The obvious maybe: water/moisture flow? Non saturation still means there is water that can move around.

The other factor (although not geochemical) would be biological activity/plant uptake?

Use of fertilizers can be another major source around agricultural lands.

There is a huge project now going on in New Hampshire at Hubbard Brook and around the state to look at nitrogen pathways in watersheds. Look for pubs by Mark Green.

Thank you all for your answers. Let me clarify that i am seeking for geochemical processes and not for physical (e.g. moisture, permeability etc) or factors (e.g. fertilizer application). 

Lisa i have tried to find ay relevant references by Mark Green (e.g. scopus) but i found too many researchers with that name.  Can you please tell me his affilliation? 

I read all above answers, but none is true and relevant to the question. First of all organic matter in dissolved and particuate forms are responsible for reproduction and mobility of the NO3. NO3 can produce from both DOM and POM and at the same time they can move with DOM along with water discharge. Second, NO3 mobility can vary based on the quantitative occurence of the of humic and fulvic acids (humic substances). These need some investigation in your study. You can also read book "Photobiogeochemistry of Organic Matter: Principles and Practices in Water Environments" that published from springer in 2013 for some more ideas from Chapter 1 and aso from some relevant reactions from other chapters.  

The description of the dominant processes affecting nitrate mobility depends on scale.  On a soil column (laboratory) scale, the obvious processes are advection-diffusion-adsorption (in both saturated and unsaturated flow to some extent). In the unsaturated zone, concentrations of ions (including nitrate) are affected by differential mass transport of water in the liquid and vapour phase. Other factors not mentioned above include nitrification/de-nitrification processes, including bacterial effects associated with other biota (flora/fauna etc.). None of these may be relevant depending on your definition of "geochemical processes", and operative scale. 

Hi take a look to my paper:

https://www.researchgate.net/publication/260434813_Modelling_nonpoint_source_pollution_by_nitrate_of_soil_in_the_Mateur_plain_northeast_of_Tunisia

Article Modelling nonpoint source pollution by nitrate of soil in th...

In your clarification you state that you look for pure geochemical parameters, but you also mention reduction under anaerobic conditions (a clearly biochemical process). In natural soil and water it will be pretty hard to distinguish between the two.