Nitrous oxide production by nitrogen-fixing, fast-growing Rhizobia
S Casella1, C Leporini, M P Nuti
1Istituto di Microbiologia agraria, Università di Pisa, 56100, Pisa.
Microbial Ecology
|November 14, 2013
Summary
Rhizobium bacteria can produce nitrous oxide (N2O) from various nitrogen sources under anoxic conditions. Organic matter and nitrate availability influence N2O production in both free-living and symbiotic states.
Area of Science:
- Microbiology
- Environmental Science
- Plant Science
Background:
- Rhizobium species are crucial nitrogen-fixing bacteria in legume symbiosis.
- Anoxic conditions can lead to denitrification, producing gaseous nitrogen compounds.
- Nitrous oxide (N2O) is a potent greenhouse gas with environmental implications.
Purpose of the Study:
- To investigate N2O production by Rhizobium species under anoxic conditions.
- To determine the influence of different nitrogen sources and organic matter on N2O evolution.
- To assess N2O production in both free-living and symbiotic (in planta) states.
Main Methods:
- Culturing Rhizobium trifolii, R. leguminosarum, and R. hedysarum ex planta in a chemically defined medium under anoxic conditions.
- Measuring N2O evolution from nitrate (NO3-), nitrite (NO2-), and ammonium nitrate ((NH4)2NO3).
- Assessing N2O production in free-living R. trifolii in peat/sand mixtures with varying organic matter.
- Detecting denitrification in planta for R. trifolii, R. leguminosarum, R. hedysarum, and R. meliloti.
- Exposing nitrogen-fixing plants to increasing nitrate concentrations and measuring N2O production in nodules.
Main Results:
- Rhizobium species evolved N2O from nitrate, nitrite, and ammonium nitrate ex planta, with approximately 0.2 μM N2O formed per mg dry weight cells after 96 hours.
- High organic matter availability enhanced N2O production from nitrate by free-living R. trifolii.
- Denitrification was observed in planta for multiple Rhizobium species.
- In planta N2O production increased almost linearly with increasing nitrate concentrations (1-12 μM) in nitrogen-fixing plants, ranging from 10-45 μM N2O per mg nodule dry weight.
Conclusions:
- Rhizobium species are capable of producing nitrous oxide from various nitrogen substrates under anoxic conditions, both independently and within plant nodules.
- Environmental factors such as organic matter and nitrate availability significantly influence N2O production.
- The findings highlight the potential contribution of Rhizobium-legume symbiosis to atmospheric N2O emissions, particularly under specific soil conditions.
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