Acidified precipitation was caused by air pollution from power plants and fossil fuel burning Sulfur dioxide (SO2) Sulfuric acid (H2SO4) Nitrous oxide (N2O) Nitric acid (HNO3) pH of rainwater fall to as low as 4
In 1970, Clean Air Act passed to reduce emissions of SO2 and particulate matter from factories and power plants. But forests did not show sign of recovery. Clear Air Act significantly reduced SO2 and N2O
pH=4 pH=5 First, trees die not because of direct effects of high hydrogen ion concentrations, but because of long- term leaching of nutrients from the soil. Second, the natural recovery of forests growing on nutrient- poor soils will require restoration of soil nutrients through the slow process of weathering. Hubbard Brook study held several important lessons for forest ecologists.
Outline (Chapter 24) 20.1 Weathering makes nutrients available in terrestrial ecosystems 20.2 Nutrient regeneration in terrestrial ecosystems occurs in the soil 20.3 Nutrient regeneration can follow many paths 20.4 Mycorrhizal associations of fungi and plant roots promote nutrient uptake 20.5 Climate affects pathways and rates of nutrient regeneration 20.6 In aquatic ecosystems, nutrients are regenerated slowly in deep water and sediments 20.7 Stratification hinders nutrient cycling in aquatic ecosystems Oxygen depletion facilitates regeneration of nutrients in deep waters 20.8 Nutrient inputs control production in freshwater and shallow- water marine ecosystems 20.9 Nutrients limit production in the oceans
Most essential nutrients are recycled within ecosystem Internal cycling Retranslocation or reabsorption Nitrogen fixation (N2) from atmosphere Fertilization Weathering from rocks and sediments Harvest Leaching to groundwater and stream runoff
20.1 Weathering makes nutrients available in terrestrial ecosystems Weathering is the physical breakdown and chemical alteration of rocks and minerals near the earth’s surface. Important nutrients, such as nitrogen, phosphorus, and sulfur, are typically scarce in parent material. Igneous rocks such as granite and basalt contain no nitrogen, only 0.3% phosphate, and only 0.1% sulfate by mass. Most sedimentary rocks contain little more. Hence, weathering adds little of these nutrients to soil. Time: Initial differentiation can be within 30 years. Formation of true soil, 2000 to 20000 years
BIOL 4120: Principles of Ecology Lecture 20: Nutrient Regeneration in Terrestrial and Aquatic Ecosystems Dafeng Hui Office: Harned Hall 320 Phone: 963-5777 Email: email@example.com
Element budget in a watershed Rain gauges are used to measure nutrient inputs. Stream gauges are used to measure nutrient outputs.
20.2 Nutrient regeneration in terrestrial ecosystems occurs in the soil
Many organisms are involved in decomposition Decomposition is the breakdown of chemical bonds formed during the construction of plant and animal tissues. Processes: leaching, fragmentation, changes in physical and chemical structure, ingestion and excretion of waste products. Microbial decomposers: Bacteria are dominant decomposer (to animal) Fungi (to plant) Aided by detritivores 4 major groups Microfauna, mesofauna, macrofauna, and megafauna
Breakdown of leaf litter occurs in four ways 1.Leaching of soluble minerals 2.Consumption by detritivores 3.Breakdown by fungi 4.Breakdown by bacteria Proteins and soluble C are decomposed very fast, then the cellulose and hemicellulose, lignin is very difficult to decompose Proteins etc. 15% Cellulose etc. 60% Lignin: 20%
Decomposition rate measurement Litterbag method Mesh bag (1- 2mm) Mass of remaining in the bag includes both original plant matter as well as bacteria and fungi that have colonized and grown on the plant litter. Carbon is lost to the atmosphere as CO2 in the process of respiration
20.3 Rates of Decomposition and influencing factors Rate at which nutrients are made available to primary producers is determined largely by rate of decomposition. influenced by: temperature, moisture, chemical compositions of leaves Decomposers
Physical conditions influence litter decomposition O2 concentration Decomposition of Spartina litter is more efficient in aerobic than anaerobic conditions Lack of fungi, which require oxygen for respiration, hinders the decomposition of lignin component, slow the decomposition rate. Valiela 1984 Litter bags on the marsh surface or buried 5-10 cm below surface
Changes in climate influence litter decomposition Decomposition of red maple litter at three sites (litter quality similar for three sites). Warm and wet conditions, decompose fast (3 studies) 7.2oC, 621 mm 12.2oC, 720 mm 14.4oC, 806 mm
Temperature influence on decomposition Diurnal changes in air temperature and decomposition in a temperature deciduous forest (Whitkamp and Frank 1969)
20.4 Nutrient regeneration can follow many paths Monomers: monomeric subunit of large organic polymers such as amino acids, nucleic acids. Depolymerization is accomplished by microorganisms secreting enzymes and other reactive substances.
Mineralization, immobilization and net mineralization rate Mineralization: a process that microbial decomposers –bacterial and fungi- transform nitrogen and other elements contained in organic matter compounds into inorganic (or mineral) forms. Organic N ammonia (waste product of microbial metabolism) Immobilization: uptake and assimilation of mineral nitrogen by microbial decomposer. N used by microbes to grow Net mineralization rate: different between the rate of mineralization and immobilization
Nitrogen remaining in the litter during decomposition Initial phase (A) leaching soluble N, then immobilized by microbes, then net N release from litter.
Chemical compositions of leaves in response to nutrients C:N ratio low C:N ratio – high protein level High C:N ratio – low in proteins, high in lignin and secondary metabolites Leaf C:N ratio is influenced by nutrients availability in the environment Leaf C:N ratio influences decomposition rate and interactions with herbivores Nutrient requirements for compensatory growth
N content influence the decomposition Under high N, the initial N can exceed the rate of immobilization from onset of experiment, N concentration will not increase
Patterns of immobilization and mineralization of sulfur (S), calcium (Ca), and manganese (Mn) in decomposing needles of Scots pine Five year litterbag experiment
20.5 Mycorrhizal associations of fungi and plant roots promote nutrient uptake Symbiotic association of fungus and root is called a Mycorrhiza: Arbuscular Mycorrhizae ( AM) penetrate cell walls in root tissue and form vesicles or branched structures in intimate contact with root cell membranes. Ectomychorrhizae ( EcM): form a dense sheath around the outsides of small roots and penetrate the spaces between the cells of the root cortical layer Function: promote plant growth, increase a plant’s uptake of minerals by penetrating a greater volume of soil than the roots.
Plant roots and mycorrhizal fungi Fungi assist the plant with the uptake of nutrient from the soil ( Fungi assist the plant with the uptake of nutrient from the soil (extended water and nutrients absorption) Plant provides the fungi with carbon, a source of energy. Endomycorrhizae (a) Ectomycorrhizae (b)
Mycorrhizal fungi work well under poor nutrient conditions
20.6 Key ecosystem processes influence the rate of nutrient cycling Primary productivity determines rate of nutrient transform from inorganic form to organic form (nutrient uptake) Decomposition determines the rate of transformation of organic to inorganic form (N mineralization) Rates of these two determine the internal cycling
Feedback between nutrient availability, NPP and N release
20.7 Climate affects pathways and rate of nutrient retention Climate affects weathering, soil properties, and rate of decomposition of detritus. Tropical soils: Deeply weathered, less clay (can’t hold nutrients) Leach out if not uptake by plants, nutrient-poor soils Why productivity is high? 1) rapid decomposition of detritus under warm, humid conditions; 2) rapid uptake by plants and other organisms from upper-most layer of soils 3) effective retention of nutrients by plants and mycorrhizal fungal associations Litter on the forest floor constitutes only 1-2% of total biomass of vegetation and detritus; <25% of carbon stored in soils Temperate forests: 1) Litter on the forest floor constitutes 20% in needle-leaved forests, 5% in hardwood forests. 2) 50% carbon stored in soils and litter.
Tropical forest ecosystems hold most of their nutrients in living vegetation In an ash-oak forest in Belgium, most of the P and N stored in the soil as detritus, decomposed organic or inorganic nutrients. But in a tropical deciduous forest in Ghana, soil:biomass ratios are much lower.
Eutrophic and oligotrophic soils Two type of soils in tropics Eutrophic soil: well-nourished soils. developed in geological active areas where erosion is high and soils are relative young. bedrock closer to soil surface, weathering adds nutrients more rapidly and soil retain nutrients more effectively Occur: Neotropics -- Andes, Center America, West Indies Oligotrophic soil: nutrient-poor soil develop in old, geologocally stable areas, intense weathering over long periods removes clay and reduce the capacity of soils to retain nutrients. Occur: Amazon Basin In oligotrophic tropical soils, plants retain nutrients by keeping leaves for long periods and by withdrawing nutrients from them before they are dropped. Dense mats of root (and fungi) to help nutrient uptake.
Recap Nutrient regeneration in terrestrial ecosystems occurs in the soil: Litter and soil organic carbon decomposition Influence factors on litter decomposition Nutrient regeneration can follow many paths Mineralization, immbilization and net minerlization Mycorrhizal associations of fungi and plant roots promote nutrient uptake Function of AM and ECM Climate affects pathways and rate of nutrient retention Tropic
Habitat conversion and soil nutrient Habitat conversion changes soil nutrient conditions Deforestation (cutting) and burning
Habitat conversion and soil nutrient Deforestration and convert from forests to crops in the tropics 1.Cutting and burning loss nutrients directly 2.Without plants, nutrients leach out quickly 3.Upward movement of water draws ion and Al3+ to the surface and form laterite (bricklike substance), as soil dries out 4.Surface runoff without plants will cause soil erosion. A study compared soil nutrient changes in three places Canada prairie site C: 8.8 kg/m2; after 65 yrs, reduced by 51%, 1% per year Brazil Forest site C: 3.4 kg/m2; after 6 yrs, decreased by 40%, 9% per year Venezuelan rain forest C: 5.1 kg/m2; after 3 yrs, decreased by 29%, 11% per year
Global warming and boreal forest Boreal forests hold 200-500 Gt of C, 80% of total C in the atmosphere Annual change from a loss of 0.7 metric Ton C to a gain of 0.1 T per ha over four years. Four year total, a slightly loss
In terrestrial ecosystem (shallow water), plants bridge the physical separation between the zones. In deep ocean, there is no direct link. Need a transport system. 20.6 In aquatic ecosystems, nutrients are regenerated slowly in deep water and sediments
Primary productivity in aquatic ecosystems is highest where nutrients regenerated in sediments can reach the photic zone
20.7 Stratification hinders nutrient cycling in aquatic ecosystems Vertical mixing of water Energy input and wind Turbulent mixing in shallow water and upwelling along coasts Thermocline Density and temperature
Turnover of water and nutrient link two zones together
Change in T, light and nutrient influence NPP, and photosynthesis also influences nutrient availability
Vertical mixing can stimulate productivity, and can also reduce productivity, as mixing bring phytoplakton far below photic zone.
20.8 Oxygen depletion facilitates regeneration of nutrients in deep waters During prolonged periods of stratification in freshwater lakes, bacteria respiration in the carbon- rich bottom sediments tend to deplete O2 supply in the hypolimnion. Bacteria switch to sulfate as an oxidizer and produce H2S (hydrogen sulfide) In O2 depleted environments, bacteria have insufficient O2 to nitrify NH4+; Iron and Mg shift to reduced forms (increase solubility). Thus, nutrients accumulate under these reducing conditions.
20.9 Nutrient inputs control production in freshwater and shallow-water marine ecosystems Phosphorus is critical to the productivity of freshwater lakes. Two parts separated, one side added carbon and nitrogen (near), another side added C, N and P (far). With P, photosynthesis cyanobacteria were developed within 2 months P caused eutrophication in a Canadian Lake experiment Productivity depends on external inputs from rainfall and streams and regeneration of nutrients in the lake.
Fertilization and eutrophication Overfertilizaiton Runoff Eutrophication: overproduction of organic matter within a lake or river Consequences: More food source for fish But if too productivity, can lead to imbalance when decomposers of this excess organic matter consume O2 faster than it can regenerated by photosynthesis. O2 depletion
Estuaries and salt marshes High productivity Due to plentiful supplies of nutrients Inputs from rivers and tidal flow and regenerated within ecosystem.
Where does the primary production go? Energy flow diagram shows energy flow in a Georgia salt marsh
Hypoxic zones in Estuaries and Shallow Marine Ecosystems Hypoxia: the depletion of O2 to the extent that acquatic organisms can no longer survive Defined as: 10) Reason: high nutrient concentrations
20.10 Nutrients limit production in the oceans Productivity in the open ocean is typically low Lack of nutrients (nitrogen and phosphorus) Ion and silicon are important and in shortage Temperature and climate factors also influence productivity
Does iron limit marine productivity? Iron addition Increased productivity in the South patch
The Redfiled ratio and nutrient limitation in the open ocean Nutrient concentrations in the ocean need to match the requirements of photosynthetic organisms; otherwise, production would be reduced and abundant nutrients go unused. Alfred Redfield In phytoplankton, N:P is approximately 16:1 (Redfield ratio) Phytoplankton incorporate 16 times as much as nitrogen as phosphorus into their biomass and release the same ratio as they decompose after death. Adding C, C:N:P =106:16:1 Stoichiometry
20.10 Water flow influences nutrient cycling in streams and rivers Nutrient spiraling Jack Webster (Virginia Tech) Because nutrients are continuously being transported downstream, a spiral rather than a cycle better represents the cycling of nutrients. One cycle in the spiral: uptake of one nutrient atom, its passage through food chain, and its return to water, where it is available for re-use. The longer the distance required, the more open the spiral.
Newbold et al. from ORNL Phosphorus at Walker Branch watershed Phosphorus movement: 10.4 m per day Cycled once every 18.4 days One spiral: 190 m