The role of throughfall nutrient losses in nutrient use efficiency across a temperate forest fertility gradient

We investigated the importance of throughfall nutrient fluxes to stand-level nutrient use efficiency across a gradient of soil fertility and tree species composition in northern Lower Michigan, USA. Potassium (K) was most susceptible to canopy leaching with annual throughfall K fluxes averaging 1.37 g m−2, compared to 0.32 g m−2 for calcium (Ca), 0.27 g m−2 for nitrogen (N), 0.13 g m−2 for magnesium (Mg) and 0.07 g m−2 for phosphorus (P). Throughfall losses of K exceeded litterfall losses and accounted for 43–77% of the total flux of K from canopy to soil. Thus, accounting for leaching losses reduced estimates of stand level K use efficiency by 2–4 times compared to estimates that only accounted for K losses in litterfall. In contrast, throughfall fluxes of Ca and N were very low, such that accounting for leaching losses of these elements had a negligible impact on estimates of stand-level Ca and N use efficiency. Throughfall fluxes of P and Mg accounted for 15–30% of total canopy fluxes, but these estimates are likely inflated by wash off of dry deposition. In summary, the K economy of these temperate deciduous forest stands cannot be understood without accounting for throughfall leaching losses.