M. Christ et al., NITROGEN-RETENTION CAPACITY OF A NORTHERN HARDWOOD FOREST SOIL UNDER AMMONIUM-SULFATE ADDITIONS, Ecological applications, 5(3), 1995, pp. 802-812
To determine the N-retaining capacity of a beech-dominated stand in th
e Hubbard Brook Experimental Forest, we added (NH4)(2)SO4 at three lev
els (40, 160 and 520 kg N . ha(-1). yr(-1)) to replicate plots in mont
hly increments for two years, and measured NH4+ and NO3- concentration
s in monthly samples from zero-tension lysimeters. The two lower treat
ment levels failed to increase N loss through the deepest soils horizo
n when compared to controls. Plots under the highest treatment level r
etained >95% of the added N; NH4+ accounted for 77% of the leaching lo
ss. The treatments caused an accumulation of exchangeable NH4+ in the
soil, but concentrations of exchangeable NO3- remained small, even aft
er samples were incubated at room temperature. Leaching, uptake by pla
nts, and the increase exchangeable NH4+ accounted for 3-25% of the add
ed N. Accumulation of available N without nitrification contradicts co
mmon expectations concerning N saturation. It was not clear why excess
N was not nitrified.