Throughfall below grassland canopies: a comparison of conventional and ion exchange methods

Sampling of canopy fluxes (throughfall and stemflow) below low structured vegetation with a small-scale, intricate canopy architecture is difficult, and representative sampling with most methods is questionable. In the present study, two sampling methods for canopy fluxes below grassland vegetation are compared. Method I sampled canopy fluxes of moisture inefficiently, because stemflow volumes were not quantitatively included. Canopy fluxes of ions calculated with method I necessitated assumptions on equal concentrations in actually sampled throughfall and non-sampled stemflow. Method II sampled canopy fluxes of ions quantitatively, because the total volume of throughfall and stemflow percolated through a mixed bed of ion exchange resins below the canopy. Ion-specific differences between the two methods were observed. For ions with foliar leaching, such as K+ and Ca2+, higher canopy fluxes were recorded with method II than with method I. In contrast, for ions with foliar uptake, such as NH4+ and NO3-, canopy fluxes were found to be less with method II than with method I. Canopy fluxes of inorganic nitrogen below Mesobrometum grassland were 2.35 and 1.52 kmol(c) ha(-1) year(-1) for methods I and II, respectively, and 2.85 and 7.90 kmol(c) ha(-1) year(-1) for K+. It is argued that these differences result from under-estimated (foliar leaching) or over-estimated (foliar uptake) concentrations in stemflow by the first method. Canopy fluxes for SO4(2-) were not statistically different, indicating that canopy exchange of SOx was quantitatively unimportant, and that both methods estimated atmospheric input equally well.