300 EMERY, ORR, RITTENBERG 



varying degrees of uncertainty. Although the required data are not as 

 precise as desired, it is believed that an attempt to set up a quantitative 

 budget of nutrients is worthwhile for pointing up discrepancies in data 

 and areas in which more work is needed. 



NITROGEN 



The geochemical cycle of nitrogen includes the atmosphere, as well 

 as the lithosphere and hydrosphere to which the cycles of phosphorus and 

 silicon are essentially confined. In fact, nearly all of the earth's nitrogen 

 is in the atmosphere, where it totals 386 x 10^^ metric tons (Rankama 

 and Sahama, 1950, p. 305). Sedimentary rocks and the hydrosphere 

 contain 7.7 x 10^^ and 2.3 x 10" tons of nitrogen, respectively. The 

 total, about 396 x 10" tons, is more than 100 times the total amount 

 that has been weathered from igneous rocks during the geological past ; 

 therefore most of the nitrogen is presumed either to have been released 

 directly into the atmosphere by volcanic activity or is a remnant of the 

 original atmosphere (Rankama and Sahama, 1950, p. 575; Goldschmidt, 

 1954, p. 443). 



In the ocean nitrogen occurs as molecular nitrogen, nitrate, nitrite, 

 ammonia, and dissolved and particulate organic matter. Although mole- 

 cular nitrogen is dominant (2.2 x 10" tons), its apparent saturation 

 at atmospheric pressure in water from all depths (Rakestraw and Emmel, 

 1938; Hamm and Thompson, 1941) means that its concentration is 

 virtually independent of chemical and biological activity in the water. 

 The quantities of nitrate, nitrite, and ammonia-nitrogen were estimated 

 from concentration-depth curves for the Pacific, Atlantic, and Indian 

 Oceans (Sverdrup, Johnson, and Fleming, 1942, pp. 242-244) by ap- 

 propriately weighting for volumes of water in 1000 m depth zones. The 

 averages are 30, 0.1, and 0.5 jug-a/L, respectively. For the whole ocean, 

 with its volume of 1.37 x 10-^ L, the total is about 5.8 x 10" tons, 

 of which nitrate-nitrogen by itself constitutes 5.7 x 10" tons. To this 

 must be added 3.4 x 10" tons of nitrogen computed from Krogh's 

 (1934) estimate of 0.244 mg/L of nitrogen in dissolved organic matter 

 in the ocean. These values total 9.2 x 10" tons, a quantity that may 

 be called the nitrogen reserve of the ocean (Table 1). To this should 

 be added the nitrogen in the standing crop of organic matter, an extremely 

 uncertain quantity owing to the difficulty of collecting the smaller forms 

 and to the variations in abundance with depth, season, latitude, and other 

 factors. Rough estimates expressed by Vinogradov (1953, p. 131) and 

 by Hutchinson (1953) correspond to w x 10^ tons of nitrogen in the 

 standing crop, an order of magnitude less than a value computed from 



