Question

At the start of the CO2 experiment, the soils contained an average inorganic N content of 10 g N m-2. Each year, 4 g N m-2 is mineralized (and this is not affected by atmospheric CO2). Assuming no new inputs from decomposition of plant detritus, N fixation or N deposition, how long (in years) would this extra NPP be supported before N became limiting?

Hint: the extra NPP is the difference between the elevated NPP (750 g C m-2 y-1) and the original NPP (600 g C m-2 y-1).

Answer 1

NPP is often calculated by determining the annual carbon storage increment.

Therefore, using the biomass and carbon storage estimate you can calculate aboveground NPP (sometimes referred to as ANPP) by taking the difference in carbon storage between years.

NPP = Carbon Stored for Year2 - Carbon Stored for Year1

= 10-4

= 6

now original NPP = Carbon Stored for Year2 - Carbon Stored for Year1

=750-600

= 150

the extra NPP is the difference between the elevated NPP and the original NPP

which is 150-6 = 144 years would extra NPP be supported before N became limiting