The Earth's atmosphere is largely a biological construct; Venus's isn't.

First, notice that, although Venus has Nitrogen and Argon as apparently minor constituents, the partial pressure of Nitrogen in the Venus atmosphere is 3.2 bars, actually more than the Earth's 0.78 bars, and the Argon partial pressures are very close, 0.009 and 0.006 bars. (The total mass of Venus and the Earth are close enough that you can regard the partial pressure as a proxy for the total mass in comparing the two atmospheres.)

The real difference is in the location of the Carbon. The Earth has 3.5 x 10^-4 bars of CO2 in the atmosphere, while Venus has 89 bars of CO2. However, it turns out that both planets have about the same amount of crustal carbon, but the Earth has most of its carbon in the ground, and some in the oceans and the biosphere. If you convert mass to partial pressure, we have the equivalent of 0.02 bars of CO2 in the oceans, 0.001 bars in the biosphere, and at least 29 bars in crustal rocks. If you could somehow heat the Earth's surface to 470 C, all of that carbon would be driven into the atmosphere as CO2, and the Earth would have an atmosphere a lot like Venus.

The real major difference (in terms of total mass) between the surface regions of Earth and Venus is in the presence or absence of water. The Earth has a lot of water, enough to cover the surface to several km depth if the surface was a perfectly smooth sphere, while Venus has very, very little (enough to cover a smooth surface to a depth of maybe 2 cm). Unless there is some unknown means of sequestering hydrogen in the hot crust of Venus, almost all of its hydrogen has presumably been lost to space. As it happens, models show that the Earth has a "cold trap," that keeps water out of our stratosphere (where solar UV would disassociate it and create free Hydrogen which could be lost to space), while Venus does not, so the current thinking is that this fairly slight difference in atmospheric dynamics is sufficient to dessicate Venus, leaving it in the dry state it is today. There has been a lot of recent debate as to how cold traps affect the "habitable zone" of exoplanets, and the real question is, how do you keep a hot planet from drying out. (I have a feeling there is a biological connection here as well, but that is just my intuition.)

Mars is a different case, which I'll cover in a separate post (unless someone beats me to it).

Marshall's answer is very complete. The role of water in the terrestrial atmosphere has been essential to recycle the atmospheric CO2, and regulate its partial pressure over time. The carbon fixation cycle in rocks really starts when the CO2 is dissolved into rain drops to reach the ground (and the hydrosphere). Several factors of Venus atmosphere evolution have participated, but certainly Venus was born too close to the Sun and water molecule is easily dissociated by UV light. In reference to N2 and O2 species, they are an obvious product of living organism metabolism. In fact, they are being proposed as signatures to detect the presence of life in exoplanets. You could be interested in our book: http://www.springer.com/astronomy/extraterrestrial+physics%2C+space+sciences/book/978-1-4614-5190-7

Another factor is the active plate tectonics on Earth. Venus and Mars have what are called "stagnant lids", which means large-scale crustal motions have ceased. Once CO2 is dissolved in water, the carbon moves through a series of reactions that create first carbonic acid, then bicarbonate, then finally calcuim carbonate, which is solid. At subduction zones, that calcium carbonate can then be cycled deep into the rocks, which is an important mechanism for locking away carbon and keeping it out of the atmosphere.

Both N2 and O2 are biogenic gases meaning that they generated by the living organisms on Earth (1). Biotically NO2 and NO are reduced to N2 by denitrifying bacteria and O2 (and O3) is result of photosynthesis. You should also note that these gases could also be made abiotically, so atmospheric lightening and photolysis by UV light of CO, CO2, SO2 (volcanic gases) lead to the formation of O2 and respective elements (C and S); and N2O, NO, NO2 to O2 and N2; similarly H2O to H2 and O2. But the overall addition of N2 and O2 is small compared to the biological activity.

Presently volcanism is absent on Mars and Venus (?). Also, solar wind has blown away almost all of the Mars’ atmosphere, because it has low gravity, leaving CO2 levels low.

Finally, plate tectonics plays a major role in the recycling of the CO2, meaning that vast majority of the CO2 is locked in the rock and oceans.

Checkout:

(1) Jheeta, S. (July 2013). “Final Frontiers: the hunt for life elsewhere in the Universe.” Astrophysics and Space Science 348(1):1-10. DOI 10.1007/s10509-013-1536-9

(2) Jheeta, S. (December 2013) “Summary of the RAS Specialist Discussion Meeting: ‘Is a moon necessary for the co-evolution of the biosphere of its host planet?’ held on Friday 8th March 2013.” The Observatory Magazine 133(1237): 13:31

In one word LIFE!

I’m not sure I agree with Kenneth’s answer because O2 is highly electronegative, as any O2 released during photodissociation would combine with the electropositive elements, especially Si to form rocks (in the form of sand). The first life forms were not phototrophs but chemolithotrophs (eg methanogens), these being made at the alkaline hydrothermal vents at the bottom of the oceans, where the conditions are anaerobic. In fact phototrophic life (~3.5 billion years, at the time stromatolites) appeared some 200 to 300 million years after the chemolithotrophs (~3.8 billion years, as evidence by sulphur fractionation). More importantly O2 is highly toxic to many life forms, especially those early chemolithotrophs, this is the reason why methanogens live in an aerobic environment. A significant amount of O2 only became detectable around 2.8 billion years ago (as evidenced by the banded iron formation). Between 3.5 and 2.8 billion years all the O2 generated due to photodissociation and phototrophic biological activity was used up to form rocks as above. Finally, photodissociation is a relatively small process compared to the biological activity even during the early life on the Earth.

The total amount of CO2 in the atmosphere of Venus is similar to the total amount of CO2 stored as carbonates in sedimentary rocks on the Earth. This similarity also extends to the Neon to Nitrogen ratios measured in both the atmosphere of Venus, the Earth and even Mars. This is a marked similarity. Beware, when you are making a trip in the distant past of the Earth, which is very tricky. The differences observed to day, in particular oxygen on the Earth, which required the birth of siderobacteria , results from their subsequent history, after the early accretion of the bulk of their atmosphere.

See Callot G., Maurette M. et al (1987) Biogenic etching of microfractures in amorphous and crystalline silicates. Nature 328, 147-151.

I collected dark mud (cryoconite) on the bottom of seasonal and shallow blue lakes that form during summer months on the melt zone of the West Greenland ice sheet. Callot and his colleagues found that they are made of mm-size cocoons of filamentary sidero-bacteria that feed their metabolism while etching out iron-rich grains in the dark sand that they incorporate in their cocoon, and such as....iron-rich carbonaceous micrometeorites. We thus found that upon disaggregation 1 kg of cryoconite release about 6 g of glacial sand. Wonderful life that adapts as to survive in cold and pure water at 0°C. They are also sidero-fungis that do the same job.

I did not quote an important sentence for colleagues with your expertise: siderobacteria are a variety of blue-green algae that adapted their buoyancy as to slowly flow on the floor of the lakes as to capture iron-rich minerals grains deposited there in the cold melt ice water. Remember that on the early Earth we had also some ponds of water on some ice fields, especially during the period of the snow ball Earth. I'll have to contact Gaby Callot to refresh my memory, because I have no collection of the journal Nature around me right now.

Part I (I'll send separately Part 2 in the next mail). This is really bad, my mail as it appears now just above your last comment was drastically shortened. In fact it was about twice longer, and now it makes no sense. II was discovering the astonishing etch patterns registered in ''barred ''cosmic spherules (composed of bars of olivine and glass) recovered from Greenland cryoconite (see our paper). First, in any ''ordinary'' chemical etching, glass is etched out much faster than single crystals of olivine. So, you should observe typical etch canals following the glass bars, with the most resistant olivine bars protruding outside (see our paper). And you observe this ''ordinary'' etch pattern: --in similar barred cosmic spherules recovered from deep sea sediments, at depth of ~5000 meters, where they have been etched for long duration in interstitial water; -- in sediments scooped on the ice surface of central Antarctica (there is no cryoconite there, because it is too cold), where they just suffered a long ''cryogenic -weathering'' in water, during the few hottest days of Antarctica summer, when the Sun preferentially heat up dark material, such as cosmic spherules and dark stony meteorites, and melt the ice around them.

Part II. But spherules extracted from the cocoon of sidero-bacteria recovered from Greenland cryoconite show very different etch canals. Now, the olivine crystals are etched out much faster than the tiny lamella of glass, which now protrude outside the spherules. We reproduced this biogenic etch pattern in the laboratory, but with a pure variety of microscopic ''sidero-fungis'' that we could get from a company (they did not offer pure variety of sidero-bacteria), and their metabolism is known because they are kinds of standard in the busines of soil formation.

So we concluded that the sidero-bacteria, like the sidero-fungis, are adapted to extract iron from the most Fe-rich component, the olivine crystals, as to feed their metabolism. They don't care about crystalline structure. Wonderful adaptive power of these two types of microscopic organisms. And this is imprinted in their genetic code.

Part III, after the talk with Callot, I'll give you additional information and/or correction. You are a good requiring partner to refresh my memory.

I know very well about all these artifacts and about the discovery of the so called fossils in a Martian meteorite by McKays et al.. Read our paper (Callot et al). It was accepted for publication in Nature. The 2 reviewers were very knowledgeable. They did like the paper. By the way, did you read our paper. I am going to have a stimulating talk with Callot, after forwarding to him your comments. Have a nice day.

Dear All,

Now, after the first portion of the common discussion, allow me to give my own answer to this question.  I take into account that, in our days, rather large deposits of niters occur near the Earth surface over the globe (about 5 mln. tons of NaNO3 are extracted in Chile, niter deposits are in western coastal regions of Central and South America, in Russia, etc.), and that each animal and plant cell contains nitrogen. The last fact means that, in the past, the near-surface nitrogen deposits were much larger than they are at present. According to the Life Origination Hydrate Theory (LOH-Theory), just the reactions that led to living matter origination led also to simultaneous formation of gaseous N2 and O2 by the reactions of the following type (below, the integrated reactions are given; detailing is available inOstrovskii V.E., Kadyshevich E.A., Thermochim. Acta, 441(2006) 69–78 and in other publications available at the RG site):

28.2 KNO3(cr) + 38 CH4(g) =

C4H4N2O2(cr)+ C4H5N3O(cr) + C5H5N5O(cr) + C5H5N5(cr)+ 4 C5H10O5(cr)+ 28.2 KOH(cr) + 32.4 H2O(lq) + 6.6 N2(g)  (∆iG0 = –8281 kJ)

15KNO3 + 18 CH4 = C4H4N2O2 + C4H5N3O + C5H5N5O + C5H5N5 + 15 KOH + 19 H2O + 3.5 O2  (∆iG0 = –3327 kJ)  

(In the right-hand side of these equations, N-bases and ribose, i.e., the DNA and RNA components, are given.) The conditions under which these reactions proceeded are given in the papers refered below. Apparently, reactions of such a type were significant, if not unique, source of the O2 and N2 simultaneous formation. These processes proceeded mainly near the ends of the glacial periods of the Earth’s history (Ostrovskii V.E., Kadyshevich E.A., EPSC Abstracts 9, EPSC2014-6, 2014; Kadyshevich E.A., Ostrovskii V.E., J. Therm. Anal. Calorim., 95(2009) 571–578). 

Kenneth,

First, don't make mistakes in arithmetic; such a quantity as 84.6 mol O2 is absent in my formulas. But I forgive you this your mistake, the more so because it is not unique in your numerous issues; I am not the first who write you about this.  Apparently, you regard yourself as a great joker and, in such a sophisticated manner, express your doubts in the fact the Earth' crust contains so large amount of NO3. You could open any geological book to make sure that the Earth contains niter in much larger quantities. If you want to know the sources of niter origination, there is a wide literature on this subject, and some references you can find in our paper: Ostrovskii V.E.,KadyshevichE.A., Life Origination Hydrate Hypothesis (LOH-Hypothesis): Original approach to solution of the problem. Global J. Sci. Frontier Res. (A), Phys & Space Sci 12(6) (2012) 1–36 (pp.12-13). As for your clownish manner of questioning, please, give it up and put your questions in the conventional form, with no «grimaces and jumps»; otherwise, I will ignore your questions. I had already notified you about this. 

You popularize yourself at different RG pages as a retired experienced and versatile scientist and editor-in-chief of scientific journals. This gives me grounds to believe that you should know that any chemical equation has stoichiometric coefficients and that a definite set of the stoichiometric coefficients responds to any redox reaction. The quantities written by me are no more and no less that stoichiometric coefficients. If you keep again something back and want to know something else, go to the trouble entering my paper cited in the previous my issue or one of 20 other papers cited there and read anything you want. As you know as an editor, one man can ask so many questions that a hundred of researchers couldn’t answer to it. If you really want to know in detail our theory of living matter origination, make it. All material is before you at the RG site. In my previous two issues, I named 4 our papers, and, I hope, you will know our theory in detail.

As for NO3, I wrote that the today living organisms contain nitrogen and they received this nitrogen from nowhere but from the near-surface layer of our planet and, after that, as I wrote in my issue, a great mass of NO3 remained in the near-surface ground. In more detail, you can read about this in the paper cited by me (Global J. …). Read these papers, and you will know answers to a great number of your questions.

Kenneth,

I am ashamed of you! You mistake again in the arithmetic for the first class of a primary school. I tried to correct your mistake in my previous issue, but you continue to persist in your mistake. What can I wait from you relative to more complicated tasks? I will try to help you once more:

We have the following equation:

28.2 KNO3(cr) + 38 CH4(g) =

C4H4N2O2(cr)+ C4H5N3O(cr) + C5H5N5O(cr) + C5H5N5(cr)+ 4 C5H10O5(cr)+ 28.2 KOH(cr) + 32.4 H2O(lq) + 6.6 N2(g)

For oxygen:

28.2 ∙ 3 = 84.6 (in the left-hand side of the equation);

2 + 1 + 1 + 4∙5 + 28.2 + 32.4 = 84.6 (in the right-hand side of the equation);

84.6 = 84.6, isn't that so? 

Why, what is the problem?

I give you the following homework: by using the above-given example, calculate, please, the balance of carbon, hydrogen, and nitrogen. 

OK, I concur with elana in all her replies. I too disagreed with Kenneth with his retort to my reply meaning I think it is he who was confused and not I; in my opinion he come across as too heavy handed. I wish not to partake  in this discussion any further and as such I will extricate myself completely - its simply  becoming silly.

Hello Kenneth, I had stimulating talks with colleagues and I found superb references that are all relevant to our recent and very confusing discussions. I want to share that with you. As I had troubles with my internet connection here in the South-East part of France, I want first  to check whether we are still in contact. Let me start just  listing a superb discovery. When I wrote my Nature paper in English, I just translated directly the French word sidéro-bactéries as sidero-bacterias. Well done in the art of...... confusion. Indeed,   this word is not listed in English dictionaries!

The total amount of CO2 injected in the early Earth atmosphere after the formation of the Moon (that blew off the earliest atmosphere) would be equivalent to about 60 bars of atmospheric pressure.  But CO2 has just vanished, because it is soluble in water. It next forms carbonic acid that reacts with rocks as to yields carbonates . They are now stored in sedimentary rocks. The difference with Mars and Venus is that the Earth also accumulated and kept a huge amount of water (about 270 bars) now mostly stored in the oceans.