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H 375 O 132 C 88 N 6 Ca 1 P 1 H 263 O 110 C 106 N 16 P 1 http://www.smithsonianmag.com/ H 263 O 110 C 106 N 16 P 1 H 375 O 132 C 88 N 6 Ca 1 P 1 N Fixation? Biological Nitrogen Fixation Evolved 3.5 billion years ago Converts N 2 into NH 3

  1. H 375 O 132 C 88 N 6 Ca 1 P 1

  2. H 263 O 110 C 106 N 16 P 1 http://www.smithsonianmag.com/

  3. H 263 O 110 C 106 N 16 P 1 H 375 O 132 C 88 N 6 Ca 1 P 1

  5. N Fixation?

  6. Biological Nitrogen Fixation Evolved ≈3.5 billion years ago Converts N 2 into NH 3 Energetically expensive (16ATP for 1 N) Poisoned by oxygen

  7. Photosynthesis requires nitrogen C 55 H 72 O 5 N 4 Mg

  8. Oxygenic photosynthesis and N fixation evolve in an anoxic world, but are so important that they remain basically unchanged for 3 billion years.

  9. Today photosynthesis and other key processes are limited by nitrogen

  10. Nitrogen Cycle Basics Transformations mediated by microbes (auto and heterotrophic) Inputs via fixation + deposition Outputs via leaching and gas losses (denitrification) + Mineral forms used by plants - and NH 4 + ) + little DON (NO 3 Losses of nitrate discriminate against 15 N, leaving it behind. The N cycle, like life, is a redox driven process.

  11. The N Cycle is Driven by Redox

  12. Ecosystem Mass Balance IF Inputs > Outputs THEN N pools grow Losses grow

  13. Ecosystem Mass Balance IF Inputs > Outputs THEN N pools grow Losses grow IF Outputs > Inputs THEN N pools shrink Losses shrink

  14. Ecosystem Mass Balance IF Inputs > Outputs THEN N pools grow Losses grow IF Outputs > Inputs THEN N pools shrink Losses shrink IF N availability > demand THEN Available N losses high

  15. Ecosystem Mass Balance IF Inputs > Outputs THEN N pools grow Losses grow IF Outputs > Inputs THEN N pools shrink Losses shrink IF N availability > demand THEN Available N losses high IF N availability < demand THEN Available N losses low

  16. Ecosystem Mass Balance IF Inputs > Outputs THEN N pools grow Losses grow IF Outputs > Inputs THEN N pools shrink Losses shrink IF N availability > demand THEN Available N losses high IF N availability < demand THEN Available N losses low

  17. Global patterns in the N cycle

  18. Fixation highest in tropics Cleveland et al., 1999

  19. NUE lower in tropics Vitousek 1982

  20. - losses higher in tropics NO 3 Brookshire et al 2012

  21. Soil δ 15 N higher in tropics -10 0 10 20 30 100 1,000 10,000 MAT ( ° C) MAP (mm/yr) Craine et al., 2015

  22. Soil δ 15 N higher in tropics Houlton and Bai 2009

  23. When, where and why does N matter in intact tropical ecosystems?

  24. “The tropics” are not one place!!! Townsend et al 2008

  25. Fertilization suggests N matters, but there aren’t enough data to suggest when, where or why. LeBauer and Tresider, 2008

  26. Controls of tropical N availability Inputs: Fixation, Deposition Outputs: Gas losses, leaching Internal cycling: mineralization, nitrification, immobilization, DNRA, FEAMMOX…

  27. Controls of tropical N availability Inputs: Fixation, Deposition Outputs: Gas losses, leaching Internal cycling: mineralization, immobilization…

  28. More fixers in the tropics Hedin 2009

  29. But are they fixing? Sullivan et al, 2014

  30. But are they fixing? Predicted from legume abundance Predicted from nodule sampling Sullivan et al, 2014

  31. But are they fixing? Nitrogen Fixation In Bahia, Brazil (kg N/ha/yr) 10 8 6 4 2 0 11 24 40 Mature (>80 years) Forest Age (years) Winbourne unpubl.

  32. Counting legumes doesn’t work. Counting nodules does 140000 120000 100000 ngN fixed 80000 60000 40000 y = 21.061x + 1097.2 R² = 0.9622 20000 0 0 1000 2000 3000 4000 5000 6000 Nodule Biomass in a Plot (mg)

  33. Our understanding of N inputs is poor Sullivan et al, 2014

  34. Controls of tropical N availability Inputs: Fixation, Deposition Outputs: Gas losses, leaching Internal cycling: mineralization, immobilization…

  35. Outputs: Gas losses Highly variable in space and time, hard to measure Highly variable in space and time, “easy”to measure Impossible to measure in the field

  36. Tropical N gas losses: high but poorly constrained, based on N 2 O Zhuang et al. 2012

  37. N 2 losses: Theory Pilegaard 2013, from Davidson 2000

  38. “Direct” measurement of N 2 emissions N 2 N 2 O Slide from M. Almaraz

  39. N 2 losses: Data From Puerto Rico Almaraz unpubl.

  40. N 2 losses ≠ N 2 O losses Almaraz unpubl.

  41. Our understanding of N outputs is poor Almaraz unpubl.

  42. Controls of tropical N availability Inputs: Fixation, Deposition Outputs: Gas losses, leaching Internal cycling: mineralization, nitrification , immobilization…

  43. “The tropics” are not one place!!! Townsend et al 2008

  44. Heterogeneity is challenging Parent Topograph Time Climate Organisms material y N availability

  45. Heterogeneity is challenging Parent Time Climate Topography Organisms material N availability

  46. How do topography, rainfall, and foliar N influence N availability?

  47. The Osa Peninsula, Costa Rica

  48. Hyperspectral-derived canopy N Lidar derived topography Digital Elevation Models Foliar Nitrogen Maps

  49. Topography Climate

  50. Variation in topography, climate. Drake Bay Piro North Piro South

  51. Topography Piro North Piro South Drake Bay Broad, flat ridge Narrow ridge Narrow ridge

  52. Topography Piro North Piro South Drake Bay Broad, flat ridge Narrow ridge Narrow ridge

  53. Climate Piro North Piro South Drake Bay

  54. Climate Piro North Piro South Drake Bay MAP ~3000 mm MAP ~6000 mm

  55. Sample collection & analyses Piro North Piro South Drake Bay • 3 regions x 4 catenas x 4 transects x 2 seasons Top Shoulder Middle Bottom

  56. N metrics measured Piro North Piro South Drake Bay - -N and NH 4 + -N NO 3 Instantaneous Net nitrification 5 days Net N mineralization δ 15 N 100s – 1000s of years

  57. Topography (δ 15 N o/oo) Piro North Piro South Drake Bay 7 a b b 6 b δ15N was b b b b 5 elevated on 4 3 broad, flat 2 δ δ ridges. 1 0 Top Shoulder Middle Bottom Top Shoulder Middle Bottom Osborne et al, in revision xcoord xcoord

  58. Climate ( δ 15 N o/oo) Piro North Piro South Drake Bay MAP ~3000 mm MAP ~6000 mm 7 mean = 3.46 mean = 4.34* 6 δ15N was 5 4 elevated 3 under drier 2 δ δ conditions. 1 prob > F 0.0363* 0 Top Shoulder Middle Bottom Top Shoulder Middle Bottom Osborne et al, in revision xcoord xcoord

  59. Topography and climate summary Piro North Piro South Drake Bay Osborne et al, in revision Climate Topography effect effect ✗ ✗ - -N and NH 4 + -N NO 3 ✗ ✗ - -N and NH 4 + -N NO 3 ✔ ✔ δ 15 N

  60. Topography Climate Organisms

  61. Organisms High N Low N Osborne et al, in revision

  62. Organisms Osborne et al, in revision

  63. Organisms scale here Osborne et al, in revision

  64. Organisms – link to soil N NO3-N (mg kg-1) NH4-N (mg kg-1) 7 7 6 6 5 5 4 4 3 3 2 2 High 1 1 Foliar N 0 0 Net Nitrification (mg kg-1 d-1) Net Mineralization mg kg-1 d-1 Low Foliar N 7 7 Net Nitrification (mg kg-1 d-1) 6 6 5 5 4 4 3 3 2 2 1 1 0 0 Osborne et al, in revision

  65. Airborne mapping may help deal with heterogeneity at large scales Osborne et al, in revision

  66. The role of trees in driving the N cycling may be more important than we know. Asner et al., 2014

  67. “The tropics” are not one place!!! Osborne et al, in revision

  68. Questions?

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