Military training activities can severely impact ecosystem structures, functions, health, and sustainability, which are intrinsically linked to ecosystem carbon (C) and nitrogen (N) cycles. Well-designed biogeochemical models can provide valuable quantitative information on the impacts of land use and climate variability on many ecosystem processes including the C and N cycles. Land and range managers at military installations can use biogeochemical cycle information as part of ecological assessments to strategically plan installation activity expansion and schedule training operations to mitigate adverse environmental impacts.

This project focused on the development of a new biogeochemical modeling system. The overarching goal of this project was to develop an advanced, spatially distributed biogeochemical cycle modeling system to simulate the dynamics of ecosystem C and N cycles under historical, current, and future land use and disturbance scenarios.

An advanced, spatially distributed biogeochemical cycle modeling system was developed for Fort Benning ecosystems and its surrounding areas. Using Fort Benning forest conditions from the forest inventory data that were collected during 1981 to 2000, and in 2006, the forest stands common in the two datasets were compared to infer recent forest changes. Relations of forest biomass to forest basal area were used to derive forest biomass change between the two inventories. Military land surface disturbances were represented in the simulation of soil erosion and deposition. The suitability of the Unit Stream Power-based Erosion Deposition (USPED) model at Fort Benning was evaluated by simulating the spatial distribution of erosion and deposition within ten watersheds using total suspended sediments measured in the runoff. The Erosion-Deposition-Carbon Model (EDCM) was used to simulate the long-term ecosystem C and N dynamics at Fort Benning under the impacts of different combinations of fire frequency, fire intensity, symbiotic and nonsymbiotic nitrogen inputs, and nitrogen deposition.

The following specific tasks were accomplished. First, the effects of cyclic prescribed burning were examined with respect to plant production, soil nutrient cycling, and net N mineralization rates. Second, the consequences of ecosystem C and N dynamics following current fire management practices were assessed. Third, the scientific basis and practical scenarios for land management decisions were provided to determine ecologically sound fire management practices and appropriate management strategies for maintaining ecosystem sustainability at Fort Benning. Finally, the General Ensemble biogeochemical Modeling System (GEMS) was used to simulate and compare ecosystem carbon sequestration between Fort Benning and the surrounding areas from 1992 to 2050.

Major findings from this project include an improved understanding of the status and trends of forest resources and C fluxes and stocks, which is essential for forest management. Carbon stocks were estimated for the inventoried forests using allometric equations and led to the conclusion that the current inventory system can be improved to provide a complete dynamic picture of forest resource change in space and in time. To better understand these trends, a number of forest field plots should be established that can be re-measured at intervals of less than five years. Measurements should include tree growth dynamics as well as ground biomass changes. Forest stand change is best analyzed when such continuous measurements are available.

Military training activities may intensify soil erosion and deposition that contribute to the degradation of ecosystems. Land cover information derived from Landsat Thematic Mapper was used to estimate surface flow accumulation. Results from ten small catchments showed that simulated net erosion rates using the USPED model were significantly correlated to the observed total suspended sediments in stream water. Erosion estimates also were related to the land disturbance index that is a measure of the intensity of military training disturbances. This suggested that the USPED model was an effective tool to quantify erosion and deposition at military installations.

Long-term ecosystem C and N dynamics at Fort Benning were simulated as to impacts of different realistic combinations of fire frequency, fire intensity, symbiotic and nonsymbiotic N inputs, and N deposition using the EDCM. Modeling results indicated that cyclic prescribed fire can have significant impacts on long-term equilibrium status of C and N fluxes and stocks. Multiple regression analysis indicated that ecosystem Net Primary Production (NPP) would be the lowest with high fire frequency and intensity without biological N inputs. Without fire and with high levels of both symbiotic and nonsymbiotic N input, understory NPP would be the lowest. How fire affects the long-term ecosystem nutrient dynamics and how the ecosystem responds to fire depend strongly on both fire regime and N input from various sources.

Military installations generally have substantially different land management strategies as compared to surrounding areas, and the carbon consequences have rarely been quantified and assessed. The results from the GEMS work indicate that the military installation sequestered more carbon than surrounding areas at present (76.7 vs. 18.5 g C m^–2 yr^–1 from 1992 to 2007) and would in the future (75.7 vs. 25.6 g C m^-2 yr^-1 from 2008 to 2050), mostly because of differences in land use activities. The results suggest that installations might play an important role in sequestering and conserving atmospheric carbon because some anthropogenic disturbances (e.g., urbanization, forest harvesting, and agriculture) can be minimal or absent on military training lands.

The modeling system and simulated results can be used to assist in the evaluation of the environmental consequences of various training and land management activities to facilitate the conservation of natural resources and thereby support future land use and training operations.