Study on soil microbial biodiversity in rhizosphere of vetiver grass for soil improvement in degradating soil   Vanlada Sunanthapongsuk Pitayakon Limtong Siangjeaw Piriyaprin Chaveevan Leaungvutivirog Nuanjun Pasda   ABSTRACT         Study on soil microbial biodiversity in rhizosphere of vetiver grass in Ta Yang soil series was conducted on the areas of Chaipattana Mae Fah Luang Forestation Project, Prachuabkhirikhan Province during 1998-1999. The randomized complete block design was used consisting of 11 treatments in 3 replications. There were 10 ecotypes growing vetiver grass which provided 2 species as Vetiveria zizanioides 5 ecotypes: South India, Fiji, Prarachathan, Monto and Surathani; the another one as Vetiveria nemoralis 5 ecotypes: Prachuabkhirikhan, Roi-Et, Kamphaenphet, Ratchaburi and Nakhonsawan comparing to non-planting vetiver grass areas. The results indicated that amount of soil microorganisms in one year age of vetiver rhizosphere at 0-60 cm of soil depth were more increase than non- planted areas. The celulolytic microorganisms were increased from 6.48 to 8.25 log no./gm of soil. For the non-symbiotic fixing nitrogen bacteria and phosphate solubilizing microorganisms were increased from 1.80 to 3.86 log no./gm of soil. Nevertheless the endo-mycorrhiza were increased from 4 to 15.5 spore/100 gm of soil. Most of microorganisms were appeared in the rhizosphere of Vetiveria zizanioides. Cellulase and phosphatase activities were raised from 0.091 and 1.21 to 0.723 and 5.81 milli unit/gm of soil. Because of densed fibrous root of vetiver which produced organic substances had effected significantly enhancing microbial number and activities of available nutrients transformation in soil. The quantitive releasing phosphorus. potassium, calcium, magnesium and sulphur were increased from 2.25, 67, 417, 362 and 1.25 to 4.81 137, 756, 562 and 2.44 ppm respectively. The pH of soil was changed from 6.2 to 6.9 and the amount of soil organic matter was raised from 0.49 to 0.75 percent. Furthermore, soil moisture content was also changed from 3.86 to 6.95 percent.         Keywords: soil microbial biodiversity, rhizosphere, vetiver grass