Effect of drought and defoliation on plant growth, symbiotic nitrogen fixation, soil nitrogen availability and soil microbial dynamics in forage legumes

Abstract

Forage legumes play a crucial role in agriculture due to their symbiotic nitrogen fixation (SNF) ability by forming a symbiotic relationship with soil rhizobia, providing high-quality forage to livestock and improving soil health. The changing global climate is predicted to increase the frequency and intensity of drought, a known stress factor that negatively impacts forage legumes. Furthermore, forage legumes frequently undergo defoliation stress through pests and grazing in managed agricultural systems, which changes the source-sink relationship between above-ground and below-ground tissues. We hypothesize that the influence of drought and defoliation can potentially influence root nodulation, plant growth, SNF, and nitrogen rhizodeposition in forage legumes. In addition, the changes in plant physiology and biochemical processes induced by these stress factors can influence the soil microbiome and enzyme activities. The main objective of this project is to evaluate the effects of drought and defoliation stresses on plant growth, plant physiological responses, SNF, and their consequential influence on soil enzyme and microbial dynamics in forage legumes. Two separate greenhouse pot experiments were conducted using alfalfa (Medicago sativa L.) and red clover (Trifolium pretense L.) to evaluate the effect of drought and defoliation stresses on the above parameters. Alfalfa and red clover seedlings were inoculated with Sinorhizobium meliloti 1021 and Rhizobium leguminosarum biovar trifolii Mj43, respectively, and were grown until the flowering stage before applying drought and defoliation treatments. Drought study was conducted by maintaining soil moisture at 20% field capacity (FC) (severe drought), 40% FC (moderate drought), and 80% FC (well-watered) for three weeks. Defoliation treatments were simulated by trimming half of the above-ground biomass (mild defoliation) or leaving only 2 cm stubble under severe defoliation treatment. Overall, drought and defoliation significantly reduced nodulation in alfalfa and red clover. Drought negatively affected both shoot and root biomass, while defoliation only negatively influenced root biomass while improving the final total shoot biomass. Drought was also found to reduce SNF, while SNF was reduced only in red clover following defoliation. On the other hand, soil available nitrogen was increased following severe drought stress and defoliation. In addition, drought significantly reduced N-acetyl-glucosaminidase and β-D cellobiosidase enzyme activities in alfalfa and red clover soil, respectively. On the contrary, defoliation positively influenced β-1, 4-glucosidase, β-D-cellobiosidase, and phosphatase enzyme activities in soil under both forage legumes. Lastly, microbiome data showed shifts in the relative abundance of some key bacterial taxa under drought and defoliation stresses. Overall results suggest that drought and defoliation induced varied influences on SNF and plant growth, eliciting different effects on nutrient cycling enzyme activities, soil nitrogen availability, and shifts in soil microbial diversity.