Cross-site comparison of ecosystem nutrient cycling in hardwood and conifer forests on contrasting soils

In the northeastern United States, both hardwood and conifer forests have developed on sites with contrasting soils, allowing an examination of the effect of site and forest type on ecosystem nutrient cycling. We measured biomass production and nutrient fluxes in northern hardwood and conifer stands at three sites differing in soil fertility. We found that leaf, root, and wood concentrations of calcium (Ca), magnesium (Mg), and potassium reflected differences in soil base cation availability, while concentrations of nitrogen (N) and phosphorus (P) were more consistent across sites. Nutrient uptake was calculated as the sum of litterfall, net throughfall (throughfall minus precipitation), root turnover, and accumulation in perennial tissues (wood). We propose a novel metric of nutrient cycling, the nutrient retention fraction (NRF), defined as the proportion of annual nutrient uptake retained in biomass accretion. Because the NRF is unitless, it can be compared across nutrients; Ca and Mg had the highest NRF and P the lowest ( p = 0.05). Across sites and elements, NRFs were lower for conifers (5.0 ± 0.6%) than for hardwoods (7.2 ± 0.5%), associated with their lower productivity. Nutrient-use efficiency (biomass production divided by nutrient uptake) tended to be high where foliar concentrations indicated low availability of that nutrient. Nutrient retention of N and P was higher where availability of the other element was high, which could be a mechanism contributing to N and P co-limitation.