Correlations between soil microbial and physicochemical variations in a rice paddy: implications for assessing soil health

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.