The Calcium, Magnesium, Potassium, and Sodium Budgets for a Small Forested Ecosystem

Abstract

The balance of calcium, magnesium, potassium, and sodium was studied in six watersheds of the Hubbard Brook Experimental Forest at West Thornton, New Hampshire. These watersheds are characterized by a near climax, northern hardwoods ecosystem, humid continental climate, and acid metamorphic rock. Bedrock in the area is considered to be watertight. Weekly samples of precipitation and stream water were collected from June 1963 through May 1965. Cation analyses were made by atomic absorption spectrophotemetry. Precipitation was about 117 cm during 1963—64 and 95 cm during 1964—65 and did not differ significantly in amount with elevation. Loss of water by runoff from the system was about 68 cm during 1963—64 and 49 cm during 1964—65. Evapotranspirational losses were calculated at 49 cm in 1963—64 and 46 cm in 1964—65. About 37% of the total annual runoff ocurred in April and less than 0.2% in September. Concentration of Ca⁺⁺, Mg⁺⁺, K⁺, and Na⁺ in weekly precipitation samples was variable. The annual weighted average values (mg/liter) were : 1963—64, Ca 0.26, mg 0.06, K 0.21, Na 0.09; 1964—65, Ca 0.30, Mg 0.12, K 0.19, Na 0.22. Differences were not related to elevation. The contribution of cations from dry fallout was negligible. Cation content of stream water was relatively very low. The weighted average content for the entire area (mg/liter) was Ca 1018, Mg 0.38, K 0.26, Na 0.87 during 1963—64; and Ca 0.80, Mg 0.38, K 0.22, Na 0.91 during 1964—65. In general, the cationic concentrations in drainage water were constant although discharge or water varied greatly. Statistically, the relationship between concentration and discharge was not significant for Ca⁺⁺ and Mg⁺⁺ in any of the watersheds, but was inversely significant for Na⁺, and was possibly direct for K+ in a few instances. The budget for dissolved cations was calculated from determinations of input (cation concentration times volume of precipitation) and output (cation concentration times volume of runoff.) During 1963—64 the average inputs in kg/ha were 3.0 Ca⁺⁺, 0.7 Mg⁺⁺, 2.5 K⁺, and 1.0 Na⁺, while the output averaged 8.0 Ca⁺⁺, 2.6 Mg⁺⁺, 1.8 K⁺, and 5.9 Na⁺. During 1964—65 the values were: input, 2.8 Ca⁺⁺, 1.1 Mg⁺⁺, 1.8 K⁺, and 2.1 Na⁺; and output, 3.9 Ca⁺⁺, 1.8 Mg⁺⁺, 1.1 K⁺, and 4.5 Na⁺. Assuming that the vegetation is near dynamic equilibrium, the net losses of Ca⁺⁺, Mg⁺⁺, and Na⁺ represent conservative measurements of geochemical weathering in the system. The K⁺ budget is probably near balance.