This study was conducted to test the hypothesis that spatial variations in soil microbial variables in a Thai rice paddy are accurately described by multivariate profiles of the soil bacterial communities. We found that community-level physiological profiles of soil bacterial communities could better describe the population density of Rhizoctonia solani in soil than the physicochemical profiles do. However, soil dehydrogenase levels were closely correlated with soil fertility (P<0.05), and these were better described by the physicochemical profiles. Hence, the hypothesis was rejected, and we suspect that soil microbial variables react differently to the same physicochemical changes. The average population density of R. solani (35 colony-forming units/g dry soil) was relatively high in the soil we studied, and the soil fertility was found to be among the poorest in Thailand. The soil quality was comparable to the most degraded bare ground soil in an adjacent bioreserve in terms of Shannon diversity index based on the communitylevel physiological profile as well as values of soil fertility indices. Overall, the soil microbial and physicochemical indicators showed that the paddy soil needs to be supplemented with soil nutrients. Otherwise, R. solani may cause a significant reduction in rice production.

Soil dehydrogenase activity is a good indicator of overall microbial activity in soil, and it can serve as a good indicator of soil condition. However, seasonal changes in soil moisture content may have an effect on soil dehydrogenase activity, making an accurate assessment of soil condition difficult. In this study, we attempted to determine the significance of soil dehydrogenase activity for assessing soil condition, and we attempted to find a way to account for the influence of soil moisture content on soil dehydrogenase activity. Soils were sampled in dry evergreen forest (original vegetation), bare ground (severely degraded) and Acacia plantation plots established on bare ground in 1986 and 1987 in Sakaerat, Thailand. Soil physico-chemical characteristics and dehydrogenase activity in the Acacia plantation soil had few differences from those in the evergreen forest soil. Soil dehydrogenase activity varied significantly between the bare ground and the forests regardless of the season (wet or dry), while the season did not produce a significant variation in soil dehydrogenase activity, as determined by repeated measures analysis of variance (p=0.077). The physico-chemical data provided the first principal component as a good measure of soil fertility. Values of soil dehydrogenase activity significantly cor- related to scores of the soil samples of the first principal component (R=0.787, p<0.001). We found that soil dehydrogenase activity is a useful indicator of the extent of soil degradation and the rehabilitative effects of reforestation in this part of Thailand. La actividad de la deshidrogenasa del suelo es un buen indicador de la actividad microbiana en el suelo en general, y que puede servir como un buen indicador del estado del suelo. Sin embargo, los cambios estacionales en el contenido de humedad del suelo pueden tener un efecto sobre la actividad de la deshidrogenasa del suelo, lo que hace difícil una evaluación precisa del estado del suelo. En este estudio, hemos tratado de determinar la importancia de la actividad deshidrogenasa del suelo para evaluar el estado de los suelos, y hemos tratado de encontrar una manera de dar cuenta de la influencia del contenido de humedad del suelo en la actividad deshidrogenasa del suelo. Los suelos fueron muestreados en el bosque seco siempre verde (vegetación original), suelo desnudo (muy degradado) y parcelas de plantación de Acacia establecidos en el suelo desnudo, en 1986 y 1987 en Sakaerat, Tailandia. Suelo características físico-químicas y la actividad deshidrogenasa en el suelo de plantación Acacia tenían pocas diferencias con los del suelo de los bosques de hoja perenne. Actividad de la deshidrogenasa del suelo varió significativamente entre el suelo desnudo y los bosques, independientemente de la temporada (húmedo o seco), mientras que la estación no produjo una variación significativa en la actividad de la deshidrogenasa del suelo, tal como se determina por análisis de medidas repetidas de la varianza (p = 0,077). Los datos físico-químicos siempre que el primer componente principal como una buena medida de la fertilidad del suelo. Los valores de actividad de la deshidrogenasa del suelo correlacionado significativamente con las puntuaciones de las muestras de suelo de la primera componente principal (R = 0,787, p <0,001). Se encontró que la actividad deshidrogenasa del suelo es un indicador útil del grado de degradación del suelo y los efectos de rehabilitación de reforestación en esta parte de Tailandia.