/runoff  nutrient loss

Import of nutrients/alteration of normal cycling Possible sources Runoff from adjacent ecosystems Agricultural Municipal/industrial Impacts Toxins – some may accumulate higher levels food chain via bio-accumulation or bio-magnification Eutrophication – nutrient enrichment in lake/pond  biological activity  stimulation of detrital food chain  anoxia Global/environmental – C,N and global climate

Two types of biogeochemical cycles based input source to ecosystems Sedimentary Rock and salt solution phases Include S, P Gaseous Global Include C, N, O Many cycles hybrid Exchange pool Reservoir

Carbon cycle: Closely tied to energy flux Major exchange pool: atm CO2 (at ~0.03% ) Uptake via photosynthesis Immobilized in carbonates of shells, fossil fuels Subject to daily + seasonal flux

The Phosphorus Cycle Phosphorus is a major constituent of nucleic acids, phospholipids, and ATP Phosphate (PO43–) is the most important inorganic form of phosphorus The largest reservoirs are sedimentary rocks of marine origin, the oceans, and organisms Phosphate binds with soil particles, and movement is often localized

Precipitation Geologic Weathering uplift of rocks Runoff Consumption Fig. 55-14d Precipitation Geologic uplift Weathering of rocks Runoff Consumption Decomposition Plant uptake of PO43– Plankton Dissolved PO43– Soil Uptake Leaching Figure 55.14 Nutrient cycles Sedimentation

Nitrogen cycle: N essential to life – amino acids, nucleic acids Atm. N2 stable, difficult bond to break Fixation largely biological (ca 90%); agricultural use requires fossil fuel input

Ammonium form available to plants Fixation of N N Free living aerobics as Azotobacter, & certain cyanobacter Lichen symbionants Mutualists associated with certain plant groups N2 N + N (NH3)2 NH4 H + energy NO3 Ammonium form available to plants Ammonia (gas) Nitrate produced by soil bacteria from ammonium may also be taken up by plants or mineralized to N2 Under acidic conditions converts to ammonium but may be lost to atmosphere

Organic nitrogen is decomposed to NH4+ by ammonification, and NH4+ is decomposed to NO3– by nitrification Denitrification converts NO3– back to N2

NO3 – NH3 NH4 + NO2 – N2 in atmosphere Assimilation Denitrifying Fig. 55-14c N2 in atmosphere Assimilation Denitrifying bacteria NO3 – Nitrogen-fixing bacteria Decomposers Nitrifying bacteria Figure 55.14 Nutrient cycles Ammonification Nitrification NH3 NH4 + NO2 – Nitrogen-fixing soil bacteria Nitrifying bacteria

Human activities now dominate most chemical cycles on Earth As the human population has grown, our activities have disrupted the trophic structure, energy flow, and chemical cycling of many ecosystems

Nutrient Enrichment In addition to transporting nutrients from one location to another, humans have added new materials, some of them toxins, to ecosystems

Agriculture and Nitrogen Cycling The quality of soil varies with the amount of organic material it contains Agriculture removes from ecosystems nutrients that would ordinarily be cycled back into the soil Nitrogen is the main nutrient lost through agriculture; thus, agriculture greatly affects the nitrogen cycle Industrially produced fertilizer is typically used to replace lost nitrogen, but effects on an ecosystem can be harmful

Fig. 55-17 Figure 55.17 Fertilization of a corn crop

Contamination of Aquatic Ecosystems Critical load for a nutrient is the amount that plants can absorb without damaging the ecosystem When excess nutrients are added to an ecosystem, the critical load is exceeded Remaining nutrients can contaminate groundwater as well as freshwater and marine ecosystems Sewage runoff causes cultural eutrophication, excessive algal growth that can greatly harm freshwater ecosystems

Fig. 55-18 Figure 55.18 The dead zone arising from nitrogen pollution in the Mississippi basin Winter Summer