Impact of transient soil water simulation to estimated nitrogen leaching and emission at high- and low-deposition forest sites in Southern California

Soil water dynamics and drainage are key abiotic factors controlling losses of atmospherically deposited N in Southern California. In this paper soil N leaching and trace gaseous emissions simulated by the DAYCENT biogeochemical model using its original semi-dynamic water flow module were compared to that coupled with a finite element transient water flow module (HYDRUS), for two mixed conifer forests with annual deposition rates of about 70 and 9 kg N ha^-1, in the San Bernardino National Forest. Numerical solution of the Richards equation implemented in HYDRUS water module could improve response of surface soil water dynamics to precipitation pattern, compared to the original, and consequently it resulted in annual N gaseous emission loss about 1.5 ∼ 2 times higher. While the two flow modules predicted similar amounts of annual water drainage, the HYDRUS water module simulated more frequent, but smaller drainage fluxes, which favors soil mineralization and downward transport. In normal precipitation years, annual leaching losses predicted by the HYDRUS coupled DAYCENT model was about 5-18 kg N ha^-1 higher due to different temporal patterns of daily water drainage. In dry and wet years, leaching losses were similar. Our analysis suggests that it is necessary to fully capture dynamics of transient water flow (e.g., by numerically solving the transient Richards equation) in order to adequately estimate soil N gaseous emissions and N transport and thus leaching, although it requires more computational resources while the uncertainty in model improvement is still large due to lack of measurements.