How to evaluate can the CO2 sink and O2 output of trees?

I am involved in the campaign to save an ancient green space, in Bristol, called Castle Park, from a development that will remove at least 20+ large, mature, deciduous trees.
1) How can we evaluate their contribution in removing CO2 and producing O2?
2) How much does their leaf litter contribute to soil fertility?
3) How can we roughly translate girth and height into these in/outputs?

Answers:
all the carbon in a tree comes from CO2 in the atmosphere, so if you can estimate the weight of a tree, and find out the percentage carbon in wood, then you can see how many kgs of CO2 this came from.
thus is you can estimate the weight from height or girth of a tree you have a measure of the rate of CO2 uptake.

not so sure about O2 production. There must be stuff out there that gives an estimate of output. try putting in carbon footprint in google.
A quick look gives 'a broad leaf tree absorbs 730kg of CO2 during it's 100 year life span'. Which doesn't seem a lot.

No idea about the extra soil fertility
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