Below-ground nitrogen transfer between different grassland species: Directquantification by N-15 leaf feeding compared with indirect dilution of soil N-15
H. Hogh-jensen et Jk. Schjoerring, Below-ground nitrogen transfer between different grassland species: Directquantification by N-15 leaf feeding compared with indirect dilution of soil N-15, PLANT SOIL, 227(1-2), 2000, pp. 171-183
Nitrogen (N) transfer from one species to another is important for the N cy
cling in low-input grassland. In the present work, estimates obtained by an
indirect N-15 dilution technique were compared with estimates obtained by
a direct N-15 leaf feeding technique over two complete growing seasons in r
ed clover-ryegrass and white clover-ryegrass mixtures under field condition
s.
The direct technique confirmed that N transfer between clovers and ryegrass
is a bi-directional process. The transfer of N from both clovers to ryegra
ss occurred within 25 days upon the first labelling event. A very high N tr
ansfer 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 exc
eeded 50% and 10%, respectively, in three out of seven harvests. The N tran
sfer from N-15 labelled grass to associated clovers constituted a relativel
y constant proportion of approx. 8% of the above-ground N content of the mi
xtures.
Estimates based on the soil N-15 dilution technique generally underestimate
d the net N transfer by more than 50% compared to the direct N-15 labelling
technique. Furthermore, the indirect N-15 dilution technique estimated onl
y marginal differences between red and white clover in the quantities of N
transferred, whereas the direct N-15 labelling technique showed the N trans
fer 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 previou
sly recognised.