Objective  Soil microorganism is a sensitive indicator of change in soil health, but how microbial groups and the sensitivity of the different indicators in paddy soils responding to continuous application of organic amendment and liming is still unknown.

  Method  The purple mud of double-cropped rice fields in mid-subtropics was selected. The “black box” indices of soil microorganism (soil microbial biomass carbon and nitrogen, microbial entropy, and soil respiration) and soil living microorganisms (PLFAs) structure of different microbial groups in response to 5-year continuous applying organic amendment or liming were studied.

  Result  Comparing to the control, the soil microbial biomass carbon (MBC), nitrogen (MBN), and soil respiration rate in organic amendment treatment were elevated by 37%, 28% and 44%-59%, respectively. Meanwhile the indices of soil microbial diversity were enhanced. The soil bacterial structure was changed, especially for the PLFAs absolute abundance of G⁻ increased by 100%, while the fungal groups were insensitive to the organic amendment. However, the soil microbial entropy and soil respiration rate was reduced by 11% and 26%-52%, respectively, as well as the decreased indices of soil microbial diversity after the liming. The absolute abundance of bacteria, actinomycetes, and most fungal groups were reduced by 30%-58%. The correlation analysis showed that soil organic matter content was significantly and positively correlated with soil microbial total PLFAs and bacterial PLFAs. However, there was a significant positive correlation between soil pH and Simpson diversity index. This study revealed that organic fertilizer application significantly increased soil organic matter content, resulting in significantly increased bacterial MBC, MBN, G⁻ and soil respiration rate. The decrease of soil microbial community structure and activity after lime application was related to soil pH.

  Conclusion  As a whole, the soil microbial indices revealed a positive and a negative response to 5-year continuous application of organic amendment and liming, respectively. Specific microbial groups (such as G⁻) were higher than those of “black box” indices, which showed great potential in soil health assessment tools.