Distribution and Cycling of Nutrients in an Aspen‐Mixed‐Hardwood‐Spodosol Ecosystem in Northern Wisconsin

Abstract

The distribution and cycling of N, P, K, Ca, Mg, S, Fe, and Zn were examined over a 2—yr period in an aspen—mixed—hardwood—Spodosol ecosystem in order to determine the roles of overstory aspen and understory sugar maple in nutrient retention. Stand biomass and net primary production (NPP) were 198 Mg/ha and 12.5 Mg°ha^—1°yr^—1, respectively. Overstory quaking aspen (Populus tremuloides) comprised 60% of the biomass and 50% of the NPP, and understory sugar maple (Acer saccharum) comprised 24 and 25% of these values, respectively. The total soil N pool and extractable soil P and Mg pools were larger than the respective vegetation nutrient pools. Nutrient cycling within this ecosystem is characterized by large uptake rates, retention of between 40 and 60% of nutrient uptake in perennial tissues, and leaching losses of —¹°yr^—1 for each nutrient. Between 22 and 40% of total stand nutrient uptake is retained in perennial tissues of quaking aspen and between 4 and 20% in perennial tissues of sugar maple. Aspen was more effective than maple in retaining nutrients because a larger proportion of its production was perennial tissues with higher nutrient concentrations. Because nutrient—rich leaves constituted a smaller proportion of aspen's production compared with sugar maple's, aspen used nutrients more efficiently in production of dry matter. Translocation of nutrients out of leaves back into perennial tissues was an important retention mechanism for N, P, and K; was less important for S, Mg, and Fe; and did not occur for Ca and Zn. Aspen translocated proportionally more of most nutrients out of green leaves than did sugar maple, but differences between the two species were often small. Uptake as rainwater passed through the canopy and immobilization with the A1 horizon of the soil were additional retention mechanisms for NH₄—N and NO₃—N. A fragipan at the base of the rooting zone also slowed leaching losses.