Contrasting stream water NO₃[superscript [-]] and Ca[superscript 2[+]] in two nearly adjacent catchments: the role of soil Ca and forest vegetation

Two nearly adjacent subcatchments, located in the Adirondack Mountains of New York State, US, with similar atmospheric inputs of N (0.6 kmol ha[superscript [-]1] yr[superscript [-]1]), but markedly different stream water solute concentrations, provided a unique opportunity to evaluate the mechanisms causing this variation. Subcatchment 14 (S14) had much greater stream water Ca[superscript 2[+]] and NO₃[superscript [-]] concentrations (851 and 73 [mu]mol[subscript c] L[superscript [-]1], respectively) than Subcatchment 15 (S15) (427 and 26 [mu]mol[subscript c] L[superscript [-]1], respectively). To elucidate factors affecting the variability in stream water concentrations, soil and forest floor samples from each subcatchment were analyzed for total elemental cations and extractable N species. Mineral soil samples were also analyzed for exchangeable cations. Tree species composition was characterized in each subcatchment and potential differences in land use history and hydrology were also assessed. Compared with S15, soils in S14 had significantly higher total elemental Ca[superscript 2[+]] in the forest floor (380 vs. 84 [mu]mol g[superscript [-]1]), Bs horizon (e.g. 1361 vs. 576 [mu]mol g[superscript [-]1]) and C horizon (1340 vs. 717 [mu]mol g[superscript [-]1]). Exchangeable Ca[superscript 2[+]] was also significantly higher in the mineral soil (64 [mu]mol g[superscript [-]1] in S14 vs. 8 [mu]mol g[superscript [-]1] in S15). Extractable NO₃[superscript [-]] was higher in S14 compared with S15 in both the forest floor (0.1 vs. 0.01 [mu]mol g[superscript [-]1]) and Bs horizon (0.2 vs. 0.07 [mu]mol g[superscript [-]1]) while extractable NH₄[superscript [+]] was higher in S14 vs. S15 in the forest floor (7 vs. 5 [mu]mol g[superscript [-]1]). The total basal area of 'base-rich indicator' tree species (e.g. sugar maple, American basswood, eastern hophornbeam) was significantly greater in S14 compared with S15, which had species characteristic of sites with lower base concentrations (e.g. American beech and eastern white pine). The disparity in stream water Ca[superscript 2[+]] and NO₃[superscript [-]], concentrations and fluxes between S14 and S15 were explained by differences in tree species composition and soil properties rather than differences in land use or hydrology. The marked difference in soil Ca[superscript 2[+]] concentrations in S14 vs. S15 corresponded to the higher stream water Ca[superscript 2[+]] and the larger contribution of base-rich tree s