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NUTRIENT CYCLING AND RETENTION Chapter 19 Molles: Ecology 2nd Ed. Chapter Concepts • • • Decomposition rate influenced by temperature, moisture, and chemical composition of litter and environment Plants and animals modify distribution and cycling of nutrients in ecosystems Disturbance increases ecosystem nutrient loss Molles: Ecology 2nd Ed. Phosphorus Cycle • Global phosphorus cycle doesn’t include substantial atmospheric pool Largest quantities of P are in mineral deposits and marine sediments Much of this not directly available to plants Slowly released in terrestrial and aquatic ecosystems – weathering of rocks Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Nitrogen Cycle • Includes major atmospheric pool – N2 Only nitrogen fixers can use atmospheric supply directly Blue-green algae, soil bacteria, bacteria of legume roots, some fungi = energy-demanding process N2 reduced to ammonia (NH3) Molles: Ecology 2nd Ed. • Nitrogen fixers fix nitrogen = anaerobic (stinks) Once N fixed – available to organisms Upon death of organism, N can be released by fungi and bacteria during decomposition Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Carbon Cycle • Moves between organisms and atmosphere due to photosynthesis and respiration In aquatic ecosystems, CO2 dissolves into water – then used by primary producers Although some C cycles rapidly, some remains stored in unavailable forms for long time Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Decomposition Rates • Rate at which nutrients are made available to primary producers is determined largely by rate of mineralization Occurs primarily during decomposition Rate in terrestrial systems influenced by temperature, moisture, and chemical compositions Molles: Ecology 2nd Ed. Decomposition in Temperate Forest Ecosystems • • Melillo et.al. (1982) Litter bags to study decomposition in temperate forests: Leaves with higher lignin:nitrogen ratios lost less mass = higher N availability in soil might have contributed to higher decomposition rates Molles: Ecology 2nd Ed. Fig. 19.7 Molles: Ecology 2nd Ed. Decomposition in Aquatic Ecosystems • • • Gessner and Chauvet (1994) Stream in French Pyrenees Leaves with more lignin decomposed slower Higher lignin inhibits fungi colonization of leaves Molles: Ecology 2nd Ed. Suberkropp and Chauvet • Tulip tree leaves degraded faster in Alabama streams with higher nitrate concentrations Molles: Ecology 2nd Ed. Nutrient Cycling in Streams • Webster (1975) – nutrients in streams are subject to downstream transport Little nutrient cycling in one place Nutrient Spiraling Fig 19.13 Molles: Ecology 2nd Ed. Webster (1975) Spiraling Length = length of stream required for a nutrient atom to complete a cycle Related to rate of nutrient cycling and velocity of downstream nutrient movement Fig 19.13 Molles: Ecology 2nd Ed. Nutrient Cycling in Streams • Spiraling Length: S = VT S = Spiraling Length V = Average velocity of nutrient atom T = Average time to complete cycle Short lengths = high nutrient retentiveness Long lengths = low nutrient retentiveness Fig 19.13 Molles: Ecology 2nd Ed. Stream Invertebrates and Spiraling Length • Grimm (1988): Sycamore Creek, AZ Collector-gatherer insect larvae Mayflies + chironomids Aquatic invertebrates increase rate of N cycling Molles: Ecology 2nd Ed. • How much N do invert’s contribute to nutrient dynamics of stream? Rapid recycling of N by macroinvertebrates increases primary production Excreted and recycled 15-70% of nitrogen pool as ammonia Molles: Ecology 2nd Ed. Fig. 19.14 Molles: Ecology 2nd Ed. • Animals and Nutrient Cycling in Terrestrial Ecosystems Huntley and Inouye (1988) Pocket gophers alter N cycle by bringing N-poor subsoil to surface Molles: Ecology 2nd Ed. MacNaughton et al. (1988) • Positive relationship between grazing intensity and rate of turnover in plant biomass in Serengeti Plain Without grazing, nutrient cycling occurs more slowly through decomposition and feeding of small herbivores Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Plants and Ecosystem Nutrient Dynamics • Fynbos is a South African temperate shrub/woodland known for high plant diversity and low soil fertility Two species of Acacia introduced to stabilize shifting sand dunes Molles: Ecology 2nd Ed. Witkowski (1991) • Compared nutrient dynamics under canopy of native shrub and introduced Acacia Amount of litter similar, but nutrient content was significantly different Acacia – N fixer Molles: Ecology 2nd Ed. Introduced Tree and Hawaiian Ecosystem • Native flora = 1,200 species > 90% endemic ~ 4,600 new species introduced to Hawaii Firetree Myrica faya Molles: Ecology 2nd Ed. Vitousek and Walker (1989) • Invading N-fixing tree Myrica faya is altering N dynamics of Hawaiian ecosystems Introduced in late 1800’s as ornamental or medicinal plant – later used for watershed reclamation Nitrogen fixation by Myrica large N input Leaves contain high N content – High decomposition rate Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. • • • Disturbance and Nutrient Loss From the Hubbard Brook Forest Vitousek et al. 19 forests around N. America 11 deciduous, 8 coniferous Acidic to neutral soils Effects of disturbance and environmental conditions on N loss Molles: Ecology 2nd Ed. Vitousek studies: • • Square meter plots Dig trenches around them, line w/plastic Trenching increased concentrations of nitrate in soil water up to 1,000X Nitrate losses higher at sites with rapid decomposition Uptake by vegetation most important in ecosystems with fertile soils and warm, moist conditions Molles: Ecology 2nd Ed. Similar study on disturbance by forest clearcutting: Fig 19.21 Molles: Ecology 2nd Ed. Flooding and Nutrient Export by Streams • Meyer and Likens found P exports were highly episodic and associated with periods of high flow Annual peak in P input associated with spring snowmelt Most export was irregular because it was driven by flooding caused by intense periodic storms Molles: Ecology 2nd Ed. Summary • • • Decomposition rate influenced by temperature, moisture, and chemical composition of litter and environment Plants and animals modify distribution and cycling of nutrients in ecosystems Disturbance increases ecosystem nutrient loss Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed.