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Maybe perpetual motion wasn't quite the right term, but I think one can understand what I'm getting at.
As you point out, photosynthesis is probably the major component contributing to the fact that we see 140% return, but it depends on the Calvin cycle which requires Nitrogens and Phosphates. Once the surplus Nitrogen and Phosphorus has been removed from the soil and burned in our cars we are going to have to start fertilizing the fields heavily, and this is where I see production of ethanol relative to the fuel input falling below the 1:1 ratio.
I guess what I'm trying to say is that I believe the 140% figure right now, but known sciences applying on Earth lead us to suspect that this figure cannot be sustained for a long period and for all practical purposes we should be looking for more efficient processes as this is a bandaid and we will need a more permanent fix in place some day.
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