The Hubbard Brook Ecosystem Study: Forest Nutrient Cycling and Element Behavior

Data from dimension analysis of forest biomass and production are combined with analyses of plant tissues, soil, and precipitation to describe nutrient cycling in a cool-temperate deciduous forest on a podzol. Mean concentrations of nutrient elements measured (N, P, S, Ca, K, Mg, Mn, Na, Fe, Zn, Cu) in tissues are relatively low, and nutrient stocks in vegetation are low compared with other forests because of the low concentrations and the relative youth of the forest (cut in 1909-17). The largest fractions of nutrients in aboveground net primary production (NPP) go into leaves-58-78% of all nutrients except Na (45%)-although leaves are only 40% of aboveground NPP. Net community (woody) growth included -30% of aboveground NPP, but smaller fractions of nutrients (7% of N and K to 21% of Ca and 28% of Na). Nitrogen, P, and some other elements are retracted from dying leaves; but concentrations of Ca, Mn, and Na increase from living to dead leaves, branches, and stem wood. Return to the soil from aboveground is primarily by leaching for K and Na but by litterfall for other elements. "Throughflow" is defined as nutrient uptake needed beyond that in NPP to provide for both leaching and increase in dead tissues before they fall. For Na, throughflow exceeds uptake into NPP; but negative throughflows for N and P reflect conservation of these elements. Belowground plant nutrient stocks are 0.25 to 0.5 x and belowground uptake rates into NPP are 0.1 to 0.2x aboveground values for most elements, but belowground values are higher for N, P, Na, and Fe. Significant amounts of some elements, especially S, Ca, K, and Na, are exuded from roots as part of the total, above- and belowground throughflow. Behaviors of nutrient elements can be characterized by ratios expressing concentration into leaves vs. woody tissues and relative leachability and throughflow vs. conservation. Relative turnover rates compare litterfall plus leaching with aboveground stocks or with nutrient contents of NPP. Turnover rates increase in the sequence: N and P; Ca; Mg, K, Fe, and Mn; S; and Na. Means and coefficients of variation of foliage nutrient concentrations express some differences between ecosystems. Com- paring different ecosystems, coefficients of variation are low for N and P, intermediate for Ca, K, Mg, and Al, and high for Si, Mn, Fe, Cl, and Na. Differences in these and other expressions of element behavior can be interpreted on the basis of function in plants.

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