How to calculate residence time for an element in a reservoir? The residence time for an element in a reservoir can be calculated by the reservoir size at steady state divided by the inflow or outflow rate. Given the following diagram, I need to calculate the mean residence times for nitrogen in the following stocks:


Atmosphere
Fixed Nitrogen on Land




But I am confused by this question, what exactly is its steady state? what exactly is the reservoir size and what it the inflow and outflow?

Thank you

EDIT: Problem-solved with the help of Camilo Rada. The way you calculate residence time is by using the formula:

$\text{Residence time} = \frac{\text{Reservoir size}}{\text{in-flow or outflow rate}}$

In the example I looked at the box in the diagram for atmosphere's content of residence, being $3 \times 10^{20}$ units, that will serve as our total reservoir size, then we can either choose the outgoing or incoming flow (as an approximation as the reservoir is not in steady state), I chose the outgoing, so we add them together: $7 \times 10^{12} + 7 \times 10^{12} = 14 \times 10^{12}$ units per year. Now plug that into the formula:

$RT_{ATMO} = \frac{3 \times 10^{20}}{14 \times 10^{12}} = 21,428,571 \, \text{years} = \, \backsim 21 \, \text{million years}$

It would be the same process for each box.
Steady state is when the size of the reservoirs doesn't change with time (because inflow=outflow). The size of each reservoir is the number inside the boxes and the flow rates are the numbers beside the arrows.
If you calculate the total inflow/outflow from each reservoir, you can use the relationship you did enunciate at the start of the question to easily calculate residence times.

However, according to the diagram those reservoirs are not in steady state. So either the question is ill posed, or you are supposed to make some approximations (or a more complex calculation, but I don't think that's the case).