Interactive Effects of Vegetation Type and Topographic Position on Nitrogen Availability and Loss in a Temperate Montane Ecosystem

• Published in: Ecosystems (New York) 2017-09, Vol.20 (6), p.1073-1088
• Main Authors: Weintraub, Samantha R., Brooks, Paul D., Bowen, Gabriel J.
• Format: Article
• Language: English
• Subjects: Analysis; Availability; Bioavailability; Biomass; Biomedical and Life Sciences; Denitrification; Depth profiling; Ecological effects; Ecological monitoring; Ecology; Ecosystem assessment; Ecosystems; Environmental Management; Fluxes; Forest biomass; Forests; Geoecology/Natural Processes; Heterogeneity; Hydrology; Hydrology/Water Resources; Isotopes; Landscape; Leaching; Life Sciences; Litter fall; Microorganisms; Mixed forests; Montane environments; Mountain ecology; Mountain ecosystems; Nitrification; Nitrogen; Original Articles; Plant Sciences; Pools; Rooting; Soils; Temperature; Terrain; Topography; Vegetation; Vegetation effects; Vegetation type; Zoology
• ISSN: 1432-9840; 1435-0629
• DOI: 10.1007/s10021-016-0094-8

Determining the fate of deposited nitrogen (N) in natural ecosystems remains a challenge. Heterogeneity of vegetation types and resulting plant–soil feedbacks interact with topo-hydrologic gradients to mediate spatial patterns of N availability and loss, yet net effects of variation in these two factors together across complex terrain remain unclear. Here we measured a suite of N-cycle pools and fluxes in sites that differed factorially in vegetation type (mixed forest vs. herbaceous) and topographic position (upslope vs. downslope) in a protected montane watershed near Salt Lake City, UT. Vegetation type was associated with large variation in N availability—herbaceous sites had larger NO₃⁻ pools, higher NO₃⁻:NH₄⁺ ratios, higher nitrification potentials, lower soil C:N values, enriched δ¹⁵N values, and lower microbial biomass compared to forests, especially those upslope. Downslope sites tended to exhibit higher N availability and indicators of N-cycle openness, but patterns were moderated by vegetation type. In downslope forest, soil NO₃⁻ depth profiles and higher foliar N content suggested trees were accessing deep soil N and transferring it to the surface via litterfall, while more deep soil NO₃⁻ but no change in surface or foliar N suggested herbaceous cover was not N limited or deeper N pools were not accessible. Soil NO₃⁻ leaching from below the rooting zone closely tracked N availability, revealing a link between N status and hydrologic loss as well as an important role for roots in N retention. NO₃⁻ isotopes did not reveal a similar link for gaseous losses (that is, denitrification), instead reflecting nitrification and/or transport dynamics. Together, these results suggest a coupled ecological, topo-hydrologic perspective can help assess the fate of N in complex landscapes.