IN INOCULANTS Nodulaid - 17th International Nitrogen Fixation ...

17 th International Congress on Nitrogen Fixation
Fremantle, Western Australia
27 November – 1 December 2011
Session Details: Thursday 1 December 2011
Concurrent Session 16 – Symbiotic Impacts & Emissions
1100 - 1230
Authors: Mark B Peoples 1 , John Brockwell 1 , John F Angus 1 , B Smith 1 , A Swan 1 and Catherine A
Osborne 2
1 CSIRO Sustainable Agriculture Flagship, CSIRO Plant Industry, GPO Box 1600,
Canberra, ACT 2601.
2 Department of Civil Engineering, Monash University, Clayton, Victoria 3800 (formerly
Microbiology Department, University of Melbourne)
Presentation Title: Effects of rhizobial strain and legume host on emissions of H2 from nodules and the
impact on soil biology and plant growth
Presentation Time: 1100 – 1120
Hydrogen (H2) is produced as an obligate by-product of nitrogenase activity in legume nodules. Some
symbioses possess a hydrogenase-uptake system (designated Hup + ) that is able to recycle almost all of the H2
evolved. However, many strains of root-nodule bacteria (rhizobia) lack the hydrogenase enzyme (Hup ─ ), or have
low Hup activity. In both these latter cases, the H2 diffuses out of the nodules into the soil. Within a very short
period, this H2 is consumed by microorganisms residing close to the nodule ─ and none of it escapes into the
atmosphere.
Experimentation undertaken to assess the Hup status of various nodulated legumes indicated that, of all the
legume x rhizobial strain combinations examined, only (i) soybean x CB1809 (Australian inoculant strain) and (ii)
soybean x USDA110 (former US inoculant strain) were Hup + . All other associations emitted H2 from their
nodules to a greater or lesser extent. Interestingly, the rates of H2 evolution from the nodules of faba bean, lupin
and subterranean clover (mean: >450 μmol H2 evolution per gram nodule dry weight per hour) were consistently
two- to five-fold greater than those observed for other legume x rhizobia combinations.
Measurements of H2 evolution from Hup ─ nodules in the field indicate that >200,000 litres per hectare of H2 gas
may be released into the soil during the life of a legume crop fixing around 200 kg nitrogen per hectare (Peoples
et al. 2008). In addition, we observed increased populations of certain species of actinomycetes in the
immediate vicinity of field-grown Hup ─ soybean nodules (Osborne et al. 2010). Some of these organisms are
known to have growth-promoting characteristics.
A number of studies in controlled-growth facilities and in the field in Australia and Canada have indicated
improvements in plant growth and yield of 10-33% when plants are grown in soil exposed to H2. These findings
need to be confirmed in other environments. If confirmation is obtained, there would be strong reason to
recommend that future decisions on the choice of rhizobial strains for inoculants should involve ensuring that the
resultant symbioses are Hup ─ in order to confer yield advantages on subsequent crops.
Osborne CA, Peoples MB, Janssen PH (2010). Detection of a reproducible, single-member
shift in soil bacterial communities exposed to low levels of hydrogen. Applied and
Environmental Microbiology 76, 1471-1479.
Peoples MB, McLennan PD, Brockwell J (2008). Hydrogen emission from nodulated
soybeans [Glycine max (L.) Merr.] and consequences for the productivity of a subsequent
maize (Zea mays L.) crop. Plant and Soil 307, 67-82.
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2011