I am going to apply sludge and Biochar as a source of nutrients on my field. For this purpose, I have thought to quantify the amount on the basis of nitrogen content in the sample. In such condition, if I want to quantify nitrogen present in my Biochar and sludge, which method of nitrogen determination would be appropriate?

N.B.- As we know, in general when we go for determining available nitrogen content in soil sample, we prefer modified Kjeldahl method. But, practically this method gives us the information about presence of Ammonium-nitrogen and organic Nitrogen present in sample whereas nitrate-nitrogen remains unconcidered. If I want to find out whole nitrogen content present in sample to quantify the "Total N content" so that we can apply it on the field, which method should I follow...

Seeking your kind advice on this topic. Please enlighten me with your thoughts.