Purpose:  Genetic modifications (GM) of commercial crops offer many benefits. However, microbial-mediated decomposition might be affected by GM crop residues in agricultural ecosystems. The objective of this study was to assess the possible impacts of cry1Ab gene transformation of rice on soil microbial community composition associated with residue decomposition in the paddy field under intensive rice cultivation.
Materials and methods:  A 276-day field trial was set up as a completely randomized design for two types of rice residues, KMD (Bt) and Xiushui 11 (non-Bt parental variety) in triplicate by conventional intensive rice cropping system. The litterbag method was used in the rice residue decomposition and a total of 120 straw and root litterbags were either placed on the soil surface or buried at 10 cm depth in the field on Dec. 24, 2005. The litterbags were sampled periodically and their soil bacterial and fungal communities were determined by terminal restriction fragment length polymorphism (T-RFLP). The additive main effects with multiplicative interaction (AMMI) model were performed for the analysis of T-RFLP on binary variables of peak presence (presence/absence). The analysis of variance and linear regressions were performed for analysis of AMMI data.
Results and discussion:  Total AMMI model analysis revealed that microbial community composition in the litterbags was affected by temporal and spatial factors. Compared with the non-Bt rice residue treatment, Bt rice straw had no significant effects on the soil bacterial and fungal community composition during the study period, regardless of the litterbags being placed on the surface or buried in the soil. There were no significant differences in the bacterial community composition profiles in root decomposition between Bt transgenic and non-Bt varieties. However, significant differences in soil fungal community composition between the buried Bt and non-Bt rice roots were observed in soils sampled on days 31, 68, and 137, indicating that Bt roots incorporated into paddy soil may affect soil fungal community during the initial stage of their decomposition.
Conclusions:  There were some significant differences in fungal community composition between Bt rice root and non-Bt root treatments at the early stage of root decomposition in the paddy field. It is important that, before Bt rice is released for commercial production, more research should be conducted to evaluate the ecological effects of the Bt rice residues returned to paddy field upon grain harvesting.