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Plant and Soil (2006) 282:209–225 DOI 10.1007/s11104-005-5847-7

Ó Springer 2006

Arbuscular mycorrhizas, microbial communities, nutrient availability, and soil aggregates in organic tomato production
T.R. Cavagnaro1,5, L.E. Jackson1, J. Six2, H. Ferris3, S. Goyal2, D. Asami4 & K.M. Scow1
Department of Land, Air and Water Resources, University of California Davis, One Shields Avenue,956168627, Davis, CA, United States. 2Department of Plant Sciences, University of California Davis, One Shields Avenue, 95616-8627, Davis, CA, United States. 3Department of Nematology, University of California Davis, One Shields Avenue, 95616-8627, Davis, CA, United States. 4Department of Human Nutrition, University of California Davis, One Shields Avenue, 95616-8627, Davis, CA, United States.5Corresponding author*
Received 5 July 2005. Accepted in revised form 12 December 2005
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Key words: aggregates, arbuscular mycorrhizal fungi, tomato mutant, nematode, organic farm, PLFA

Abstract Effects of arbuscular mycorrhzal (AM) fungi on plant growth and nutrition are well-known, but their effects on the wider soil biota are less clear. This is in part due to difficulties with establishingappropriate non-mycorrhizal controls in the field. Here we present results of a field experiment using a new approach to overcome this problem. A previously well-characterized mycorrhizal defective tomato mutant (rmc) and its mycorrhizal wildtype progenitor (76R MYC+) were grown at an organic fresh market tomato farm (Yolo County, CA). At the time of planting, root in-growth cores amended with differentlevels of N and P, were installed between experimental plants to study localized effects of mycorrhizal and non-mycorrhizal tomato roots on soil ecology. Whilst fruit yield and vegetative production of the two genotypes were very similar at harvest, there were large positive effects of colonization of roots by AM fungi on plant nutrient contents, especially P and Zn. The presence of roots colonized byAM fungi also resulted in improved aggregate stability by increasing the fraction of small macroaggregates, but only when N was added. Effects on the wider soil community including nematodes, fungal biomass as indicated by ergosterol, microbial biomass C, and phospholipid fatty acid (PLFA) profiles were less pronounced. Taken together, these data show that AM fungi provide important ecosystemfunctions in terms of plant nutrition and aggregate stability, but that a change in this one functional group had only a small effect on the wider soil biota. This indicates a high degree of stability in soil communities of this organic farm. Introduction Ecological processes provide key ecosystem services for agriculture. Organically managed agroecosystems offer a unique context in which to study theecological relationships between soil biology and biogeochemistry. Microbial processes are crucial for nutrient availability because only organic sources of nutrients can be used
* FAX No: +1-530-752-1552. E-mail: trcavagnaro@ucdavis.edu

(USDA National Organic Program Standards). Organic farmers manage for high soil organic matter (SOM) and net N mineralization rates. Furthermore, use of syntheticpesticides, which can negatively impact soil biota (e.g., Miller and Jackson, 1997; Sukarno et al., 1993), is also prohibited (USDA National Organic Program Standards). As a consequence, microbial activity is often higher in organic than conventionally-managed soils (Carpenter-Boggs et al., 2000; Drinkwater et al., 1995; Lundquist et al., 1999), community composition can be more complex

210(Mader et al., 2002), and in some instances, ¨ invertebrate density and biodiversity increases (Hole et al., 2005). Abundance of arbuscular mycorrhizal (AM) fungi may also increase under organic management (Mader et al., 2002; Smu¨ kler and Jackson, personal communication). AM fungi effectively increase the absorptive surface of the plant root system thereby providing access to soil-derived...
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