Rhizodeposition of nitrogen by red clover, white clover and ryegrass leys

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. All rights reserved.