LONG-TERM EFFECTS OF EXPERIMENTAL NITROGEN ADDITIONS ON FOLIAR LITTERDECAY AND HUMUS FORMATION IN FOREST ECOSYSTEMS

Decomposition rates and N dynamics of foliar litter from 4 tree specie s were measured over a 72 month period on the Chronic Nitrogen Additio n plots at the Harvard Forest, Petersham MA, beginning in November 198 8. Plots received nitrogen additions of 0, 5 and 15 g N m(-2).yr(-1) i n two different stand types: red pine and mixed hardwood. Bags were co llected in August and November of each year and litter analysed for ma ss remaining, nitrogen, cellulose and lignin content. Mass remaining w as significantly greater for litter in nitrogen treated plots than in control plots after 48 months. Lignin content of litter was significan tly higher with nitrogen treatments but there was little effect of tre atment on cellulose content. N concentration was similar between treat ments, but greater mass remaining in treated plots resulted in a highe r total amount of N in humus produced in the high N plot. This mechani sm could be a sink for up to 1.5 g N.m(-2).yr(-1) of the 1.5 g N.m(-2) .yr(-1) added annually to the high N plots. Reduced decomposition rate s in conjunction with increased lignin accumulation could impact globa l carbon sequestration as well.