Let those microbes interact with weeds , after all ,  tree ecosystem is like that only . Incidently , microbial diversity alongwith microbial load are considered most responsive to any management input in a terrestrial ecosystem.. ..

Dear Singson,

To determine the impact of different trees on soil, you may use methods such as Transect, Zone or Sectors around trees. Bayala et al 2015 explains well some of the methods. Ref:

Bayala, J., Sanou, J., Teklehaimanot, Z., Ouedraogo, S. J., Kalinganire, A., Coe, R., & van Noordwijk, M. (2015). Advances in knowledge of processes in soil–tree–crop interactions in parkland systems in the West African Sahel: A review. Agriculture, Ecosystems & Environment, 205, 25-35.

Dear Alain..thank you ...i actually mean to ask what procedue...such as study has to be undertaken...to know the effects..other than the normal soil studies such as c sequestration etc

The methods employed by Hydrologic Research stations such as Coweeta in NC are a good example on selecting similar catchments in topography, soils, rainfall, but differences in vegetation, management, etc. and measuring them through time.such as differences in pine type and hardwood, farming, grasslands, etc.  Small catchment study can be a powerful tool is looking for something beyond plot studies.  There are many hydrological and ecological studies that emulate from this approach if you are looking for effects and installing instrumentation and collecting samples to determine them.  

I think , please look at the rhizosphere microbial diversity ..

Srivastava Sir, Thank you...yes microbial diversity is also a good indicator...but will it be highly dependent of the weed community and management...

Let those microbes interact with weeds , after all ,  tree ecosystem is like that only . Incidently , microbial diversity alongwith microbial load are considered most responsive to any management input in a terrestrial ecosystem.. ..

Apart from above Nitrogen and phosphorus are the two most important limiting nutrients of terrestrial, freshwater, and marine ecosystems.

Yes , i agree with you , since they are all the part of such dynamic ecosystem... 

long-term study is surely a best way. However, it is not so often such kind of sites. So, Paird sampling is a candidate method. some people also used chronosequence method. please check reference.s

Thank you Sirs for your valuable answers.. What do you think regarding assessment through the litter studies through incubation

Thats a good procedure to determine the rate of mineralization of litter( called litter bag incubation)  from different trees to facilitate you know the nutrient release pattern , but eventual impact of such litter variation is to be worked out using some effective indices representing rhizosphere environment . ..

Studying its litter fall effect on soil property to know nutrient depletion status may be  one method. Other methods will be studying land use competition with adjacent lands and water depletion status.

Thank you..i feel that if the litters can be studied through incubation for atleast 150 days mixing with soil at a constant moisture may be one good option....where other factors will be absent such as topography, etccc

What impact on tree rhizosphere , you anticipate...?