Influence of Organic Farming on Arbuscular Mycorrhizal Fungal Populations in a Mediterranean

16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at

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 agro-ecosystem 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 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 Agri-environmental 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.

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 long- term 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 copper-based fungicides, may be detrimental to AM fungal populations.

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”.

References

Bàrberi P., Mazzoncini M. (2006). The MASCOT (Mediterranean Arable Systems COmparison Trial) long-term experiment (Pisa, Italy). In Raupp, J., Pekrun, C., Oltmanns, M., Köpke U. (eds.): Long-term field experiments in organic farming. ISOFAR Scientific Series, Verlag, Berlin, p. 1-14.

Bedini S., Avio L., Argese E., Giovanetti M. (2007). Effects of long-term land use on arbuscular mycorrhizal fungi and glomalin-related soil protein. Agr. Ecosys. Environ.120:463-466.

Bending G. D., Turner M. K., Rayns F., Marx M. C., Wood M. (2004). Microbial and biochemical soil quality indicators and their potential for differentiating areas under contrasting agricultural management regimes. Soil Biol. Biochem. 36:1785–1792.

Daniell T.J., Husband R., Fitter A.H., Young J.P.W. (2001). Molecular diversity of arbuscular mycorrhizal fungi colonising arable crops. FEMS Microbiol. Ecol. 36:203-209.

Franke-Snyder M., Douds D. D., Galvez L., Phillips J. G., Wagoner P., Drinkwater L., Morton J. B. (2001): Diversity of communities of arbuscular mycorrhizal (AM) fungi present in conventional versus low-input agricultural sites in eastern Pennsylvania, USA. Appl. Soil. Ecol. 16:35–48.

Galvez L, Douds D.D., Drinkwater L.E., Wagoner P. (2001). Effect of tillage and farming system upon VAM fungus populations and mycorrhizas and nutrient uptake of maize. Plant Soil 228:299–308.

Giovannetti M., Gianinazzi-Pearson V. (1994). Biodiversity in arbuscular mycorrhizal fungi. Mycol. Res. 98:705-715.

Gosling P., Hodge A., Goodlass G., Bending G. D. (2006). Arbuscular mycorrhizal fungi and organic farming. Agr. Ecosys. Environ. 113:17-35.

Helgason T., Daniell, T.J., Husband, R., Fitter, A.H., Young, J.P.W. (1998). Ploughing up the wood-wide web? Nature 394:431.

Oehl F., Sieverding E., Ineichen K., Mader P., Boller T., Wiemken A. (2003): Impact of land use intensity on the species diversity of arbuscular mycorrhizal fungi in agroecosystems of central Europe. Appl. Environ. Microbiol. 69:2816–2824.

Oehl F., Sieverding, E., Mader, P., Dubois, D., Ineichen, K., Boller, T., Wiemken, A., 2004. Impact of long-term conventional and organic farming on the diversity of arbuscular mycorrhizal fungi. Oecologia 138:574–583.

Purin S., Filhoa O. K., Sturmer S. L., (2006). Mycorrhizae activity and diversity in conventional and organic apple orchards from Brazil. Soil Biol. Biochem. 38:1831–1839.

Wright S. F., Green V. S., Cavigelli M.A. (2007). Glomalin in aggregate size classes from three different farming systems. Soil Tillage Research. 94:546–549.

van der Heijden M. G. A., Klironomos J. N., Ursic M., Moutoglis P., Streitwolf-Engel R., Boller T., Wiemken A., Sanders I. R.(1998). Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity. Nature 396:69–72.

[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 , Internet