What are the mechanisms whereby a) plants affect weathering, and b) weathering affects atmospheric co2concentrations? write the major equations involved for the most important reactions.

Explained  below.

Explanation:

Plants roots break the rocks and soil from the bottom and derive up nutrient form the steam to leaves and its due to this factor the planet are considered as an agent in biological weathering like animals that bring up the soil particles and displace the soil matter and expose the rocks at greater depths. Weathering is directly affected by the atmospheric conditions such as the concentration of atmospheric pollutants by the human-induced factors and the weathering created due to the erosion of base rocks and this happens in chemical weathering phases.

An equation is given for reference  

2(HCl) + CaCO3 ---> Ca+2 +2Cl- + H2O + CO2.

Here chemical distinction and breakdown of elements on earth surface take place.