Long-term nitrogen additions and nitrogen saturation in two temperate forests

This article reports responses of two different forest ecosystems to 9 year s (1988-96) of chronic nitrogen (N) additions at the Harvard Forest, Peters ham, Massachusetts. Ammonium nitrate (NH,NO,) was applied to a pine plantat ion and a native deciduous broad-leaved (hardwood) forest in six equal mont hly doses (May-September) at four rates: control (no fertilizer addition), low N (5 g N m(-2) y(-1)), high N (15 g N m(-2) y(-1)), and low N + sulfur (5 g N m(-2) y(-1) plus 7.4 g S m(-2) y(-1)). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations o f dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the p ine stand, nitrate losses were measured after the first year of additions ( 1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (p ercentage of added N retained) over the 9-year period was 97-100% in the co ntrol and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, an d soil extractable pools accounted for only 16-32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil o rganic matter, contains the remaining 88-84%. Short-term redistribution of N-15 tracer at natural abundance levels showed similar division between pla nt and soil pools. Direct measurements of changes in total soil C and N poo ls were inconclusive due to high variation in both stands. Woody biomass pr oduction increased in the hardwood high N plot but was significantly reduce d in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor lead ing to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996.