Plant Nutrients PDF

A) Mineral Nutrients NUTRIENTS Key Points Water, nutrients come from soil; C from air Nutrients are generally actively taken up as dissolved inorganic ions Decomposition supplies N, but plants must compete with microbes C:N Ratio determines soil fertility Symbiotic N fixation, mycorrhizas Read Chapt...

A) Mineral Nutrients NUTRIENTS Key Points Water, nutrients come from soil; C from air Nutrients are generally actively taken up as dissolved inorganic ions Decomposition supplies N, but plants must compete with microbes C:N Ratio determines soil fertility Symbiotic N fixation, mycorrhizas Read Chapter 4,5; also see pp. 325-331 1 2 Internal recycling Sources of essential elements - plants also aggressively recycle most nutrients internally; majority of N recycled - nutrients removed from old, senescing leaves and translocated to new ones - major part of nutrient budget - one reason herbivory is damaging! - except for C,O, and H, almost all are taken up by root as ions dissolved in soil water (mineral nutrients) - CO2 from air; H2O from soil - organic compounds not generally absorbed by roots (except amino acids) - N, P (and K) most important Plantago major (common plantain) and Dibolia borealis 3 4 1 B) C:N Ratio Uptake by Roots - N commonly most important nutrient - but atmospheric N2 is useless to most plants - instead, N is taken up from soil - most of N in soil comes from decomposition of dead organic material (litter) - but large organic molecules also useless - plants need NO3-, NH4+, sometimes amino acids - so organic matter must first be broken down (mineralized) into these forms by microorganisms, usually bacteria or fungi (but occasionally by animals like foxes!) - most nutrients taken up from soil water as inorganic ions - NH4+, K+, Ca2+, NO3-, H2PO4-, H2PO42-, SO42- requires energy, but can move nutrients against concentration gradients - important since most mineral nutrients are very scarce in soil; must be moved against concentration gradient into the plant - also requires specialized uptake mechanisms, ion channels - positive ions (cations) often exchanged for H+; negative ones for OH- 5 6 PROBLEM Nutrient Cycling - plants need N - but so do decomposers - decomposers get C, N from soil - retain N (immobilization) - but respirate some C as CO2 - but if N is taken up by microbes, how does it get to plants? - key is that N is recycled by microbes but C declines over time Soil [C,N] - depends on soil food web N C Time 7 8 2 RESOLUTION Form of C matters as well - optimal ratio (stoichimetry) for decomposers is C:N ratio of about 25 - as long as soil C:N >25, decomposers have all the C they need, but their growth tends to be limited by N - initially, C:N ratio of dead leaf litter is very high (25:1 to 150:1) (much lower for microbes) - but as microbes respire away CO2, C:N drops below 25 - N becomes relatively overabundant, and decomposers start to free more N than they can use; excess inorganic N is lost to the soil as a waste product plants can now use! Warning: model works best when C is in forms that are easy to break down; recalcitrant forms (lignin) can slow down decomposition, reduce N cycling 9 - cellulose, especially lignin very slow to break down very slowly - cellulose: specialized fungi, some bacteria, some protists - lignin: very few decomposers (fungi) 10 Fungal decomposers of wood Brown (Cuboidal) Rots (degrade cellulose) White Rots (degrade lignin) As a result, decomposition rate depends on litter quality - faster for high N litter - slower for high-cellulose, lignin litter - for example, litter from conifers often decays slower than litter from deciduous plants (less N, more recalcitrant resins, structural materials) - slows rate of transfer of mineral N to plants Wikipedia 11 12 3 Tollund Man C) Peatlands: what happens when the system fails - peat = partially decomposed organic soil (e.g., bogs, fens) - very low available nutrients, since organics not broken down - reason: high rainfall, cool temperatures, poor drainage - result: excess water, low water movement - result: low O2 (water conducts O2 poorly, decomposition of organics uses up any that's available) - result: reduced microbial activity, anaerobic respiration - result: low pH (since partial decomposition, acid fermentation; acidity further inhibits microbes) - final result: decomposition interrupted, peat accumulates, nutrients not released [Also true for pickles: high acidity, low oxygen] - An Iron-age body (c. 400 BC) found in a bog in Denmark - Believed to have been sacrificed (several hundred similar bodies have been found in Danish bogs) - Preserved by the lack of decomposition - When originally discovered in 1950, was initially thought to be a recent murder victim Algonquin Park Wikipedia 13 Hudson Bay Lowlands (N Ontario, Manitoba): largely peatlands - trees mainly on drier ridges where O2 is available in soil, or near rivers where dissolved nutrients are accessible - huge storehouse of C - may be released as CO2 if climate change results in drying (and fire), restarts decomposition 15 14 D) Nutrient Uptake Symbioses Biological Nitrogen Fixation - some prokaryotes can use atmospheric N2 (e.g., many free-living cyanobacteria) - use lots of energy to convert it to NH3 (biological N fixation) - process is poisoned by O2; takes place in specialized cells (heterocysts) under anaerobic conditions 16 4 2) Mycorrhizae D) Nutrient Uptake Symbioses - associations between roots and fungi - several different types that differ in morphology, taxa involved, details of interaction 1) Symbiotic N fixation - some plants exploit this in a directly symbiotic relationship - only a few plant groups: best-known example is root nodules of legumes (peas, beans, etc.), but others exist (e.g., alders) - legumes provide energy (carbohydrates) to Rhizobium or Bradyrhizobium in root nodules in exchange for NH3 - also protect N-fixing bacteria from O2, which poisons N fixation - can allow them to survive in very N-poor soils - unusual, but important: major input of N into ecosystems, soils But two types are especially important... 17 18 2) Mycorrhizae Arbuscular mycorrhizae (= AM, Endomycorrhizae) - penetrate root - used to be considered zygomycetes (glomales) - now in own phylum (Glomeromycota) - hundreds of species; relatively non-host-specific - 75%-80% of terrestrial plants, including herbaceous species, many tropical trees, some temperate trees (like maples) Ectomycorrhizae = (ECM) - surround root - largely basidiomycetes; thousands of species (many mushrooms) - relatively host-specific - 3% plants BUT many woody species - especially at high latitudes; more cold-tolerant Russula sp. www.environment.gov.au/biodiversity/abrs/ publications/fungi/glomus-sporocarps.html 19 20 5 3) Function - improve plant performance, but rarely obligate - usually obligate for fungi - carbohydrates from plant exchanged for nutrients, especially P, but also N - AM fungi work largely by vastly increasing area for nutrient uptake; some ECM also can degrade organic material for N - water uptake and conservation - ectomycorrhizae protect from pathogens; AM much less so 4) Cheaters - orchid seeds lack storage tissues; depend on mycorrhizae as seedlings - in some cases, mature orchid remains a parasite (Coralroots) - true of some other plants as well (Indian Pipes: Ericaceae) - avoid costs of photosynthesis (N demand, water loss, etc.) Coralroot (Orchid) Indian Pipes common ragweed (Ambrosia artemisiifolia): AM-mycorrhizal MacKay & Kotanen (2008) J Ecol 96: 1152-1161 21 Corallorrhiza striata - USDA PLANTS database 22 One more oddity - carnivorous plants Cryptic parasitism by AM fungi - mycorrhizae are most useful in nutrient-poor soils; may not be profitable in rich soils - the wrong combination of fungi and plant can even have negative effects > 600 species in several families - Typically in very nutrient-poor sites - Insects are rich in N! Byblis - traps food on sticky hairs, but Sundew (Drosera) Pitcher Plant (Sarracenia) - traps and absorbs - traps and absorbs insects predatory bugs then digest them, fertilize plant with their insects with glue-tipped in water-filled leaves faeces hairs on leaf - in Ontario - Australia - in Ontario Source: Klironomos (2003) Ecology 84: 2292-2301 23 Monotropa uniflora - USDA PLANTS database USDA PLANTS 24 6