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Understanding Nutrient Limitation in Ecosystems: Implications for Productivity and Sustainability

This article explores the importance of studying nutrients in ecosystems, how nutrients control vegetation response to environmental changes, and the impacts of nutrient limitation on ecosystem productivity and food production. It also discusses nutrient cycling, micronutrients, and the balance of nutrients required for maximum plant growth. The article concludes with an overview of nutrient uptake and use in plants, nutrient loss mechanisms, and plant nutrient use efficiency.

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Understanding Nutrient Limitation in Ecosystems: Implications for Productivity and Sustainability

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  1. What is a nutrient?

  2. Why study nutrients? • Basic control over ecosystem structure and function • Constrains productivity of the biosphere • Constrains production of food and fiber • Agriculture and N deposition has altered global N cycle • Nutrients control vegetation response to elevated CO2, climate change, etc.

  3. Nutrient cycling: The exchange of “mineral elements” among plants, soil, and soil microbes From the plant’s perspective, belowground resources limit GPP, NPP, and decomposition

  4. Nutrient limitation: Plant demand is greater than supply from biotic and abiotic reservoirs Short-term: Transformations of nutrients from “unavailable” organic forms to “available” organic and mineral forms do not meet plant demand Long-term: Balances between nutrient inputs and outputs do not meet demand of potential biota

  5. Micronutrients: Cl, Fe, B, Mn, Zn, Cu, Mo

  6. The balance of nutrients required to support max. growth is similar for most plants • Redfield ratio: 16 N : 1 P (mass based) • Any nutrient present at less than the optimal balance is likely to limit growth • Plants invest preferentially in gain of the nutrient that most strongly limits growth

  7. Nutrient limitation is operationally defined: If nutrient availability increases, plant production increases Indirect evidence: Element ratios (N:P) of plants and soil N isotopes Direct evidence: Experimental addition of nutrients to meet plant demand Most common response to: N, P and N x P

  8. “Historic” fertilization experiment • 10 gN and 5 gP•m-2 •yr-1 added to replicate plots of tussock tundra since 1981

  9. 450 400 Control Fertilized 350 300 250 200 ANPP (g m-2 yr-1) 150 100 50 0 1982 1987 1992 1997 Year Effect of fertilization on aboveground net primary productivity of vascular plants Cumulative vascular ANPP over 20 years (g m-2): C=2537 F=5506

  10. Global patterns of nutrient limitation • N stimulates NPP in tundra, boreal forest, correlated with NPP in temperate forests • P stimulates NPP in weathered tropical soils (not many studies) • Some temperate forests may be limited by base cations, especially when they receive/d high loads of acid N and S deposition • Wetlands: some respond to N, some to P

  11. Will N deposition alter the global C cycle?

  12. 4. Use 3. Loss 2. Uptake 1. Movement to the root Plant nutrient uptake and use

  13. 1. Nutrient movement to the root: diffusion and massflow • Diffusion is most important for N (NO3-), P, K • Mass flow is most important for Ca, Mg, S, micronutrients • Root interception is not important

  14. Active transport moves ions across root cell membranes 2. Nutrient uptake Up a concentration gradient Ion specific carriers Large component of root respiration

  15. Controlled by supply rate at “steady state” 2. Nutrient uptake • After disturbance, controlled by root length and root activity • Enhanced by mycorrhizae

  16. 80% angiosperms, all gymnosperms 2. Nutrient uptake: Mycorrhizae • C exchanged for nutrients (4-20% GPP) • Extend root surface into bulk soil • Increase surface area • Diffusion through mycorrhizae more rapid than through soil water Ectos, AM, ericoid, orchid

  17. Form • NH4+, NO3- , Amino acids • PO42- 2. Nutrient uptake

  18. Plant demand for nutrients increases allocation to nutrient uptake • Increase specific root length • Increase density of ion carriers • C investment in mycorrzhiae 2. Nutrient uptake • But at an ecosystem scale, nutrient supply is the ultimate constraint on nutrient uptake

  19. 4. Use 3. Loss 2. Uptake 1. Movement to the root Plant nutrient uptake and use

  20. Litterfall > leaching > herbivory > exudates 3. Nutrient Loss • Plants resorb 0-90% of N&P at leaf senescence (roots? wood?) • Resorption is not clearly linked to plant nutrient status (but is to water status) • Resorption is sometimes linked to growthform

  21. Nutrient supply effects growth more than it effects nutrient concentration 4. Plant nutrient use (THINK: difference between uptake and loss) • Plant nutrient use efficiency: • Biomass produced per unit nutrient • Mean residence time of nutrient Plant NUE = N productivity x N Turnover Time A* Tn

  22. 4. Plant nutrient use • Infertile sites •  N TT x  N productivity • Fertile sites •  TT x  N productivity • Result:Less difference in NUE across fertility gradients than you would expect given nutrient supply rates

  23. 5. Ecosystem NUE • Ratio of biomass:N lost in litterfall • (Vitousek 1982) • Greatest where production is nutrient limited • Plants maximize Ecosystem NUE in poor soils by reducing nutrient loss through longer-lived tissues, not through increased resorption

  24. Vitousek 1982

  25. 4. Use 3. Loss 2. Uptake 1. Movement to the root Plant nutrient uptake and use

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