Interactive Effects of Vegetation Type and Topographic Position on Nitrogen Availability and Loss in a Temperate Montane Ecosystem
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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.
Keywordssoil nitrogen topographic wetness nitrate leaching plant–soil interactions
We are grateful to Suvankar Chakraborty for assistance with stable isotope analyses, and to Christina Woltz, Dave Eiriksson, Steven Hall, Brett Boyer, Simone Jackson, Rachel Gabor, and Mallory Millington for their help with field and laboratory work. Funding provided by US National Science Foundation Grant DBI-1337947 was awarded to G. Bowen, and EPSCoR Grant IIA 1208732 was awarded to Utah State University. Friends of Red Butte Creek at the University of Utah also provided funds that supported this work.
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