Nitrogen cycling in low oxygen regions: how do autonomous, sustained observations factor into the debate on the role of classical denitrification versus anammox Ken Johnson MBARI

Where does NO3- go?

Free energy change (ΔG° kJ/4 e-) Oxic Respiration: -402 O2 + ½C2H3O2-  HCO3- + ½ H+ Denitrification: -398 4/5NO3- + ½C2H3O2- + 3/10H+  2/5N2 + HCO3- + 2/5H2O DNRA: -257 ½NO3- + ½C2H3O2- + 1/2H+ + 1/2H2O  1/2NH4+ + HCO3- Anammox: -358 NH4+ + NO2-  N2 + 2H2O

Mean Conc Input/ Loss Process Residence Time PO43- 2 µmol/L 2x1010 mol/y River input (Paytan & McLaughlin 2009) 100,000 years NO3- 30 µmol/L 230 Tg/y x 14 g/mol Global denitrification (Devries et al 2012) 2,500 years Ocean volume 1.4x1021 Liters Residence time = time required to add (or remove) the amount now present in the ocean.

Eastern Tropical North Pacific (ETNP) 6960, 7558 Arabian Sea 6381

Rates measured with incubations using 15N labeled substrates.

Arabian Sea Denitrification dominates Eastern Tropical South Pacific Anammox dominates

Anammox rate Denitrification rate

To get N*, we need PO43-, which we don’t measure! (PO43-) = (PO43-)preformed + AOU * 1/150 (PO43-)preformed = 0.9545 - 0.10875 x T + 0.002298 x T2 + 0.00264 x Longitude - 0.00420 x Latitude

N* at 200 m (Gruber and Sarmiento, 1997)

Float 6960 ETNP

Float 7558 ETNP

Float 6381 Arabian Sea

Arabian Sea

Arabian Sea Denitrification dominates Eastern Tropical South Pacific Anammox dominates

Anammox rate (mmol N/m2/d) Anammox rate (mmol N/m2/d). Dashed line is region with high NO2- (Naqvi, 1991).

Arabian Sea -5 µmol/L/ 45 days = 50 nmol N2/L/d -5 µmol/L/ 45 days x 200 m = 22 mmol/m2/d

Float 6960 ETNP

5 µmol/L is a typical deficit in the N* minima If they have a time scale of about 2 weeks 5 µmol/L /14 d = 14 nmol/L/h

Anammox rate Denitrification rate

Physics has a big influence on observations (Float 7558 ETNP, 220 to 240 m)

Arabian Sea -5 µmol/L/ 45 days = 50 nmol N2/L/d -5 µmol/L/ 45 days x 200 m = 22 mmol/m2/d

The N-Cycle is highly perturbed The N-Cycle is highly perturbed. Human production of fixed nitrogen (nitrate, ammonia, urea) now exceeds the natural rate on land. The Economist, May 26, 2011

Norman Borlaug, father of the “Green Revolution” and 1970 Nobel Peace Prize winner -- “the application of low-cost nitrogen derived from synthetic ammonia, has become an indispensable component of modern agricultural production (nearly 80 million tonnes consumed annually). It is estimated that 40% of today’s 6 billion people are alive, thanks to the Haber-Bosch process of synthesizing ammonia” (Borlaug, 2000). The Borlaug “Hypothesis”: Higher agricultural yields are critical to preventing further deforestation – (but more generally high yields can limit NET environmental impact because less land is converted to agriculture, my interpretation).

Distance from Coast (km) Number of Papers % Negative O2 Trend 0-30 41 70.7 30-100 19 68.4 100+ 40 77.5 Published time series Distance from Coast (km) Number of Sites % Negative O2 Trend 0-30 424 64.2 30-100 632 57.6 100+ 2330 49.1 Analyze entire data set (all time series)