Are there figures for how much nitrogen biochar can absorb?

I'm trying to figure out how much nitrogen is absorbed by biochar before it is saturated and won't take up any more. Does anyone have any ideas for a source of such information? I realize that any such numbers would be a rough approximation.

I did a few quick searches but nothing was very telling.  

Is there a reason why you are specifically wanting numbers on nitrogen?

One thing that a person could do is take some biochar and some compost tea, and measure the volumes or masses of both of the substances separately and then mix the two, and see how much of the volume or mass remains in the char after it has drained on a screen.  This is not just nitrogen, but a more balanced mix of microbes and nutrients.

One number I've read is that Biochar can absorb 90 times it's volume of ammonia gas....very roughly that's about 1% of the biochar's mass.  I don't have a reference for that, but thought I'd toss it out there anyway.  I'll try and dig through my Biochar texts to see if I can get you a better number.

The reason I'm interested in this is that I will be charging biochar to use for an experiment, and it is important that there is not a lot of free nitrogen around which would mess up the experiment. In other words, I don't want the biochar to be a nitrogen sink when I incorporate it in the soil, but neither do I want it to be a source.

The problem may be that nitrogen is not just found as ammonia NH4+ ions but in nitrates, nitrites, amino acids, isocyanates ,cyanides  etc etc a whole host of organic and inorganic chemicals  so actually measuring the nitrogen content as such may not be helpful .

David

In other words, I don't want the biochar to be a nitrogen sink when I incorporate it in the soil, but neither do I want it to be a source.

 As far as the former, if you have a large volume of nitrogen in saturated solution and you immerse the char in it, then you will max out what liquid the char can hold unless you have more solids involved that can fill voids.  The inclusion of more solid materials would be directly related to the amount of surface area that the solids could involve themselves with, thus your biochar and soil particle size would be an important factor, as would your levels of organic matter in the soil.  At any rate, you will likely not experience nitrogen draw down in your soil if you have provided a saturated nitrogen solution to your char.  I'm not sure that you will have much luck with the latter.  If the char is saturated with nitrogen, then it will likely be a source of nitrogen to the soil as some of the soil community will no doubt be incorporated inside the char and thus with the nitrogen.  

Out of curiosity, if this is an experiment, how are you going to set up your controls?    

Out of curiosity, if this is an experiment, how are you going to set up your controls?  

It is a potting soil experiment. I'm testing different base ingredients for potting mixes. Ideally, I'd use the same fertilizer on all the variations. However, biochar without added nitrogen would stunt growth; biochar with surplus nitrogen could boost it. The results of greater aeration, etc. would be impossible to sort out.