Associative nitrogen fixation (ANF) in switchgrass (Panicum virgatum) across a nitrogen input gradient

Area of Science:

• Ecology
• Microbiology
• Plant Science

Background:

• Associative N fixation (ANF) is the conversion of dinitrogen gas to ammonia by bacteria in association with plants.
• ANF in temperate ecosystems, particularly for grasses like switchgrass (Panicum virgatum L.), remains understudied.
• Switchgrass productivity often shows little response to nitrogen (N) fertilizer, suggesting alternative N sources.

Purpose of the Study:

• To investigate the potential for ANF in switchgrass across varying soil fertility and N fertilization levels.
• To determine how ANF potential changes throughout the growing season and in response to plant senescence.
• To assess the contribution of ANF to the overall N budget of unfertilized switchgrass and temperate grasslands.

Main Methods:

• Short-term 15N2 incubations of switchgrass rhizosphere soils and excised roots were conducted.
• Measurements were taken across N fertilization gradients at two sites with contrasting soil types (Mollisols and Alfisols).
• ANF potential was measured four times during the growing season.

Main Results:

• ANF potentials generally decreased with long-term N addition, correlating with higher soil N availability.
• The highest rates of root ANF were observed after plant senescence, not during peak growing season.
• Diazotrophs appear to utilize carbon released during senescence for N fixation.

Conclusions:

• ANF is a significant source of nitrogen for unfertilized switchgrass.
• The findings suggest ANF is an important N source for temperate grasslands.
• Plant senescence plays a critical role in supporting ANF activity through carbon availability.