You have two options Sheetal. One is to do the amendments by mass and the other on a surface area basis. For publication I suggest the former, Paul.

Who ! good Question

Slightly Correction

In your question

Biochar Mass M in kg= (kg/m2) ×Depth (m)× Container area (m2) =Kg. m

Not Fit, you must be known Mass Kg=Density X Volume

Biochar amount m (kg)= m = density(ρ) X Volume (V)

Biochar application rate (kg/m3) X {(Depth (m)× Container area (m2)} =m (Kg)

=2 X (1/0.2) X 0.2 X 2.25

4.45 Kg

That means i need to know the bulk density of biochar and soil both, and calculate the biochar amount needed for soil volume (in my case soil volume of 0-20 cm depth) , is it right? thank to both of you.

As per your requirement if you required Biochar gram per kg of soil then both density required,

If you want gram of Biochar per unit area(m2) then NO density required.

If you want Gram of Biochar per unit Volume of Soil(m3) then only Soil volume required not density, the density of Biochar you can calculate with your answer,

e.g.

2 kg /m2(only surface area no depth)

2 kg/m2 in 0.2 m depth means, =2 kg/(1 m2X 0.2 m)=2 kg/(0.2 m3)

Now Biochar density in soil=2 kg/0.2 m3, i.e. 5 kg/m3

If you want Biochar Kg per kg of soil, then Soil density is required

Suppose Soil density is 1.2 gram/cm3 OR 1200 kg/m3

Then Biochar, =5 Kg Biochar /1200kg of soil.

=4.16 Gram Biochar /kg of Soil.

Biochar= 4.16 gram/kg