Forest ecosystem response to four years of chronic nitrate and sulfate additions at Bear Brooks Watershed, Maine, USA

Nitrogen deposition to forest ecosystems is of growing concern, as total N emissions to the atmosphere continue to increase globally. Potential negative effects on forests and surface waters include soil and water acidification, mobilization and leaching of aluminum and heavy metals, and nutrient imbalances in trees, In this paper we report the results of a chronic nitrogen amendment experiment at the Bear Brooks Watersheds in northern Maine (BBWM), and compare them with results from similar studies conducted in Massachusetts and Vermont. Treatments included 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 replicates 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 in treated plots were equal to or higher than control plot rates for all but the low S treatment while in 1990, measured rates for all treatments 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 concentration in nitrogen treated plots was consistently higher than in the controls, and those differences generally increased with time, Differences in woody biomass increment and foliar litterfall were not statistically significant, although tree mortality did increase substantially in all but the low S treatment. In general, N leaching losses increased with increasing N additions. Nitrogen retention ranged from 93 to 97% of inputs in the control and N amended sites. Measurement of ecosystem pools shows that 70-92% of inputs to the N treated plots were retained in the soil pool, similar to estimates obtained by N-15 analyses. Results from the external plots at Bear Brooks are similar to those from other nitrogen manipulation experiments at the Harvard Forest, MA 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 differences in cumulative N deposition.