SULFUR, NITROGEN, AND CARBON IN THE BIOSPHERE 187 



If we take the net fixation of carbon on earth as 78 X 10 9 tons/year and 

 divide by the area of the earth, carbon metabolism comes out at 15.3 mg cm" e 2 

 year -1 . For nitrogen assimilation, I use the Hitchcock and Wechsler figure, 

 7.9 X 10 9 tons/year, even though I am cited as the authority for the terrestrial 

 half, but the number really comes from Rodin and Bazilevich. 2 l Nitrogen is then 

 a tenth as mobile as carbon on an areal basis, 1.5 mg cm e year . 



For sulfur, I will use a figure which Hitchcock and Wechsler regard as very 

 high but which seems to me about right. Lloyd ' has been looking at the 

 18 : 16 ratio in seawater sulfate and finds it so grossly out of equilibrium 

 with the O of the oxygen as to imply an annual reduction within the ocean of 

 7.8 X 10 9 tons of sulfur, reoxidation of which would yield the observed value of 

 9 18 in oceanic sulfate, 9.7 per mil. The argument is indirect, and the data are 

 scanty, but I cannot help noticing that on an areal basis the metabolism of sulfur 

 comes out at 1.5 mg cm" e 2 year -1 , about the same as on the floor of Linsley Pond 

 and about the same as the metabolized nitrogen. It is now generally accepted 

 that about half the annual sulfur budget of the atmosphere must come from the 

 sea in nonsulfate form. If Lloyd's figures are right, only about 1% of the reduced 

 marine sulfur need reach the atmosphere by any mechanism to make the budget 

 balance. 



So, provisionally, we can set the relative mobilities of metabolized C : N : S 

 as 10 : 1 : 1. These ratios are very different from the proportions occurring in 

 the biosphere, 554 : 7: 1 (by weight); and if we look (Table 2) at the mobile 

 reservoirs from which these proportions are drawn at these relative rates, we find 

 them not even in the same order of rank. Carbon has the smallest mobile 

 (atmospheric plus marine) reservoir, 7.03 g/cm e ; sulfur is next with 240 g/cm e ; 

 and nitrogen has an enormous reservoir, 720g/cm e . Coupling between these 

 reservoirs is clearly very nonlinear. 



The problem of the sulfur balance to which I have been alluding is the old 

 problem of the excess sulfate in continental runoff. A large fraction of the sulfur 

 in rainwater clearly comes from industrial pollution, i.e., from the combustion 

 of fossil fuels and the pollution of rivers by spent sulfuric acid. Another large 

 fraction certainly comes from sea spray, as chloride does, although there are 

 some second-order arguments about the S : CI ratio to be used in estimating 

 how much rainwater sulfate is marine in this sense and how much is excess. The 

 largest fraction — about half the atmospheric budget, according to Kellogg 

 et al. 1 — presumably also comes from the sea, but its isotopic ratio, now 

 measured in several places, shows convincingly that its origin was not in sulfate 

 form. It is this S-depleted fraction that is now presumed to be biogenic, of 

 microbial origin in a reducing environment, as suggested many years ago by E. J. 

 Conway. 



Apart from the lack of direct evidence of H 2 S emission on any scale, let 

 alone the massive scale required, there are two main sources of uncertainty in 

 this picture. One is the fact that much industrial sulfur is as depleted in S as 



