NPP in a grassland 1000 g m2 yr1 of which 40 is aboveground

NPP in a grassland = 1000 g m^-2 yr^-1, of which 40% is aboveground, in leaf tissue with 2% N; the other 60% is belowground, in roots with 1% N. The grassland receives 1 g N m-2 yr-1 in atmospheric deposition, and has no symbiotic N fixers. Assuming that free-living N fixation rates are negligible, compute how much N mineralization is needed to support plant annual N demand (annual uptake). Show your calculations.

Solution

NPP by grassland =1000 g in which 40% or 400g is above in form of leaves tissues. The net nitrogen demand in leaves is 2% therefore 2% of 400g is required nitrogen in leaves.

Required nitrogen in leaves = (400 x 2) /100= 8g

NPP storage in belowground= 1000-400=600g, required nitrogen here is 1%.

Required nitrogen in roots = (600 x 1) /100= 6g

Net amount of nitrogen requires = 8+6 =14g.

Rate of nitrogen uptake per year per m² area = 1g.

Therefore, required mineralization = 14-1 = 13g nitrogen.