An unexpected nitrate decline in New Hampshire streams

Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream NO(3) concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376-381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and chronic N deposition. Between 1973-74 and 1996-97, mean NO(3) concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 mumol/L), Ca(2+) decreased by 24% (8 mumol/L), and Mg(2+) decreased by 22% (5 mumol/L). Nitrate concentrations decreased in every stream in every season, but spatial patterns among streams persisted: Streams draining old-growth stands maintained higher NO(3) concentrations than those draining successional stands. The cause of the NO(3) decline is not evident. Nitrogen deposition has changed little, and although mechanisms such as insect defoliation and soil frost may contribute to the temporal patterns of nitrate loss, they do not appear to fully explain the NO(3) decline across the region. Although the role of Climate remains uncertain, interannual climate variation and its effects on biotic N retention may be responsible for the synchronous decrease in NO(3) across all streams, overriding expected increases due to chronic N deposition and forest aging.