Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2009
Publication Date: 4/1/2010
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2277.pdf
Citation: Ibekwe, A.M., Poss, J.A., Grattan, S.R., Grieve, C.M., Suarez, D.L. 2010. Bacterial diversity in cucumber (Cucumis sativus) rhizosphere in response to salinity, soil pH and boron. Soil Biology and Biochemistry. 42(4):567-575. Interpretive Summary: Environmental stress is a major limitation in saline agricultural especially where high salinity from irrigation practices may affect the selection, growth and yield of crops. This is the case in the drainage impacted areas on the Westside of the San Joaquin valley of CA where there are serious limitations in implementing drainage water reuse system due to high boron and salinity. In this study, the polymerase chain reaction (PCR) assay was used to determine how the incorporation of pH into the experimental variables may provide insights into how salinity-boron interactions may affect rhizosphere microbial population. Data from the study showed that salinity and pH were the most influential factors affecting the growth of plants and the effect of boron on plant was more severe under slightly acidic conditions. Our results indicate that the effects of salinity, boron, and pH were more severe on the rhizosphere bacterial population during the first week of growing cucumber, with decreasing impacts with plant growth. Also, salinity-boron-pH interaction effects were more apparent during week seven with significant interaction effects on the bacterial diversity indices compared to insignificant interaction effects on plant. This suggests that changes in microbial diversity may be the first indicator of stress in salinity-boron-pH affected soils. Therefore, if stress can be detected early enough in salinity-boron-pH affected soils, then some remedial action may be possible to improve soil quality and crop performances.
Technical Abstract: Soil salinity is a major factor relating microbial communities to environmental stress in the microbial selection process as stress can reduce bacterial diversity. In the San Joaquin Valley (SJV) of California, the problem of increasing salinity and consequently, decreasing crop productivity, due to reuse of saline drainage water are major concerns. An experiment was conducted in a closed, recirculating volumetric lysimeter system (VLS) consisting of 24 experimental plant growth units to determine the interactive effects of salinity, boron and pH on rhizosphere and non-rhizosphere microbial composition of cucumber (Cucumis sativus L. cv. Seminis Turbo hybrid). Plants in the VLS were irrigated from individual reservoirs containing a modified half-strength Hoagland's nutrient solution combined with salinity, boron (B), and pH treatments. The results indicated that salinity and pH were the most influential factors affecting the growth of plants and the effect of boron on the plant was more severe under slightly acidic conditions. Total bacterial DNA was extracted from rhizosphere and non-rhizosphere samples, and a 236-bp DNA fragment in the V3 region of the small subunit ribosomal RNA genes of eubacteria was amplified. The 16S rRNA and the products were subjected to denaturing gradient gel electrophoresis (DGGE) and sequencing. Analyses of bacterial diversity showed that the effects of salinity, boron, and pH were more severe on the rhizosphere bacterial population during the first week of growing cucumber, with decreasing impacts with plant growth. However, there was no salinityeBepH interaction effects on plant biomass, but the effects were seen in the number of heterotrophic bacteria in the rhizosphere and on species richness and diversity during week seven of the study. These suggest that the effects of salinityeBepH interactions may influence microorganisms first before plants and may pose long term effects on soil quality.