USING FIELD LYSIMETERS IN WATERSHED RESEARCH: RESULTS FROM THE HUBBARD BROOK SANDBOX STUDY

Water, gas and nutrient concentrations have been measured for six years in field lysimeters (“sandboxes”) at the Hubbard Brook Experimental Forest. These results provide intriguing hypotheses regarding nutrient fluxes and carbon cycling that should be tested at the watershed scale. Chemical denudation was largest beneath a nonvascular plant cover and approximately 3-5 times larger than denudation from a red pine sandbox. Mineral weathering estimates, however, were larger beneath red pine. Weathering in the red pine sandbox consumed ~5000 mol_c /ha/yr of H⁺ for the first 5 years of tree growth and averaged only 500 mol_c /ha/yr the following decade. Mineral weathering consumed approximately 1000 mol_c /ha/yr of H⁺ in the nonvascular sandbox. Our findings suggest that while growing vascular plants may increase primary mineral weathering, they do not necessarily accelerate the transfer of carbon (as bicarbonate) to the oceans via denudation from terrestrial systems. We have also studied processes controlling nutrient fluxes in response to a young-tree harvest. Four years ago the red pines were cut and removed (roots & stem left intact in the sandbox) after 17 years of growth. Rapid decomposition of the litter layer released K to solution within two months of the harvest; Ca and Mg were not released until the onset of nitrification (~9 months after harvest). In contrast to the dynamic nature of the porewater/exchange system, declines in soil respiration after harvest were gradual. There was no change in carbon loss from the system during the first year after harvest; subsurface CO₂ concentrations have decreased by only 20% over four years and continue to be positively correlated with soil temperature. Carbon loss from the system via diffusion of CO₂ was ~300 times larger than aqueous carbonic acid/bicarbonate fluxes.