Modeling response of soil productivity to biogeochemical cycling and atmospheric acid deposition in the Hayden Brook watershed (Canada) using the ForNBM model

A process-based biogeochemistry model, the ForNBM, was calibrated to simulate stream flows, nutrient concentrations and loadings on a monthly time scale using the field data collected from 1973 to 1985 at the Hayden Brook watershed, a natural forested mixed-hardwood watershed, in central New Brunswick, Canada. The model was then used to assess the major biogeochemical processes that influence soil productivity and the sensitivity of soil acidity and nutrient leaching to different scenarios of atmospheric acid deposition in the Hayden Brook watershed. sults of the model calibration showed that the ForNBM model reproduced the field data well with the Nash–Sutcliffe coefficient of efficiency ranking from 0.63 to 0.99 for the simulations of stream flows, from −0.11 to 0.14 for the stream nutrient concentrations, from 0.03 to 0.63 for the monthly stream nutrient loadings, and from 0.94 to 0.99 for the cumulative stream nutrient loadings. measurements and model predictions indicated that 90% of atmospheric deposited nitrogen (N) was retained in the soils. Atmospheric N deposition may be the most important source of N for the long-term watershed soil N accumulation in the Hayden Brook watershed. Biomass had low nutrient retention rates and high nutrient return rates, which demonstrated that the biogeochemical cycling was fast in the mixed-hardwood forest. The results also showed that annual N supply for biomass uptake was mainly from soil organic matter mineralization, and base cations (Ca2+, Mg2+, and K+) from both soil organic matter mineralization and mineral weathering. Results of sensitivity analysis indicated that the soil pH and base cation leaching were more sensitive to atmospheric acid deposition than N. A 100% increase of current atmospheric acid deposition may cause a 0.6 unit decrease of soil pH, 44%, 39%, and 30% increase of soil Ca2+, Mg2+, and K+ leaching, respectively. Long-term soil productivity at the Hayden Brook watershed may also be declined by soil acidity and depletion of base cations due to increased leaching in response to increased atmospheric acid deposition.