Arbuscular Mycorrhizas, Microbial Communities, Nutrient Availability, and Soil Aggregates in Organic Tomato Production

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 establishing appropriate 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 different levels 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 by AM 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 ecosystem functions 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.

Growth, nutrition, and soil respiration of a mycorrhiza-defective tomato mutant and its mycorrhizal wild-type progenitor

“…76R (76R MYC+) (Barker et al 1998), were surface-sterilised and pre-germinated following Cavagnaro et al (2006). Seeds germinated within~5 days, after which they were planted in peat moss in seedling trays and grown in a glasshouse for 26 days, followed by hardening in a lath house for 21 days.…”

“…Shoot and fruit B, Ca, Fe, K, Mg, Mn, Na, P and Zn contents were determined on plant material that was microwave-digested with nitric acid (Sah and Miller 1992) and analysed by ICP-AES (Thermo Jarrell Ash Corp., Franklin, MA, USA). Stable isotope ratios, and concentrations, of C and N were measured by mass spectrometry at the University of California Davis Stable Isotope Facility (http://stableisotopefaci lity.ucdavis.edu/, verified 17 March 2008) as described by Cavagnaro et al (2006). Soil was removed by cutting the pot away from the soil mass, which retained its form.…”

“…Methodological artefacts may also contribute to the observed variation in the effects of AMF on belowground respiration. Sterilisation or fumigation of soils is commonly used to establish controls in mycorrhizal studies, yet can cause elimination of non-target members of the soil biota (see Cavagnaro et al 2006;for discussion). This may lead to underestimation of the heterotrophic component of soil respiration, and hence, overestimation of the mycorrhizal contribution to soil respiration, with an uncertain influence on total belowground respiration.…”

“…This may lead to underestimation of the heterotrophic component of soil respiration, and hence, overestimation of the mycorrhizal contribution to soil respiration, with an uncertain influence on total belowground respiration. To avoid such potential problems, a mycorrhiza-defective tomato mutant (rmc) and its mycorrhrizal wild-type progenitor (76R MYC+) (Barker et al 1998) have been used in both glasshouse and field settings to study the effects of AMF on plant growth and nutrition (Gao 2002;Poulsen et al 2005;Cavagnaro et al 2006), plant ecology , and responses to elevated atmospheric CO 2 concentrations (Cavagnaro et al 2007b). The growth of the two genotypes has been found to be very similar under a range of circumstances (Cavagnaro et al , 2006Poulsen et al 2005), including non-mycorrhizal conditions, suggesting that the mutation affecting colonisation of rmc by AMF has no pleiotropic effects on other plant processes .…”

“…This unique experimental system allows for averaging of diurnal effects on soil respiration, and assessing the impacts of both abiotic and biotic factors on soil respiration. Using this system, we compared plant growth and nutrition, soil microbial biomass and community profiles, and belowground respiration in mesocosms, of rmc and 76R MYC+ tomato genotypes to avoid disturbance of the wider soil biota (Cavagnaro et al 2006). In order to influence the potential nutritional benefits of mycorrhizal status, we crossed two levels of phosphorus availability with the two tomato genotypes.…”