I don't have the time to get into this discussion as deeply as I would like, but MrSmith figured it out in his earlier comment regarding field curvature.
I've tested (extensively so) the Sigma 35/1.4 as well as the Nikon 35/1.4G and 35/1.8G FX lenses, although the 35/1.4G was sold after I chose the Sigma. Field curvature is definitely different - and will impact how we view a test image. In the case of the OP, the test scene favors the Nikons field curvature since that particular type of curvature has objects closer to the camera relative to the focused point being sharper, and of course, vice versa, objects further behind the focus points won't do as well. By only looking at the near grass blades in the example images, it appears the Nikon is the winner. However, if the test scene had included objects of varying distances behind the focused plane as well, you'd see that in some cases the Sigma is the winner. An example I could give would be if you were to photograph a tree at mid distance; a vertical shot, there is subject matter with detail in front of the tree (closer to camera) and behind the tree (ridge line of trees, etc). The tree itself takes up most of the frame vertically. Assuming focus is set on the center of the tree (vertical and horizontally), you'd find the Nikon is sharper in the subject matter closer to you, and the Sigma in subject matter further behind the focus point. In the case where the tree took up most of the frame, the top edges of the tree would be sharper in the Sigma shot as by nature of geometry the top is further away from you than the focus point. The point I'd wish to make in the short time I have is that a single scene, single distance, single iteration test can be misleading and I strongly caution one from reading too much into it. With cameras as highly discerning as the Nikon D800 or D800E, one has to look closer and harder at aspects between comparitive lenses such as field curvature and how the lens performs at various distances. In the case of the Sigma, it performs best at closer distances, while at more typical landscape distances it becomes a case of how one views the field curvature differences relative to the types of work one mostly does. There may not be a single "winner" as it may be more of a case of which lens works the best for each situation or photographer.
Personally I have chosen the Sigma 35/1.4 for closer in distances (where it has no competition) and the Nikon 35/1.8G FX for landscape work, although there are times I will take both lenses along as I've found at longer subject distances the "answer" is not as clear cut. I think the strength of the 35/1.4G is closer in shooting at semi-wide apertures where bokeh is of importance, and I personally didn't find the lens to be quite as good as either the Sigma or the 1.8G version for landscape work, but again, that is within the context of my own work, preferences and acceptance of tradeoffs. I should note the Nikon 35/1.8G FX is stupendously flare resistant. None of these options is a 'bad lens'; rather, one will likely fit one kind of scene better than the other.
