16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
Influence of organic farming on arbuscular mycorrhizal fungal
populations in a Mediterranean agro-ecosystem.
Bedini, S.1, Cristani, C.2, Avio, L.3, Sbrana, C.4, Turrini, A.5 & Giovannetti, M.6
Key words: arbuscular mycorrhizal fungi, glomalin-related soil protein, spore
population, biodiversity, organic farming.
Abstract
Arbuscular mycorrhizal fungi (AMF) are key components of the soil microbiota,
fundamental for soil fertility, plant nutrition and functioning of agroecosystems. Data on
the interactions between organic practices and AMF populations are limited and
inconsistent. Here we compared AM fungal communities and glomalin-related soil
protein (GRSP) content occurring in a recently converted organically farmed soil with
those occurring in a conventionally managed soil. The results show that the two
farming systems did not significantly differ in AM fungal spore populations and
glomalin-related soil protein. We hypothesize that in our experimental system, which
was converted from conventional to organic farming only recently (5 years), there may
not have been enough time to allow the establishment of differentiated AM fungal
populations.
Introduction
Soil microbial communities are considered a vital factor for the functioning of
agroecosystems and success in organic farming (Gosling et al., 2006).
Glomeromycotan fungi form arbuscular mycorrhizal (AM) symbioses with most crop
plants and are fundamental for soil fertility and plant nutrition and health. Since
different species and isolates of AM fungi (AMF) show differences in plant growth
responses and quality, any change in their populations may result in changes of agroecosystem productivity (van der Heijden et al., 1998). AMF are strongly affected by
anthropogenic activities (Giovannetti and Gianinazzi-Pearson, 1994), and intensive
agricultural practices, such as crop rotation fertilization pest control and tillage impact
AMF, reducing population biodiversity (Helgason et al., 1998; Daniell et al., 2001).
Organic agriculture has been shown to increase AMF root colonization and propagule
numbers (Galvez et al. 2001; Oehl et al. 2003), although low input practices used in
such management system do not always allow the level of biodiversity to increase,
even after a long time (Franke-Snyder et al., 2001). Hence, understanding the
structure and the dynamics of AMF populations as affected by diverse agricultural
practices represents an important prerequisite for the success of organic farming. This
1
Dipartimento di Biologia delle Piante Agrarie, Università di Pisa, Via del Borghetto 80, 56124
Pisa, Italy
2
As Above
3
Istituto di Biologia e Biotecnologia Agraria, C.N.R., Via del Borghetto 80, 56124 Pisa, Italy
4
As Above
5
Dipartimento di Biologia delle Piante Agrarie, Università di Pisa, Via del Borghetto 80, 56124
Pisa, Italy
6
As Above, E-Mail mgiova@agr.unipi.it, Internet www.agr.unipi.it/dbpa/giovannetti
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
work was intended to describe the AM fungal communities occurring in field trials
comparing conventional and stockless organic farming.
Materials and methods
The experimental plots are located in the CIRAA (Interdepartmental Centre for Agrienvironmental Research “E. Avanzi”, University of Pisa) experimental centre near Pisa
(Italy), and are a part of the long-term experimental system MASCOT (Mediterranean
Arable Systems COmparison Trial). MASCOT was established in Autumn 2001 as a
long-term comparison between organic and conventional management system for a
typical rotation of coastal Tuscany, characterized by the absence of livestock, and
including sugar beet, common wheat, sunflower, pigeon bean, and durum wheat. In
spring 2006 sugar beet was replaced by maize.
In the conventional management, crops were minerally fertilized with 602 kg ha-1 N ,
487 kg ha-1 P2O5, 346 kg ha-1 K2O distributed over the five years of rotation. In organic
management all crops were supplied with 30 kg ha -1 N, 30 kg ha-1 P2O5, 0 kg ha-1 K2O
each year. Additionally, in the organic system, red clover was interseeded in common
and durum wheat and used as a green manure for subsequent crops.
The five crops in the rotation were allocated to five fields and managed organically or
conventionally (each group of five fields represents a system within a block; each crop
is present every year). Systems were replicated three times according to a
randomized complete block design. Additional information on the agricultural practices
used in the organic and conventional systems can be found in Bàrberi and Mazzoncini
(2006).
Soil samples consisted of seven random cores collected from each of three plots
under organic and conventional management after the harvesting of maize. Samples
were collected in the second half of June 2006 in the plots. AM fungal spores were
extracted from 50 g soil samples by wet-sieving and decanting, down to a mesh size
of 50 μm. Spores and sporocarps were examined under a dissecting microscope and
the numbers and types of AM fungal spores were recorded. Only intact, healthy
spores were counted. Spore identification was performed under the light microscope,
after mounting the spores in polyvinyl alcohol lacto-glycerol (PVLG) on microslides.
We determined species richness and calculated the frequency of occurrence. Relative
abundance was calculated as the number of spores of each species divided by the
total number of spores. We also calculated the Shannon diversity index and the Pielou
evenness index.
Results
Nine species of AMF were found in the experimental sites, eight belonging to Glomus
and only one to Scutellospora. Some differences in species occurrence and frequency
were observed: S. calospora spores were more frequent in the organic plots than in
the conventional ones, and G. rubiforme spores were more frequent in conventional
plots. (Fig.1).
Species richness and other diversity indexes in the two agricultural systems did not
differ statistically.
The organic and conventional farming systems did not show statistical differences in
EE-GRSP, whose values were 172.5 to 170.3 g g-1 soil, respectively.
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
500
Spores 100 g-1 soil
400
300
200
***
100
*
2
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ca
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sp
o
1
p.
p.
S.
G
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G
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ub
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Figure 1: Distribution of fungal spores per AM fungal species, in organic
(hatched bars) and conventional (dotted bars) plots. *, P<0.05; ***, P<0.01
Discussion
The analyses of AM fungal spores of the experimental plots show a quantitative and
qualitative uniformity of AM fungal populations in the conventional and organic farming
systems, 5 years after the beginning of the experiment. Our data are in agreement
with previous reports showing only slight differences in AM fungal populations
between conventional and organic farming systems a few year after conversion (Purin
et al., 2006). Slight differences in the levels of diversity between the two systems were
found after 15 years of cultivation (Franke-Snyder et al., 2001). By contrast, in a longterm comparison trial in central Europe, higher species richness and diversity of AMF
were reported (Oehl et al., 2004), while other authors found an increase of AM fungal
inoculum in organic farmed soils (Bending et al., 2004). Overall we assessed the
occurrence of nine AM fungal morphospecies. Such level of biodiversity is lower than
that found in other organically farmed soils, while it is consistent with data from
conventional agriculture (Oehl et al., 2003).
A few differences were also recorded in GRSP concentrations, in agreement with
previous observations concerning conventional versus organically managed soils
(Purin et al., 2006; Wright et al., 2007). Our present data contradict previous findings
obtained in a 50-year-old corn monocrop subject to conventional high-input
agriculture, where GRSP concentration was 7.5-fold lower than in an organically
grown grassland (Bedini et al., 2007).
On the basis of the present results we can hypothesize that in our experimental
system, only recently converted from conventional to organic farming (5 years), there
was not enough time to allow the establishment of differentiated AM fungal
populations. Further investigations are needed to understand whether other factors,
such as residual phosphate from previous conventional management, or organic
farming management practices such as tillage for weed control and the use of copperbased fungicides, may be detrimental to AM fungal populations.
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
Conclusions
Our data on the characterization of AM fungal spore populations and on GRSP
content of two differentially managed field trials, 5 years after conversion from
conventional to organic farming, represent the reference point for future assessment
of putative AM fungal population shifts.
Acknowledgments
This work was funded by the MIUR-FISR Strategic Project SIMBIOVEG 2005-2008
“Food quality and health”.
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