I haven't used Decagon only prenart or soil moisture company but I was interested in your question.  Looks like the SK20 is best for the analyses you need to run:

http://www.decagon.com/products/hydrology/pore-water-sampling/pore-water-samplers/   They also have a chemical compatibility sheet.

We've used lysimeters made by SoilMoisture Equipment. A couple of things to be aware of. If you use packing material around your lysimeter, this may affect your samples. Watch out for your hose material, because that may affect your TOC (you may have to use teflon tubes which are expensive).

Marija,

I have used SoilMoisture Equipment lysimeters, too, with success:  http://www.soilmoisture.com/prod_details.asp?prod_id=967&cat_id=16

The large size of this lysimeter was convenient as it would yield 1L of water and I had many bottles to fill for different lab analyses.  I sampled for the constituents you mention, and felt the results were good.  One thing to keep in mind is dissolved versus particulates.  The ceramic cup undoubtedly must filter out some particulates (you can get specs on ceramic pore size), so you don't get a true "whole" water sample, if that's important.  Seems like it could be for TN, TOC, NH4.

As Tracy indicated, it's a good idea to be aware of the packing material.  I used silica flour to pack the lysimeters, and I wondered if there could be some effects, e.g. silica concentrations were highest for the initial sample and generally had a downward trend over time (although I didn't investigate these effects further). I did use Teflon tubing.

Best regards,

Andy