Continental scale patterns of soil nitrogen pools and transformations

Abstract:

Nitrogen is a key limiting nutrient and its availability influences carbon sequestration along with air and water quality. A better understanding of spatiotemporal trends in N pools and cycling rates can help shed light into ecosystem processes and assist with land management in the face of global change. Here, measurements from the National Ecological Observatory Network (NEON) are used to explore N dynamics across a set of continentally distributed sites. Wet deposition inputs of inorganic N vary widely in amount and composition across the NEON, but in many places are only weakly related to N forms in the soil, highlighting the importance of soil biogeochemical transformations. Dominant soil inorganic N forms change across the network, yet do not always conform to conventional wisdom. For instance, tundra and boreal sites exhibit surprising dominance of NO₃^- compared to NH₄⁺, even though they should be strongly N-limited, while certain eastern and northern forests are dominated by NH₄⁺ but also exhibit relatively high rates of net nitrification. Sites show variable degrees of N cycling seasonality, with peak greenness serving as a ‘hot moment’ for N mineralization in specific regions but the winter-spring transition in others. Soil geochemical variables like percent carbon and pH appear to be fairly weak predictors of N dynamics, while land cover type and soil order play a more important role. These data are valuable as a benchmark of the N status of a continental set of sites. They will be useful for tracking changes in ecosystem N dynamics over time and can be leveraged to improve N cycle modeling.