Correct assessment of the rhizodeposition of N in grassland is essential fo
r the evaluation of biological Nz-fixation of legumes, for the total N bala
nce of agro-ecosystems, and for the pre-cropping value of grasslands. Using
a leaf-feeding technique by which plants were N-15 labelled while growing
in mezotrons in the field, the rhizodeposition of N by unfertilised red clo
ver, white clover and perennial ryegrass growing in pure stands was shown t
o amount to 64, 71 and 9 g N m(-2), respectively, over two complete growing
seasons. The corresponding values for red clover and white clover growing
in mixtures with ryegrass were 89 and 12 g N m(-2). respectively. The rhizo
deposited N compounds. including fine roots, constituted more than 80% of t
he total plant-derived N in the soil, and in all cases exceeded the amount
of N present in stubble. In the mixtures of red clover-ryegrass and white c
lover-ryegrass and the pure stands of red clover. white clover and ryegrass
. respectively, the rhizodeposition constituted a 1.05. 1.5, 1.26, 2.21 and
2.77 fold increase over the total N in the shoots harvested during the two
production years. In pure stands and mixtures of clover, 84 and 92%, respe
ctively, of this N derived from biological N-2 fixation. It is concluded th
at rhizodeposition provides a very substantial input of N to the legume-bas
ed grassland systems with great consequences for ecosystem N balance and tu
rnover. Furthermore, the amount of atmospheric-derived N in the rhizodeposi
ts may exceed that in the harvested shoots. (C) 2001 Elsevier Science Ltd.
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