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

17 th International Congress on Nitrogen Fixation
Fremantle, Western Australia
27 November – 1 December 2011
Session Details: Wednesday 30 November 2011
Concurrent Session 15 – Quantification of N-Fixation
1600 – 1740
Authors: Georg Carlsson 1 & Kerstin Huss-Danell 2
1 Department of Agrosystems, Swedish University of Agricultural Sciences, SE-23053
Alnarp, Sweden.
2 Department of Agricultural Research for Northern Sweden, Swedish University of
Agricultural Sciences, SE-90183 Umeå, Sweden.
Presentation Title: Does nitrogen transfer confound 15 N-based quantifications of N2 fixation?
Presentation Time: 1600 – 1620
Nitrogen transfer between neighbouring plants is a well-known phenomenon, mediated via e.g. rhizodeposition
and mycorrhizae. Transfer of fixed N from legumes to non-legume reference plants has been suggested to
confound N2 fixation estimates obtained by the 15N natural abundance and isotope dilution methods, because a
reference plant taking up fixed N will supposedly not correctly reflect the 15 N signature of plant-available soil N.
On the other hand, experiments using direct leaf-feeding of 15 N, which allows for direct detection of N transfer
with high precision, have shown that N transfer also occurs in the direction from non-legume to legume. In order
to establish whether N transfer causes problems in N2 fixation measurements with 15 N-based methods when
using reference plants grown together with the legume, we labelled both legumes and non-legumes with 15 N and
performed detailed measurements of N transfer in mixed plant communities in the field, including measurements
of potential N transfer from legume to legume. N transfer occurred in all directions: from legume to non-legume,
from non-legume to legume and from legume to legume. These results, together with analyses of previously
published data on N transfer, were used to calculate nitrogen fixation with different N transfer scenarios, showing
the effects of using reference plants in mixture with the legume versus in pure stand. We conclude that the most
important mechanism of N transfer is likely to be indirect, via rhizodeposition and litter degradation, and that fixed
N transferred to neighbouring reference plants also modifies the 15 N signature of the soil N available to the N2fixing
legume. This provides strong support for using reference plants growing as close as possible to the N2fixing
legume for reliable N2 fixation quantifications.
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2011