Transfer of Nitrogen from Legumes to Grasses in Perennial Forages

Abstract

Nitrogen (N)-fixing legumes are important in forage production not only for the high-quality herbage they provide, but also for their ability to provide a portion of their fixed N to surrounding non-fixing plants, such as grasses. This general process, known as ‘N transfer’, involves the net transfer of N from one plant (N-donor) to another (N-receiver). While much of this transfer is accomplished through the decomposition of root tissue, recent studies have suggested that a more direct route, via exudation of low-molecular weight N (LMW N) compounds by N-donors and subsequent uptake by N-receivers, may also be important. To test what effect that plant species and cultivars have on the efficiency of direct N-transfer via exudates in temperate forage species, three key areas involved in its efficiency were examined: legume nodulation (Chapter 2), legume exudation (Chapter 3) and legume-grass compatibility (Chapter 4). Two genotypically different cultivars each of two common legume N-donors, alfalfa (Medicago sativa) and red clover (Trifolium pratense), as well as two N-receiver grasses, perennial ryegrass (Lolium perenne) and timothy (Phleum pratense) were chosen. Legume nodulation rate varied mostly between species, with red clover nodulating much higher at lower N-fertility levels than alfalfa, but growth and N-accumulation were similar between the species, suggesting nodulation number might not be indicative of fixation rate. In testing legume N exudation, one specific clover cultivar released much more LMW N than other cultivars tested, which may have been related to its high nodule to dry matter (DM) ratio. Ultimately, however, exudation rate was a poor predictor of N transfer, as grasses grown with alfalfa tended to accumulate more N. Grass species and cultivar were also important, with more aggressively-growing grasses (ryegrass cultivars) accumulating more N. Ultimately, it was found that two important legume traits for transfer (nodulation and exudation) varied both within and between species, but that the competitive aspects of the legume-grass relationship may limit the amount of N transferred via the exudation pathway. Further research into complimentary growth patterns in N-donor/-receiver pairs may help improve the efficiency in this complex relationship