MODELING MINERAL NITROGEN EXPORT FROM A FOREST TERRESTRIAL ECOSYSTEM TO STREAMS

Abstract

Terrestrial ecosystems are major sources of N pollution to aquatic ecosystems. Predicting N export to streams isa critical goal of nonpoint-source modeling. This study was conducted to assess the effect of terrestrial N cycling on streamN export using long-term monitoring data from Hubbard Brook Experimental Forest (HBEF) in New Hampshire. The field-scale DAYCENT model was used to quantify N pools and long-term annual streamflow and mineral N export for six subwatershedsat the HBEF. By combining DAYCENT with the Soil and Water Assessment Tool (SWAT) watershed model, mineralN export simulations were extended to the watershed scale. Our study indicated that only 13% of external N input was exportedto streams during 1951-2000 at HBEF. As much as 4763 kg/ha of N was stored in forest litter, soil organic matter(SOM), and living plant biomass. Net N mineralization of SOM and forest litter contributed 93% of total available N for exportwithin the HBEF ecosystem. The Nash-Sutcliffe coefficient (Ens) evaluating model performance of DAYCENT at sixsubwatersheds ranged from 0.72 to 0.82 for simulating annual streamflow (1964-2000) and from 0.48 to 0.67 for annualmineral N export (1971-1995), indicating reasonable simulated values. DAYCENT successfully predicted the effect ofecosystem disturbance such as forest cut and insect invasion on stream mineral N export. The watershed-scale simulationsuggested that soil spatial variability affects stream N export in addition to the accepted controls of land cover, external Ninput, climate, and ecosystem disturbance.