Soil sterilization and organic carbon, but not microbial inoculants, change microbial communities in replanted peach orchards

Abstract

Methyl bromide is highly effective in reducing soil pathogens. Although its use was to be phased out completely in the United States by 1 January 2005, due to environmental concerns, a 1-year critical-use exemption will allow tree fruit growers to use the fumigant through the end of the year. To explore possible replacements for methyl bromide, we compared the effects of pre- and postplant treatments and amendments on soil microbial communities and tree vigour in a replanted peach orchard site north of Modesto, California, USA. This orchard suffered from bacterial canker and had been removed 2 years prior. The soil at this site is classified as a Delhi sand. The preplant soil fumigants include methyl bromide, metam sodium and 1,3-dichloropropene, while the postplant treatments are sodium tetrathiocarbonate, black polyethylene film mulch, compost+microbial inoculant and compost+kelp extract+humic acid. Compost alone (preplant) and compost+calcium (pre- and postplant) were also included. Both soil sterilization treatments and organic carbon amendments changed the composition of microbial communities in the soil. High microbial biomass is generally considered beneficial to agricultural soils; we found that it was usually highest in soils that received the organic carbon amendment and lowest in those with soil sterilization. However, tree vigour was highest with the sterilization treatments. The effects of a microbial inoculants/organic carbon combination on microbial communities and plant vigour were no different from simply adding organic carbon.