Predicting dissolved organic nitrogen export from a drained loblolly pine plantation

Abstract

[1] Dissolved organic nitrogen (DON) export from terrestrial ecosystems influences the ecology of receiving surface waters. The soil carbon (C) and nitrogen (N) model, DRAINMOD-N II, was modified to simulate key processes associated with DON transformations and transport in the soil profile. DON production is modeled by tracking dynamic C:N ratios of dissolved organic matter originating from various organic matter pools. The Langmuir isotherm was used to quantify the assumed instantaneous equilibrium between potentially soluble organic N in solid and aqueous phases. DON transport with soil water was simulated using a numerical solution to the advection-dispersion reaction equation. The modified model was used for simulating temporal variations of DON export from three loblolly pine (Pinus taeda L.) plantations located in eastern North Carolina.Results showed that the model can accurately predict DON export dynamics during storm events with Nash-Sutcliffe efficiency (E) of 0.5, seasonal DON losses with E above 0.6, and annual DON losses with E above 0.7. In addition to the well-recognized role of hydrological processes, reasonable quantifications of the seasonal changes in the potentially soluble soil organic matter, the DON sorption to soil particles, and the dynamic C:N ratios of dissolved organic matter were found to be essential for mechanistic representation of DON export dynamics. Specifically, adapting the dynamic C:N ratios enabled the model to reasonably describe the temporal variations of correlations between DON and dissolved organic carbon in drainage water.

Global sensitivity analysis of DRAINMOD-FOREST, an integrated forest ecosystem model

“…This finding is similar to the results of sensitivity analysis of other process-based models (Neff and Asner, 2001;Yurova et al, 2008), which highlight the importance of accurately parameterizing the sorption algorithm for simulating DON dynamics. This is further supported by an uncertainty analysis of DRAINMOD-FOREST in predicting long-term DON losses (Tian et al, 2013). Additionally, hydrological processes (represented by the maximum stomatal conductance (gsmax)) were also important in predicting long-term annual DON losses.…”

“…In DRAINMOD-N II, the temperature constraint function is based on a form of the Van't Hoff equation with variable Q10. DRAINMOD-N II was recently modified to simulate biogeochemical and physical processes of DON, including production of DON from diverse SOM pools, microbial consumption, partitioning between solid and aqueous phases, and physical transport along the soil profile (Tian et al, 2013).…”

“…Challenges still exist for accurately predicting DON losses using process-based models because of the current limited understanding of mechanisms controlling sources and sinks of DON, as well as its transport in the soil profile. DRAINMOD-FOREST was modified to simulate key processes associated with transformations and transport of DON in the soil profile (Tian et al, 2013). In the modified model, DON production rates were empirically linked with organic matter pools on forest floor and in forest soil.…”

“…DRAINMOD-FOREST is an integrated forest ecosystem model for simulating water, soil carbon (C), and nitrogen (N) dynamics and plant growth in natural and managed forests on shallow water table soils (Tian et al, 2012a(Tian et al, ,2012b. Previous studies successfully tested DRAINMOD-FOREST for predicting hydrological processes, nitrate and dissolved organic nitrogen (DON) losses in drainage, and plant growth for three artificially drained loblolly pine (Pinus taeda L.) plantations under various management practices over a 21-year period (Tian et al, 2012a(Tian et al, ,2012b(Tian et al, , 2013. The model is highly nonlinear and nonadditive because of the tight interactions among its three components: hydrology, biogeochemistry, and plant growth.…”

Development and preliminary evaluation of an integrated field scale model for perennial bioenergy grass ecosystems in lowland areas

“…The model was developed by adding a perennial grass growth component to DRAINMOD and DRAINMOD-NII models. This strategy is similar to DRAINMOD-FOREST, which was developed as a field-scale, mechanistic, fully integrated ecosystem model for simulating water, soil C and N dynamics, and tree growth in artificially-drained lowland area forests under various management practices (Tian et al, 2012(Tian et al, , 2013. Previous model evaluations and applications (Tian et al, 2012(Tian et al, , 2012b(Tian et al, , 2013 demonstrate that DRAINMOD-FOREST, with its three integrated components (hydrology, soil C/N, and vegetation growth) provides a valid template for developing a model for simulating hydrological and biogeochemical processes and plant growth in bioenergy grass systems with high water table soils.…”