INTRODUCTION

Both beneficial and pathogenic bacteria exist in the soil. Many of the beneficial bacteria live on or near the root surface (termed rhizobacteria) and are instrumental in plant growth and the biological control of crop pests. How a crop is produced may affect crop rooting patterns and the number and types of rhizobacteria. Factors that may influence rhizosphere population dynamics include the crop grown, soil tillage, soil type, and the number and types of weeds present. If the effects of crop management on rhizobacteria ecology were understood, rhizobacteria populations could potentially be manipulated to enhance crop productivity.

OBJECTIVE

To identify the major genera and species of root bacteria and to monitor the potential ecological shifts of rhizobacteria between soil types, under traditional versus innovative crop management, and during the growing season.

APPROACH

Sampling Site and Schedule. Rhizobacteria from corn grown in a split-landscape study at the Pee Dee Research and Education Center was sampled from a Rains (Fine-loamy, siliceous, semiactive, thermic Typic Paleaquults) and Bonneau (loamy, siliceous, subactive, thermic Arenic Paleudults) loamy sand soils in May and July 1999. The split-landscape study is a field-size comparison of a traditional cropping system to a more innovative cropping system centered on new cropping practices and technologies. Selected bacterial isolates were identified by GC/FAME analyses. A mixture of root and soil, 15 cm radius from the plant and 15 cm deep from the top, was collected from each corn plant. For non-rhizosphere control soil, soil samples were collected from soil without vegetation. The samples were kept on "blue ice" until being processed within 48 hours.

Experimental Protocol. Plant roots were separated from soil, placed in a dilution buffer, and shaken for 30 min at 200 rpm on a rotary shaker. For non-rhizosphere control, soil without roots was used. The resulting suspensions were subjected to serial dilution and plating using standardized techniques and medium (Fig. 1). A 1/10 tryptic soy broth agar (TSBA) supplemented with cycloheximide (100 mg/L) to inhibit fungi was used for total bacterial populations. From the 1/10 TSBA plates, we randomly selected 40 isolates/plant species or non-rhizosphere control to be identified using the GC/FAME analysis (Figure 1).

Gas Chromatography Fatty Acid Methyl Ester (GC-FAME) Analysis. Identification of the bacterial isolates was determined using the gas chromatography/MIDI Microbial Identification System MIS (MIDI,Inc.,Newark, DE) in the Multiuser lab at Clemson University.

RESULTS

Because results from the Innovative and Traditional sides were similar, only results from Innovative side are presented (Tables 1 and 2). Bacillus, Burkholderia, or Arthrobacter were major genera from corn roots in the Rains soil in both the May and July samples and the Bonneau soil in May. Burkholderia was not a major genus on corn roots from the Bonneau soil in July.

As expected, diversity (measured here as the number of different genera) was higher on roots than in non-rhizosphere soil. Rains soil had more microbial diversity on corn roots than the Bonneau soil, except for rhizobacteria in July with 11 vs. 14 different genera, found respectively. However, diversity in rhizobacteria from the Bonneau soil in July was unbalanced with only 2 genera (Bacillus and Enterobacter) found representing over 70% of all isolated bacteria.

Also, diversity decreased from May to July, probably because of a decrease in soil moisture. In dry conditions, especially in non-rhizosphere soil, the proportion of spore-formers increased, e.g., 92.5% of Bacillus in non-rhizosphere Bonneau soil in July.

With respect to species, the May samples contained similar numbers for Rains (30) and Bonneau (29) rhizosphere soils and for non-rhizosphere soils (14-16), as shown in Figures 2 through 4. However, in July the number of species decreased to 18-19 in the rhizosphere soils. In the non-rhizosphere Rains soil, a decrease to 16 species was noted; whereas, only 5 species were recorded in the Bonneau non-rhizosphere soil. B. megaterium and B. cereus accounted for 92% of these 5 species (Figures 5 through 8).

SUMMARY OF RESEARCH RESULTS

In May samples, the number of species from both the Rains and Bonneau rhizosphere soils were approximately 2-fold (100% increase) higher than non-rhizosphere samples.

In July samples, the number of species under drought conditions decreased slightly for the Rains non-rhizosphere soil, compared to the rhizosphere soil.

However, Bonneau non-rhizosphere had 5 species versus 19 species in the Bonneau rhizosphere samples in July.

Decreases in bacterial genera over time were less drastic than for species. Again, Bonneau non-rhizosphere had 4 genera versus 14 in rhizosphere in July samples.

The bacterial diversity for genera and species in the more productive soil, Rains, was more resistant to drought conditions than in the less productive Bonneau soil.

CONCLUSIONS

Soil bacteria populations were much higher on corn roots (rhizosphere) than in root-free soil. The number of genera and species of bacteria were much higher in the more productive Rains soil than the sandy Bonneau soil, suggesting that soil quality may be linked to specific soil bacteria populations on the corn roots. It is surprising that there was no effect of cropping system (Innovative versus Traditional) on soil bacteria populations within a soil type. We are finding that soil fertility and carbon differences between the two cropping systems primarily occurring in the top inch of soil (see Improvements in Soil Quality - Soil Fertility). On the other hand, we are examining soil bacteria populations from the rhizosphere of corn roots collected as a composite from the top 8 inches of soil. Research is needed to determine whether the effects of cropping practices, such conservation tillage, on rhizosphere bacteria populations are dependent on soil depth.

For further information about this research, please contact: Dr. Horace Skipper 864-656-3525 email