Synchrony of Nitrogen Wet Deposition Inputs and Watershed Nitrogen Outputs Using Information Theory

Nitrogen (N) wet deposition chemistry impacts watershed biogeochemical cycling. The timescale and magnitude of (a)synchrony between wet deposition N inputs and watershed N outputs remains unresolved. We quantify deposition-river N (a)synchrony with transfer entropy (TE), an information theory metric enabling quantification of lag-dependent feedbacks in a hydrologic system by calculating directional information flow between variables. Synchrony is defined as a significant amount of TE-calculated reduction in uncertainty of river N from wet deposition N after conditioning for antecedent river N conditions. Using long-term timeseries of wet deposition and river DON, NO3-, and NH4+ concentrations from the Lamprey River watershed, New Hampshire (USA), we constrain the role of wet deposition N to watershed biogeochemistry. Wet deposition N contributed information to river N at timescales greater than quick-flow runoff generation, indicating that river N losses are a lagged non-linear function of hydro-biogeochemical forcings. River DON received the most information from all three wet deposition N solutes while wet deposition DON and NH4+ contributed the most information to all three river N solutes. Information theoretic algorithms facilitated data-driven inferences on the hydro-biogeochemical processes influencing the fate of N wet deposition. For example, signals of mineralization and assimilation at a timescale of 12 to 21-weeks lag display greater synchrony than nitrification, and we find that N assimilation is a positive lagged function of increasing N wet deposition. Although wet deposition N is not the main driver of river N, it contributes a significant amount of information resolvable at time scales of transport and transformations.

Automated summary

This study investigates the impact of nitrogen wet deposition on watershed biogeochemical cycling using the transfer entropy method. The results show that wet deposition N has a lagged nonlinear effect on river N, with river DON being mostly influenced by wet deposition N. Additionally, the processes of mineralization and assimilation display stronger synchrony than nitrification.