REVIEW OF NITROGEN FATE MODELS APPLICABLE TO FOREST LANDSCAPES IN THE SOUTHERN U.S.

Assessing the environmental impacts of fertilizer nitrogen (N) used to increase productivity in managed forests is complex due to a wide range of abiotic and biotic factors affecting its forms and movement. Models developed to predict fertilizer N fate (e.g., cycling processes) and water quality impacts vary widely in their design, scope, and potential application. We review the applicability of five commonly used eco-hydrologic models (APEX, MIKESHE-DNDC, DRAINMOD-FOREST, REMM, and SWAT) in assessing N fate and transport in southern forest landscapes (< 50 km(2)) because of their comprehensiveness and multi-scale predictions. The field-scale models DRAINMOD-FOREST and REMM contain process-level components characterizing hydrology, forest growth, and N dynamics, but they have limited capability to describe transport processes at the landscape scale. APEX can describe hydrology, forest growth, N fate processes, and plant competition at the landscape and small watershed scales mostly for upland SWAT is best suited to hydrologic simulations at watershed scale (> 50 km(2)), although N routing below the subbasin level does not yet exist. Similarly, the distributed MIKESHE-DNDC model has been used to assess N cycles across different spatial scales, on both uplands and lowlands, but was not intended to model lateral N transport. However, MIKESHE alone is capable of describing the hydrology and N transport. The strengths of each of the models reflect their original design and scope intent. Based on this review, none of the five models that we considered is independently adequate to address the fate of N fertilizers applied to forest stands at both small and large scales, including uplands and lowlands. While efforts are underway to extend these tools' capabilities and address their various limitations, the models must be validated using experimental data before using their outputs, together with uncertainty analysis, for developing forest fertilization guidelines and the fate and transport of N.