Methodological considerations for elucidating low-level liming effects onwhite clover symbiosis establishment in an acidic soil model system

摘要

Because acidic soils limit the establishment, growth, and persistence of forage legumes, new information about symbiotic development in these soils, particularly during the early stages, will likely contribute to the solution of this worldwide problem. A previously described laboratory model system (Staley, T. E. and D. R. Morris. 1998. Soil Sci. 163: 230-240) using an acidic (pH, 4.62), fresh Ultisol showed the ability to discern enhancement of growth and nodulation of white clover (Trifolium repens L.) as early as 3 weeks after planting, even when limed to only py, 4.83. We now report on the reproducibility of soil chemical alterations and plant and nodule responses using the identical system with soil collected from the same field plot about 1 year later. The effects of low-level liming rates (pH, 4.54-4.94) and rhizobial (Rhizobium leguminosarum by. trifolii) inoculation levels (50-5000 CFU g(-1) dry soil), of both fresh and re-wet soil on earlier (2 weeks) and later (4 weeks) growth and nodulation of Huia white clover were determined. Plant parameters (shoot mass, shoot N uptake, and root mass) were generally unaffected by liming at any of the inoculation levels at Cut 1 (2 weeks) in both soils, whereas most of these parameters were positively affected by liming at all inoculation levels at Cut 2 (4 weeks), especially in the fresh soil. Positive lime rate effects on nodulation were found at both cuts in both soils, but almost exclusively at the highest inoculation level. Thus, the interactive effects of liming and rhizobial population on nodulation were demonstrated, at 2 weeks, before significant Nz;fixation. Growth rates of asymbiotically grown roots, determined by short-term (hours) assays in soil-over-agar, paralleled the root mass responses to liming in the soil model system. Rhizobial viability, assessed in soil solutions extracted from the limed soils, decreased from about 10(7) to 10(6) CFU mL(-1) in 21 h. These results suggest that although seedling root growth and likely rhizobial viability are compromised within hours after planting and inoculation by soil acidity-related chemical factors, maintenance of a threshold level of viable rhizobia can lead to improved nodulation of white clover even at very low soil pH values.