A Forest Nutrient Cycling and Biomass Model (ForNBM) based on year-round,monthly weather conditions: Part II: Calibration,verification, and application

The ForNBM was applied to the Nashwaak Experimental Watershed Project in central New Brunswick, Canada. The data represented a mixed hardwood site and included information about nutrient leaching, foliage and wood biomass, leaf fall, and ancillary information required for model initialization. Ancillary information included forest cover type, stand density, forest floor depth, soil rooting depth, soil texture, soil substrate type, initial amounts of biomass and N, S, Ca, Mg, and K content in foliage, wood, and roots, and mineral soil nutrient contents in soil solution, on ion-exchange sites, and in the soil.The authors were able to calibrate the model with existing data. The model simulated observed monthly leaching reasonably well. The r²-values of model simulations compared with field observations of monthly leaching of NO₃^?_N, NH₄⁺_N, Ca, Mg, and K were 0.78, 0.7, 0.78, 0.84, and 0.75, respectively. Modeled multi-year cumulative leaching of NO₃^?_N, NH₄⁺_N, Ca, Mg, and K compared with actual values gave r²-values close to one for all cases considered.The results also showed that soil nutrient leaching had increased approximately five-fold for NO₃^?_N, 80% for NH₄⁺_N, 71% for K, 20% for Ca, and 14% for Mg during the 3 years following a stem-only harvest operation applied watershed-wide. However, the model simulation showed that increased nutrient leaching during the 11 years following the stem-only harvest was small compared with the amounts removed in the biomass during the harvest. Increased nutrient leaching following harvesting did not appear to impact site productivity over the long term.