Humans have greatly increased rates of reactive nitrogen (N) deposition to the biosphere, yet the fate of added N in complex landscapes remains poorly resolved. Heterogeneity of vegetation types (and resulting plant-soil feedbacks) are likely to interact with topo-hydrologic gradients to mediate spatial patterns of N availability and loss, yet the net effects of variation in these two factors are unclear. Here, we ask: how does vegetation type interact with topographic position to influence soil N status and the potential fate of N? To shed light on this issue, we measured a suite of N cycle pools and processes in a protected montane watershed located 12 km from Salt Lake City, UT. In plots that differed in vegetation type (mixed forest vs. herbaceous) and topographic positions (upslope vs downslope), we measured inorganic and organic N pools, potential nitrification rates, nitrate isotopic composition, and N leaching losses.
Results/Conclusions
We found that vegetation was linked to large spatial variation in N availability, with herbaceous sites having bigger nitrate pools, higher nitrate to ammonium ratios, higher nitrification potentials, lower soil carbon:N values, and enriched δ15N values compared to forests – especially those upslope. Terrain exerted a second-order control, with forests in downslope positions displaying more open N cycling and having higher foliar N content compared to upslope forests. Links between soil organic matter stoichiometry and N availability in upslope plots suggested plant-soil feedbacks and litter quality shaped N dynamics, while lack of such relationships, as well as more nitrate at depth, in downslope areas implicated the importance of N subsidies and transport from upper zones. Leaching rates, assessed with and without excluding plant roots, closely tracked N availability, suggesting N status played an important role in regulating hydrologic losses. However, leached nitrate was isotopically distinct from soil extractable nitrate, indicating hydrologic separation of mobile and immobile N pools. Moreover, leaching rates were greatly elevated when roots were excluded, demonstrating the critical role of plants in soil N retention. These results suggest a landscape-integrated, coupled ecological-hydrological perspective is needed for assessing the fate of N in complex landscapes. Disturbances or climatic changes that affect vegetation dynamics and hydrology in the montane zone must be considered when projecting the future fate of deposited N.