Contrasting stream water NO₃⁻ and Ca²⁺ in two nearly adjacent catchments: the role of soil Ca and forest vegetation

Abstract

Two nearly adjacent subcatchments, located in the Adirondack Mountains of New York State, US, with similar atmospheric inputs of N (0.6 kmol ha À1 yr À1 ), 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 2 1 and NO 3 À concentrations (851 and 73 lmol c L À1 , respectively) than Subcatchment 15 (S15) (427 and 26 lmol c L À1 , 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 2 1 in the forest floor (380 vs. 84 lmol g À1 ), Bs horizon (e.g. 1361 vs. 576 lmol g À1 ) and C horizon (1340 vs. 717 lmol g À1 ).Exchangeable Ca 2 1 was also significantly higher in the mineral soil (64 lmol g À1 in S14 vs. 8 lmol g À1 in S15). Extractable NO 3 À was higher in S14 compared with S15 in both the forest floor (0.1 vs. 0.01 lmol g À1 ) and Bs horizon (0.2 vs. 0.07 lmol g À1 ) while extractable NH 4 1 was higher in S14 vs. S15 in the forest floor (7 vs. 5 lmol g À1 ). 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 2 1 and NO 3 À , 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 2 1 concentrations in S14 vs. S15 corresponded to the higher stream water Ca 2 1 and the larger contribution of base-rich tree species to the overstory biomass in S14. Soil under such species is associated with higher net mineralization and nitrification and likely contributed to the higher NO 3 À concentrations in the drainage waters of S14 vs. S15. Studies investigating differences in spatial and temporal patterns of the effects of chronic N deposition on surface water chemistry need to account for changes in tree species composition and how vegetation composition is influenced by soil properties, as well as climatic and biotic changes.

High elevation watersheds in the southern Appalachians: Indicators of sensitivity to acidic deposition and the potential for restoration through liming

“…K. Koch) resulted in greater stream Ca and NO 3 concentrations; whereas, concentrations were lower in the watershed dominated by Fagus grandifolia Ehrhart and Pinus strobus L. McLaughlin (2014) found that although hardwood sites contained significantly greater pools of Ca compared to mixed hardwoodconifer sites, they also lost a greater amount of Ca from the Ohorizon and to stream export. In addition, vegetation that stimulates nitrification, resulting in increased soil and soil solution NO 3 , can also result in increased cation leaching (Lovett and Mitchell, 2004;Christopher et al, 2006). We identified catchment Quercus spp.…”

“…Other mineral soil total cation parameters (Fe:Al ratio, Felsic:Mafic, extractable K:total K), at both the catchment and riparian zone scale, were significantly related to stream pH and Ca:Al molar ratio (Tables 3 and 4). The role of riparian zone characteristics differs among stream acidity indices, perhaps due to differences in location of exchangeable cations and anions within the soil profile (Fölster et al, 2003;Lutz et al, 2012) or differences among watersheds and catchments (Christopher et al, 2006;Talhelm et al, 2012). and Sullivan et al (2011) modeled sensitivity of wilderness areas in western North Carolina USA to SO 4 deposition, they found that sites with soils derived from parent material with low base cation concentrations were more sensitive to SO 4 deposition.…”

“…The effects of vegetation composition on stream chemistry have been noted in several ecosystems (Knoepp and Swank, 1998;Lovett and Mitchell, 2004;Christopher et al, 2006;Wurzburger and Hendrick, 2007;Knoepp et al, 2008). For example, Christopher et al (2006) found that the presence of base rich tree species (Acer saccharum Marsh., Tilia americana Mill., Ostrya virginiana (Mill.)…”

“…For example, Christopher et al (2006) found that the presence of base rich tree species (Acer saccharum Marsh., Tilia americana Mill., Ostrya virginiana (Mill.) K. Koch) resulted in greater stream Ca and NO 3 concentrations; whereas, concentrations were lower in the watershed dominated by Fagus grandifolia Ehrhart and Pinus strobus L. McLaughlin (2014) found that although hardwood sites contained significantly greater pools of Ca compared to mixed hardwoodconifer sites, they also lost a greater amount of Ca from the Ohorizon and to stream export.…”

“…For example, Lutz et al (2012) found evidence that NO 3 is transported to the stream via subsurface pathways across a range of precipitation event sizes, whereas surface soil DOC is only mobilized when soils become connected with the stream after large precipitation events. Research has also shown that seasonal differences in stream chemistry are often due to differences in hydrologic flow paths and soil/stream connectivity (Christopher et al, 2006;Murdoch and Shanley, 2006a,b). We conducted our stream collections seasonally to account for the differences in soil/stream connectivity that are evident due to greater stream baseflow in March-May in the southern Appalachians (Swift et al, 1988).…”