Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 9/7/2010
Publication Date: 5/30/2011
Citation: Toro, M., Nichols, K.A. 2011. Glomalin as an indicator of mycorrhizae in tropical agroecosystems. IN: Microbial Ecology of Tropical Soils. A.S.F. de Araujo M. do Vale Barreeto Figueiredo (Eds.). Book Chapter. p. 207-247. Interpretive Summary: Given growing interest in environmentally-friendly techniques to improve agriculture, such as the use of cover crops and application of arbuscular mycorrhizal (AM) inoculants to soil, a study of Venezuelan agriculture was conducted to better understand management effects on soil quality and AM fungi in tropical agroecosystems. Results of this study suggest conservation management techniques, such as cover crop usage and biological fertilization with AM fungi, promote the stabilization of soil aggregates. Additionally, AM mycelium and glomalin increased under these treatments compared to more traditional treatments. Many studies have shown a corollary relationship between these mycorrhizal parameters and soil aggregation. This study also highlighted the role of glomalin as an indicator of AM fungal presence in agricultural systems, thus showing that alterations in symbiosis and/or propagules may result from modifications in agricultural practices. Long-term studies are needed to better understand biological effects from AM fungi on soil physical properties. These studies should include analysis of additional soil types under contrasting management practices in order to refine conclusions regarding the ecological importance of AM fungi in Venezuelan agricultural soils. Although results obtained in this study provided useful information on soil biophysical properties in tropical agroecosystems, no conclusive statements could be made regarding differences among treatments. Complexity associated with the measured parameters and tropical agroecosystems suggests future evaluations on soil biophysical properties would benefit from investigations of less complex agroecosystems.
Technical Abstract: Arbuscular mycorrhizae (AM) are symbiotic mutualistic associations established between the roots of most plants and certain soil fungi. This symbiosis has positive effects on the development and nutrition of plants as it provides them with low mobility soil elements such as P, Zn and Cu. It also improves the soil physical condition by fostering the formation of stable aggregates which enhance soil structure, plant establishment, and soil biological conditions. AM are naturally found in most ecosystems, however, their presence and abundance may be affected by management or level of soil disturbance. Some management examples include traditional agricultural practices, such as tillage, chemical fertilization and the use of pesticides. Many of these agricultural systems show low biodiversity and low amounts of mycelium extending out from colonized roots. Spore amounts of these fungi also are affected by management with negative impacts on the establishment of symbiosis in the plant, thus leading eventually to the virtual extinction of AM propagules. This paper presents a study of Venezuelan agricultural systems where conservation or non-conservation agricultural management techniques have been applied. Its main objective is to explain whether AM fungal mycelium outside the root and glomalin amount (mg / g soil) are affected by soil management and whether these factors are related to stable soil aggregates (water stable aggregates, WSA). Two of the systems studied are located in the Venezuelan central plains: Chaguaramas and Santa María de Ipire, and a third system is located in the semi-arid area of the Falcon state in the northeast side of Venezuela. In the semi-arid area of Falcon state, the following values were obtained: immunoreactive glomalin between 0.52 and 1.6 mg / g soil and EM length between 0.14 and 0.24 m / g soil. A low correlation was calculated between glomalin and EM, but no correlation was measured with WSA, suggesting factors different from AM could be affecting aggregate stability in the semi-arid area of Falcon. The Buffel plant species (Cenchrus ciliaris) showed the best glomalin-EM relationship and a higher production of both parameters. The Chaguaramas agrosystem had glomalin values between 0.34 and 0.6 mg / g soil with EM values between 0.7 and 1.8 m / g soil. In the presence of the leguminous cover crop, Crotalaria juncea, higher contents of glomalin and WSA were observed which suggests the application of a cover crop favoured both parameters. In the Santa María de Ipire ecosystem, the presence of Centrosema macrocapum favoured glomalin production (up to 0.68 mg / g soil) as well as EM values (between 1.8 and 8.4 mg / g soil). The application of AM inoculants favoured the formation of stable aggregates between 250 and 53 µm, thus suggesting the beginning of the agglutinative effect of the AM hyphae over smaller particles for the formation of WSA. Future studies using regular measurements through time on systems where conservation methods have been applied may be used to determine more directly the positive interactions of AM fungi on soil aggregation and structure.