Cutting regime determines allocation of fixed nitrogen in white clover

Leguminous leys are important sources of nitrogen (N), especially in forage-based animal production and organic cropping. Models for estimating total N(2) fixation of leys-including below-ground plant-derived N (BGN)-are based on grazed or harvested leys. However, green manure leys can have different proportions of above-ground plant-derived N (AGN) and BGN when subjected to different cutting regimes. To investigate the effects of cutting on N distribution in white clover, a pot experiment was carried out using (15)N techniques to determine N(2) fixation, N rhizodeposition and root C and N content of cut and uncut white clover (Trifolium repens L. cv. Ramona) plants. Percentage N derived from air (% Ndfa) was lower in uncut (63%) than in cut (72%) plants, but total Ndfa was not significantly affected by cutting. The higher reliance on N(2) fixation in cut plants was thus counterbalanced by lower biomass and total N content. With BGN taken into account, total plant-derived N increased by approximately 50% compared with AGN only. Cutting did not affect the proportion of BGN to standing shoot biomass N after regrowth, but decreased the proportion of BGN to total shoot biomass production during the entire growth period. Thus, estimates of N fixation in green manure leys should consider management practices such as cutting regime, as this can result in differences in above-and below-ground proportions of plant-derived N.