Nitrogen cycle — fixation, nitrification, denitrification

{
  "nodes": [
    { "id": "n2", "type": "cloud", "label": "N₂ pool — 78% of the atmosphere", "x": 40, "y": 40, "width": 150, "height": 104, "fill": "#dbeafe", "stroke": "#1d4ed8", "bold": true },
    { "id": "lightning", "type": "star", "label": "Lightning fixes 5 Tg/yr as NOₓ", "x": 40, "y": 200, "width": 150, "height": 104, "fill": "#ede9fe", "stroke": "#6d28d9" },
    { "id": "haber", "type": "conveyor", "label": "Haber–Bosch H: out(H) = 120 Tg/yr applied", "x": 40, "y": 360, "width": 150, "height": 104, "fill": "#ede9fe", "stroke": "#6d28d9", "bold": true },
    { "id": "rhizobium", "type": "leaf", "label": "Rhizobium in legume root nodules", "x": 215, "y": 40, "width": 150, "height": 104, "fill": "#dcfce7", "stroke": "#15803d", "bold": true },
    { "id": "nitrogenase", "type": "gear", "label": "Nitrogenase — 16 ATP per N₂ fixed", "x": 215, "y": 200, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309", "bold": true },
    { "id": "freeliving", "type": "sphere", "label": "Free-living Azotobacter, cyanobacteria", "x": 215, "y": 360, "width": 150, "height": 104, "fill": "#dcfce7", "stroke": "#15803d" },
    { "id": "ammonium", "type": "hexagon", "label": "NH₄⁺ pool N_a = 200 Tg held on clay", "x": 390, "y": 40, "width": 150, "height": 104, "fill": "#dbeafe", "stroke": "#1d4ed8", "bold": true },
    { "id": "volatilise", "type": "cloud", "label": "NH₃ loss V: in(V) = 40 Tg/yr off wet urea", "x": 390, "y": 200, "width": 150, "height": 104, "fill": "#fce7f3", "stroke": "#be185d" },
    { "id": "decomposers", "type": "petri-dish", "label": "Organic N pool N_o = 100000 Tg in soil", "x": 390, "y": 360, "width": 150, "height": 104, "fill": "#dbeafe", "stroke": "#1d4ed8", "bold": true },
    { "id": "nitrosomonas", "type": "collate", "label": "Nitrosomonas: NH₄⁺ → NO₂⁻ (aerobic)", "x": 565, "y": 40, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309", "bold": true },
    { "id": "nitrite", "type": "hexagon", "label": "Nitrite NO₂⁻ — short-lived, toxic", "x": 565, "y": 200, "width": 150, "height": 104, "fill": "#dbeafe", "stroke": "#1d4ed8" },
    { "id": "nitrobacter", "type": "collate", "label": "Nitrobacter: NO₂⁻ → NO₃⁻", "x": 565, "y": 360, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309", "bold": true },
    { "id": "nitrate", "type": "water-drop", "label": "NO₃⁻ pool N_n = 100 Tg, mobile in water", "x": 740, "y": 40, "width": 150, "height": 104, "fill": "#dbeafe", "stroke": "#1d4ed8", "bold": true },
    { "id": "leaching", "type": "funnel", "label": "Rivers R: in(R) = 45 Tg/yr leached away", "x": 740, "y": 200, "width": 150, "height": 104, "fill": "#fee2e2", "stroke": "#b91c1c", "bold": true },
    { "id": "eutrophication", "type": "ocean-wave", "label": "Eutrophication — EU limit 50 mg NO₃⁻ L⁻¹", "x": 740, "y": 360, "width": 150, "height": 104, "fill": "#fee2e2", "stroke": "#b91c1c", "bold": true },
    { "id": "plants", "type": "leaf", "label": "Plant N N_p = 4000 Tg in protein", "x": 915, "y": 40, "width": 150, "height": 104, "fill": "#dcfce7", "stroke": "#15803d", "bold": true },
    { "id": "herbivores", "type": "pentagon", "label": "Herbivores eat the plant protein", "x": 915, "y": 200, "width": 150, "height": 104, "fill": "#dcfce7", "stroke": "#15803d" },
    { "id": "excreta", "type": "bucket", "label": "Urea and faeces returned to the soil", "x": 915, "y": 360, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309" },
    { "id": "budget", "type": "clipboard", "label": "Budget: net(total) = 0 Tg/yr at steady state", "x": 1090, "y": 40, "width": 150, "height": 104, "fill": "#ccfbf1", "stroke": "#0f766e", "bold": true },
    { "id": "denitrifiers", "type": "summing-junction", "label": "Denitrifiers: NO₃⁻ → N₂ (waterlogged)", "x": 1090, "y": 200, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309", "bold": true },
    { "id": "n2o", "type": "cloud", "label": "N₂O leak — 273× CO₂ over 100 years", "x": 1090, "y": 360, "width": 150, "height": 104, "fill": "#fee2e2", "stroke": "#b91c1c", "bold": true }
  ],
  "edges": [
    { "id": "e1", "source": "n2", "target": "rhizobium", "label": "symbiotic fixation", "thickness": 2.5 },
    { "id": "e2", "source": "rhizobium", "target": "nitrogenase", "thickness": 2.5 },
    { "id": "e3", "source": "nitrogenase", "target": "ammonium", "label": "N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂", "thickness": 2.5 },
    { "id": "e4", "source": "n2", "target": "freeliving", "label": "free-living fixation" },
    { "id": "e5", "source": "freeliving", "target": "ammonium" },
    { "id": "e6", "source": "n2", "target": "ammonium", "label": "B = 120 Tg/yr — biological fixation", "thickness": 2.5 },
    { "id": "e7", "source": "haber", "target": "ammonium", "label": "H_f = 120 Tg/yr — urea and ammonium nitrate", "thickness": 2.5 },
    { "id": "e8", "source": "lightning", "target": "nitrate", "label": "NOₓ oxidised, falls as HNO₃", "style": "dashed" },
    { "id": "e9", "source": "n2", "target": "nitrate", "label": "L = 5 Tg/yr — lightning" },
    { "id": "e10", "source": "ammonium", "target": "volatilise", "label": "V_a = 40 Tg/yr — alkaline soils lose NH₃" },
    { "id": "e11", "source": "ammonium", "target": "nitrosomonas", "thickness": 2.5 },
    { "id": "e12", "source": "nitrosomonas", "target": "nitrite", "thickness": 2.5 },
    { "id": "e13", "source": "nitrite", "target": "nitrobacter", "thickness": 2.5 },
    { "id": "e14", "source": "nitrobacter", "target": "nitrate", "thickness": 2.5 },
    { "id": "e15", "source": "ammonium", "target": "nitrate", "label": "N_f = 1000 Tg/yr — nitrification" },
    { "id": "e16", "source": "nitrate", "target": "plants", "label": "U_n = 800 Tg/yr — uptake as NO₃⁻", "thickness": 2.5 },
    { "id": "e17", "source": "ammonium", "target": "plants", "label": "U_a = 400 Tg/yr — uptake as NH₄⁺" },
    { "id": "e18", "source": "nitrate", "target": "leaching", "label": "R_l = 45 Tg/yr — leaching" },
    { "id": "e19", "source": "leaching", "target": "eutrophication", "thickness": 2.5 },
    { "id": "e20", "source": "plants", "target": "herbivores" },
    { "id": "e21", "source": "herbivores", "target": "excreta" },
    { "id": "e22", "source": "excreta", "target": "decomposers" },
    { "id": "e23", "source": "plants", "target": "decomposers", "label": "L_t = 1200 Tg/yr — litter and excreta" },
    { "id": "e24", "source": "decomposers", "target": "ammonium", "label": "A_m = 1200 Tg/yr — ammonification", "thickness": 2.5 },
    { "id": "e25", "source": "nitrate", "target": "denitrifiers", "thickness": 2.5 },
    { "id": "e26", "source": "denitrifiers", "target": "n2", "label": "closes the cycle", "thickness": 2.5 },
    { "id": "e27", "source": "nitrate", "target": "n2", "label": "D_n = 160 Tg/yr — denitrification" },
    { "id": "e28", "source": "denitrifiers", "target": "n2o", "label": "incomplete reduction", "style": "dashed" },
    { "id": "e29", "source": "budget", "target": "n2", "label": "245 Tg/yr in, 245 Tg/yr out", "style": "dashed" }
  ]
}