Dollete, D., R.A. Lumactud, . 2024.
Background and Aims Forage legumes fix atmospheric nitrogen through a mutualistic relationship with soil rhizobia bacteria. However, frequent defoliation stress from grazing and mowing can alter the source-sink relationship between above-ground and below-ground tissues, potentially impacting their nitrogen-fixing ability. Methods In this study, we evaluated the effects of defoliation intensities on nodulation, root phenotypic traits, plant biomass, symbiotic nitrogen fixation, soil available nitrogen, soil enzyme activities, and soil microbial community structure of alfalfa (Medicago sativa L.) and red clover (Trifolium pretense L.). Two defoliation intensities were applied at flowering: mild (removing half of above-ground biomass) and severe (leaving only 2 cm stubble), along with non-defoliated controls. Results Mild defoliation positively affected final shoot biomass in both legumes but had negative effects on nodulation and non-symbiotic root traits, including root biomass. The symbiotic nitrogen fixation capacity was reduced in red clover under severe defoliation stress, whereas it was unaffected in alfalfa. Soil available nitrogen content was greater following severe defoliation in red clover compared to the mild and non-defoliation, but no changes were observed in alfalfa following defoliation. Severe defoliation significantly increased soil enzyme activities of β-1, 4-glucosidase, β-D-cellobiosidase, and phosphatase enzymes in both legumes. Defoliation had no significant effect on shifting soil bacterial diversity or taxonomic composition. Conclusion Overall results suggest that defoliation intensity had a deleterious effect on root traits, a positive influence on C and P extracellular enzyme activities, but varied influence on the shoot growth, symbiotic nitrogen fixation, and soil available nitrogen based on the forage legume.
Alfalfa Red clover Grazing Nodulation Soil enzymes Nitrogen Rhizodeposition Soil bacteria
