Below-ground nitrogen transfer between different grassland species: Direct quantification by 15N leaf feeding compared with indirect dilution of soil 15N

Summary

Nitrogen (N) transfer from one species to another is important for the N cycling in low-input grassland. The accuracy of measurements based on indirect dilution of soil 15N has been questioned. In the present work, estimates obtained by this technique were compared with estimates obtained by a direct 15N leaf feeding technique over two complete growing seasons in red clover-ryegrass and white clover-ryegrass mixtures under field conditions.
The direct technique documented that N transfer between clovers and ryegrass is a bi-directional process. The transfer of N from both clovers to ryegrass occurred within 25 days upon the first labelling event. A very high N transfer occurred from white clover to the associated ryegrass, 4.5 and 7.5 g m-2 in the 1st and 2nd production year, respectively. The corresponding values for transfer from red clover to the associated ryegrass were 1.7 and 3.6 g m-2. Quantified relatively to the total above-ground N content of white clover- ryegrass and red clover-ryegrass mixtures, the N transfer exceeded 50% and 10%, respectively, in three out of seven harvests. The N transfer from 15N labelled grass to associated clovers constituted a relatively constant proportion of approx. 8% of the above-ground N content of the mixtures.
Estimates based on the soil 15N dilution technique generally underestimated the net N transfer by more than 50% compared to the direct 15N labelling technique. Furthermore, the indirect 15N dilution technique estimated only marginal differences between red and white clover in the quantities of N transferred, whereas the direct 15N labelling technique showed the N transfer from white clover to the associated ryegrass to be significantly higher than that involving red clover. It is concluded that N transfer is a much more dynamic and quantitatively important process in grassland than previously recognised.