FOREST ECOSYSTEM RESPONSE TO 4 YEARS OF CHRONIC NITRATE AND SULFATE ADDITIONS AT BEAR BROOKS WATERSHED, MAINE, USA

Nitrogen deposition to forest ecosystems is of growing concern, as tot al N emissions to the atmosphere continue to increase globally. Potent ial negative effects on forests and surface waters include soil and wa ter acidification, mobilization and leaching of aluminum and heavy met als, and nutrient imbalances in trees, In this paper we report the res ults of a chronic nitrogen amendment experiment at the Bear Brooks Wat ersheds in northern Maine (BBWM), and compare them with results from s imilar studies conducted in Massachusetts and Vermont. Treatments incl uded low and high nitrogen (2000 and 4000 eq ha(-1) year(-1) as HNO3), low and high sulfur (2000 and 4000 eq ha(-1) year(-1) as H2SO4), and nitrogen plus sulfur (2000 eq ha(-1) year(-1) each), with three replic ates per treatment. Initial net N mineralization rates were similar in all plots, and net nitrification rates varied between 4 and 9 kg ha(- 1) year(-1) in the control plots over the 3 years of measurement (4-12 % of net annual mineralization). In 1989, net N mineralization rates i n treated plots were equal to or higher than control plot rates for al l but the low S treatment while in 1990, measured rates for all treatm ents were lower than controls, Net nitrification increased in all but the control and low S plots by 1990, representing from 8 to 25% of net annual N mineralization across treatments in that year. Foliar N conc entration in nitrogen treated plots was consistently higher than in th e controls, and those differences generally increased with time, Diffe rences in woody biomass increment and foliar litterfall were not stati stically significant, although tree mortality did increase substantial ly in all but the low S treatment. In general, N leaching losses incre ased with increasing N additions. Nitrogen retention ranged from 93 to 97% of inputs in the control and N amended sites. Measurement of ecos ystem pools shows that 70-92% of inputs to the N treated plots were re tained in the soil pool, similar to estimates obtained by N-15 analyse s. Results from the external plots at Bear Brooks are similar to those from other nitrogen manipulation experiments at the Harvard Forest, M A and Mt. Ascutney, VT in several ways, but N retention was less than expected, We hypothesize that differences in previous land use history have had a greater effect on current N cycling rates than have differ ences in cumulative N deposition.