There are two prominent issues emerge from your proposed experiment. I don't know the soil pH. I suppose it is alkaline where the effect of biochar alone may not  be so conspicuous but being sandy texture , biochar will improve the soil aggregate size and it's stability to trigger  improvement in whole range  of water transmission properties of soil. Use of green manure will not only improve the efficacy of biochar but cut down the dose of biochar as well.

There are two prominent issues emerge from your proposed experiment. I don't know the soil pH. I suppose it is alkaline where the effect of biochar alone may not  be so conspicuous but being sandy texture , biochar will improve the soil aggregate size and it's stability to trigger  improvement in whole range  of water transmission properties of soil. Use of green manure will not only improve the efficacy of biochar but cut down the dose of biochar as well.

Biochar will increase the cation exchange capacity (CEC) of the soil and thereby have a better 'holding capacity' for nutrients in cationic form, such as potassium (K) and ammonium (NH4) as well as essential trace elements (such as Cu, Zn)...

Thanks for your quick reply,

but this query doesn't sufficient for my research,

1. with increasing concentration of biochar, what are the effect of soil and plant nutrient level? Is it increasing with the time period or not?

2. Is soil and plant nutrient retention level depends upon soil CEC due to the presence of biochar or not?

Yes increasing level of biochar 8s likely to improve soil aggregate size , water retention capacity , better soil nutrient reserve and eventually a much better soil microbial load..Regarding second query, biochar has very specific surface and in sandy soils, both soil water and nutrient retention capacity will improve with time..since biochar possess most recalcitrant form of carbon.. 

Combine application of biochar with other source of essential nutrients is expected to be most beneficial to plants, soil and soil microorganisms. Moreover, increasing sole application of biochar of high C:N though may leads to improved soil structure however can results into Immobilization of N especially on low nutrient soil such as sandy soils. This can reduce the growth and yields of crops with short gestation period.

Varinder , you have raised some very good points , let me try to respond...

* With increasing concentration of biochar , there is likelihood of increase in soil aggregate size ,, water holding capacity and other water transmission properties of soil treated with biochar , but the magnitude of these responses will depend upon  texture of untreated soil and to some extent pH...Yes, with time , biochar will display more pronounced effects on soil chemical and biological properties as well...

* Yes, nutrient retention will depend upon soil CEC , finer is the texture of the soil , greater will be the retention of applied nutrients. which will further increase with application of biochar   , depending upon the initial soil texture ...and soil pH will further dictate the changes in soil chemical properties..