We measured throughfall nutrient fluxes across a gradient of soil fertility and tree species composition in the northern Lower Peninsula of Michigan, USA. We collected throughfall from April through November in 2006 and 2007 across five stands ranging from the poorest site on sandy, infertile outwash sediments dominated by black oak (Quercus velutina) and white oak (Quercus alba), to moderate-fertility ice-contact origin sites dominated by red oak (Quercus rubra) and red maple (Acer rubrum), to the highest fertility site on fertile moraine dominated by sugar maple (Acer saccharum) and basswood (Tilia americana). Collectors were sampled whenever cumulative precipitation exceeded 2 cm. At each site we had five, randomly located throughfall collectors, and a single throughfall collector in an open field centrally located relative to the five study sites. All samples were anlyzed for total nirtogen (N), calcium (Ca), potassium (K), magnesium (Mg) and phosphorus (P). The net exchange of nutrients in throughfall was calculated for each collector as the difference in nutrient concentration between the throughfall sample and the open precipitation collector, multiplied by the volume of throughfall collected. The data set is organized by site and by sampling date with throughfall fluxes of N, Ca, K, Mg and P expressed in units of g of nutrient per m².

The five stands were selected from a pool of sites that had been classified into ecological land units based on landform, soil properties, vegetation, and N cycling rates and were numbered sequentially in order of increasing fertility/productivity. Site 1 was dominated by black oak (Quercus velutina) and white oak (Quercus alba); site 2 was dominated by white oak, red oak (Quercus rubra) and red maple (Acer rubrum); site 3 was dominated by red oak and sugar maple (Acer saccharum); site 4 was dominated by sugar maple, red oak, American basswood (Tilia americana), and American beech (Fagus grandifolia); and site 5 was dominated by sugar maple and American basswood.

Within each site, a 100 x 40-meter study area was established with at least 100 meters from any forest edge and free of any canopy gaps or other disturbances. A single sampling station was located at a random point within each 20 x 40 m segment of the 100 x 40 m plot at each site, for a total of five sampling stations per site. A single throughfall collector was also installed at each sampling location. Throughfall collectors consisted of a 219 cm² high-density polyethylene (HDPE) funnel that was attached to a 2 L HDPE bottle. We placed a glass wool plug within each funnel to filter out debris; plugs were changed regularly throughout the study. The collectors were strapped to a PVC post 1.5 m above the ground, and the collecting bottles were spray painted on the outside with silver metallic paint to block light transmission into the collector. In addition, a single TF collector was installed in an open field at 44.224°N, 85.819°W which was centrally located relative to the five study sites. We collected throughfall samples from April 1 through November 30 in both 2006 and 2007. We sampled throughfall collectors whenever cumulative precipitation exceeded 2 cm based on our monitoring of daily precipitation from a nearby weather station.

Total throughfall volumes were measured with a graduated cylinder and 50-mL subsamples were collected, filtered through GF/A Whatman glass microfiber filters and then stored frozen (-20°C) prior to analysis. Throughfall samples were analyzed for concentrations of total dissolved N (TDN) by oxidative combustion- chemiluminescence (TOC/ TN analyzer, Shimadzu Corp., Kyoto, Japan) and for P, K, Ca, and Mg by inductively coupled plasma-optical emission spectrometry (ICP-OES; Optima 2100 DV, Perkin-Elmer, Bridgeport, CT). The net exchange of nutrients in throughfall was calculated for each collector as the difference in nutrient concentration between the throughfall sample and the open precipitation collector, multiplied by the volume of throughfall collected. Throughfall fluxes were expressed in units of g of nutrient per m².