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Culture media, whether they be soils, nutrient solutions for higher plants, or bacteriological cultures, are subject to fluctuations, often of considerable magnitude, in their H-ion concentrations. Plant and animal fluids and contents of living cells also fluctuate in reaction. One phase of the problem upon which the writer is working involves changes in the reaction of soils, apparently due in part to the changes in the nitrogen cycle. It seemed desirable to investigate in a theoretical way the principal changes that could arise from these and other transformations. The results secured as well as the methods used offer the possibility of wider application than to this specific problem, being perhaps most applicable to the fields of plant physiology, soil chemistry, and bacteriological chemistry. For that reason, they have been prepared for publication. Certain changes in the nitrogen cycle produce acidity, other changes produce alkalinity, and still other changes in the nitrogen cycle have little or no effect on the pH of the medium. Many of these changes have been worked out quantitatively, others only qualitatively.2 Confusion exists in some plant and soil literature bearing on these changes. This paper is an attempt to develop by theoretical reasoning, from the fields of inorganic and biological chemistry, a quantitative expression of the acidity and alkalinity produced by changes in the nitrogen cycle, culminating in a chart which shows these relations graphically. Some attention will be given also to production of acidity and alkalinity resulting from transformations within the carbon and sulphur cycles. A subsequent paper will give experimental data bearing on a part of this cycle. It will be convenient in the following development to speak of the production of one equivalent of "acidity" as meaning also the disappearance of one equivalent of alkalinity, or a fraction of each, such that the sum of the fractional equivalents of acid produced and alkalinity disappearing will equal one; and conversely, with the production of one equivalent of alkalinity. 1 Contribution from the Division of Agronomy, University of California, Davis,, California.

This paper is a compilation of the essential results of our experimental work in the determination of the path of carbon in photosynthesis. There are discussions of the dark fixation of photosynthesis and methods of separation and identification including paper chromatography and radioautography. The definition of the path of carbon in photosynthesis by the distribution of radioactivity Within the compounds is described.

Past treatments of the variation of δS 34 in marine evaporites have either assumed a steady-state ocean or have invoked rather simplified ocean input-output models. This paper derives more completely the relationships between the parameters that influence the time variation of δS 34 in ocean water and the relationship between δS 34 in ocean water and net gains and losses of atmospheric oxygen due to the operation of the sulfur cycle. The lower and mid-Paleozoic are shown to have been periods of net gain of atmospheric oxygen by the operation of the sulfur cycle; the upper Paleozoic, particularly the Permian, a period of oxygen loss. It is difficult to relate these oxygen gains and losses to variations in the oxygen content of the atmosphere, because the oxygen flux due to the operation of the carbon cycle is approximately twice as large as the flux due to the operation of the sulfur cycle. Data for the organic carbon and sulfide content of sedimentary rocks of the Russian Platform suggest that a decrease in sulfide from the Paleozoic to the Mesozoic and Cenozoic Era was roughly balanced by an increase in the proportion of organic carbon; however, such data are insufficient to define the abundance of atmospheric oxygen during the Phanerozoic. Biologic data and a better understanding of controls on atmospheric P o 2 are more likely to produce convincing evidence regarding variations of atmospheric oxygen in the past.

It is generally accepted that the combustion of fossil fuels over the period 1860 to 1954 has produced an amount of carbon dioxide, containing no radiocarbon, that is equal to approxi­mately 13% of the carbon dioxide in the atmosphere. The addition of this 4 'old' carbon dioxide to the atmosphere has observably disturbed the steady-state distribution of carbon-14 in nature. In the present paper measurements are described of the carbon-14 concentration in sets of wood samples from the northern and southern hemispheres, and these show that the carbon-14 specific activity of atmospheric carbon dioxide has decreased by 2.03 ± 0.15% over the period 1860 to 1954, and that the present-day difference between the decrease in the northern and southern hemispheres is less than 0.50%. The response of various mathematical models of the carbon cycle in nature to the addition of ‘old’ carbon dioxide at an exponential rate has been considered. Using the above data in conjunction with these models it is deduced that: (1) The mean life of a carbon dioxide molecule in the atmosphere before it is absorbed into other reservoirs of carbon must be less than 7 yr, and is probably of the order of 2 yr. (2) The exchange time for mixing of the atmospheres of the two hemispheres (i. e. the mean life of a carbon dioxide molecule in the atmosphere of one hemisphere before trans-ferrence to the other hemisphere) is less than 2 yr.

A summary is presented of results obtained in the study of the organic matter contained in sediments. The evolution of this organic matter with age is discussed and its importance is emphasized for the understanding of processes occurring during diagenesis. Organic Geochemistry is a recent Science: a well-documented review published in 1971' lists some 300 references, half a dozen of which refer to papers published before 1945. Reviews"2 and monographs3 have put on record present results and the philosophy of the work, so that we can dispense with a balanced presentation. I shall only give a personal description of our present approach to organic geochemical problems, with no attempt at showing how much more the field owes to other groups, such as those of Calvin, of Eglinton, and of many petroleum companies. A global survey. Most argillaceous sediments studied so far contain some one or two per cent of organic carbon, be it a 'recent' sediment (say, from the Tertiary) or a 'very old' one (say, from the late Precambrian). A small mountain, 100 m high and 1 x 1 km large, may contain some 4 x 106 tons of organic carbon in its minerals; on its surface, if densely covered by vegetation, it may bear only some iO tons of carbon in living trees, plants, animals and micro-organisms: less than one thousandth of its fossil carbon. Such is the result of imperfect recycling. As organisms die, their organic matter decays, and their carbon returns to its most stable form in an oxygencontaining atmosphere: not dust, but carbon dioxide. This leads to the first, short lived, carbon cycle, followed in months or years. Recycling would be accomplished through re-use by photosynthesis, were it not for a flaw: this 'quick' cycle leaks into a second cycle, a long-lived one, followed in aeons. A small portion of organic matter escapes decay, and is buried with carbonates or silicates into sediments. It is preserved until orogenesis and erosion bring it back to the surface (Figure 1). One estimates the total mass of organic carbon accumulated in sediments at some 6 x 1015 tons—the mass of organic carbon present in living organisms at 3 x 1012 tons. Organic Chemists should therefore mostly study rocks! I shall restrict this report to our work on two lacustrine sediments, oil shales found near Strasbourg, in Messel (Germany) and Bouxwiller (France).

The conclusions which have been drawn from the results of C{sup 14}O{sub 2} fixation experiments with a variety of plants are developed in this paper. The evidence for thermochemical reduction of carbon dioxide fixation intermediates is presented and the results are interpreted from such a viewpoint.

The separation of the phenomenon of photosynthesis in green plants into a photochemical reaction and into the light-dependent reduction of carbon dioxide is discussed, The reduction of carbon dioxide and the fate of the assimilated carbon were investigated with the help of the tracer technique (exposure of the planks to the radioactive C{sup 14}O{sub 2}) and of paper chromatography. A reaction cycle is proposed in which phosphoglyceric acid is the first isolable assimilations product. Analyses of the algal extracts which had assimilated radioactive carbon dioxide in a stationary condition ('steady-state' photosynthesis) for a long time provided further information concerning the proposed cycle and permitted the approximate estimation, for a number of compounds of what fraction of each compound was taking part in the cycle. The earlier supposition that light influences the respiration cycle was confirmed. The possibility of the assistance of {alpha}-lipoic acid, or of a related substance, in this influence and in the photosynthesis cycle, is discussed.

Photosynthesis of leafy shoots was determined under a range of CO2 concentrations that spanned past and likely future levels of atmospheric CO2. The temperature regime was also manipulated. Response of photosynthesis to CO2 was pronounced, and differed markedly among species. A substantial rise in atmospheric CO2 is likely to produce shifts in species composition of southern California mountain and foothill vegetation, with several conifer species being disadvantaged. INTRODUCTION How much has the well-advertised increase in atmospheric CO2 affected photosynthesis of land plants? What sorts of response can we expect from further increases in CO2? What might be the outcome in a forest? These questions are explored in this paper. In the late 1800's, concentration of atmospheric CO2 was ca. 285 ppm (Callendar, 1958). According to recent careful measurements (Keeling, 1970), the current mean world CO2 level is close to 324 ppm. Annual rate of CO2 increase is approaching 1 ppm and is rising. Levels of 400 ppm by the year 2000, and 500 ppm by 2020, are predicted (Peterson, 1969). Increased burning of fossil fuel is at the root of this increase. Carbon that was gradually withdrawn from global circulation over several hundred million years is suddenly being returned to the carbon cycle. It can be absorbed by sea water, or even contribute to deposition of new fossil fuel, but such processes take thousands to millions of years (Bolin, 1970). If the earth's fossil fuels are fully exploited over the next several hundred years, we face a precipitous rise in atmospheric C02, followed by an almost equally precipitous drop (Revelle, 1965). It is well known that CO2 is one of the principal limiting factors governing rates of photosynthesis on land. A number of studies have shown that plants respond to increased CO2 up to several thousand ppm (Hesketh, 1963; Madsen, 1968; Gordon and Gatherum, 1969). Enhancement of CO2 in greenhouses by artificial means is now accepted practice in many places. However, little is recorded concerning the response of native vegetation to small increments in atmospheric C02, such as have been experienced in the past 100 years or may be expected in the 50 years to come. A field study in the oak-pine forest on Long Island, New York, indicated that enhancement of CO2 up to about 100 ppm above ambient levels produced a sharp linear rise in photosynthesis of leafy twigs (Wright and Woodwell, 1970). The present study was designed to explore the response of a number of western species to modest CO2 enhancement, and to assess the relation of CO2 response to composition of the vegetation.