For N, use Kjeldahl, for P calcination or organic matter desstruction by acis as ujsual.

Dry ash method for plants is the best to exract N,and  P . Then boil with double acids method before read on ICP-MS.Please refer the attached paper.

Cordially.

Article Sediment Quality in Arkansas Bait Minnow Ponds

Dear Rezania,

There are many article in literature with standartd methods for both determinations. For nitrogen there are:

1 - Horneck, Donald A., and Robert O. Miller. "Determination of total nitrogen in plant tissue." Handbook of reference methods for plant analysis (1998): 75-83.;

2 - Nelson, D. W., & Sommers, L. E. (1973). Determination of total nitrogen in plant material. Agronomy Journal, 65(1), 109-112.

And, for phosphorus we use these references:

1 - Richards, John E., and M. R. Carter. "Chemical characterization of plant tissue." Soil sampling and methods of analysis (1993): 115-139.

2 - Fiske, C. H., & Subbarow, Y. (1925). The colorimetric determination of phosphorus. J. biol. Chem, 66(2), 375-400.

I hope it helped you and good job

Dr.Salazar,good references suggested.For total nitrogen the standard Kjeldahl method with modification to include nitrate and nitrite(if present  in considerable amounts in the plant materiel) using salicylic acid thiosulphate modification.For total  phosphorus the diacid digestion(9:4 mixture of HNO3:HClO4)followed by colorimetric determination by vanado-molybdate yellow color method can be followed.Total N after digestion can be estimated by distillation or colorimetric methods.