Spatial and temporal variation in N transfer in grass–white clover mixtures at three Northern European field sites

The N flow dynamics in grass–clover mixtures are not well understood. Spatial distributions and temporal differences in inter- and intra-species N transfer were investigated at field sites in Iceland, Germany, and Denmark, with three different managements at the Danish site. Both grass and white clover were leaf-labeled four times during a full year and the short-term N transfer to neighboring grass and white clover was determined in 10 cm intervals up to 1 m from the labeled plant. In addition 15N-injection of inorganic N at 5, 15, and 25 cm soil depth was carried out at the Danish site to compare spatial and temporal N transfer patterns to soil inorganic N uptake. The short-term N transfer from white clover to the closest companion grass reached levels of more than 50% of N from labeled white clover late in the growing season, thus questioning whether longer-term root turnover processes are dominating N transfer. The horizontal N transfer to grass exceeded 50 cm from the labeled plant at one site. The study showed that the competitive ability of white clover is as important for N dynamics in grass–white clover mixtures as that of the companion grass. Intra-species N transfer showed that both grass and white clover have reuptake of deposited N corresponding to at least 1% of the N found in leaf biomass. The study also showed that N transfer cannot be explained simply by competition for soil inorganic N, which indicates transfer via organic N forms.