| Transgenic Bacillus thuringiensis (Bt) rice cropping is an effective way to control the damaging effect of insects. Transgenic Bt rice released toxic Cry protein to soil ecosystem by means of residues and root exudates. Those Bt protein may maintain its toxic insect-killing properties, causing negative effect to soil processes and non-target organisms. Soil fauna played significant roles in material cycling and energy flow. There’re abundant organisms in rhizosphere (root-soil interface) and detritusphere (residue-soil interface), which took part in soil processes actively but affected by plants directly and significantly. Based on the fact that transgenic Bt crops affected soil ecosystem by routes of roots growing and decomposition of residues, this study investigated the effect of Bt rice on soil fauna community by means of integrating field investigation and hot spots of greenhouse experiment.A one-year field trial was conducted to investigate the effect of two Bt rice varieties (HH1and KMD) and their corresponding parent lines (MH63and XSD) on soil nematode assemblage after three years cultivation. The main results from our field investigation were as follows:1) Transgenic Bt rice HH1did not consistently cause significant changes of soil Bt protein concentration at three sampling dates (seedling, elongation and maturing stages). Under planting of KMD, concentration of soil Bt protein of seedling stage was significantly higher than that of elongation and maturing stage (P<0.05).2) Transgenic Bt rice had a transient effect on soil available resource. Significant negative effect of KMD on soil dissolved organic carbon (DOC) at seedling and elongation stages was observed, while there was significant lower DOC at maturing stage and dissolved organic nitrogen (DON) and ammonium nitrogen content for soil under HH1planting at seedling stage. Compared with corresponding parent lines, significant negative effect on soil microbial biomass was found under the two transgenic Bt rice varieties at seedling stage. HH1increased basal respiration at elongation and maturing stages, and HH1and KMD increased microbial metabolic quotient at seedling stage.3) As compared with XSD, KMD increased nematode numbers at elongation stage. Nematode total numbers at elongation stage was significantly lower than that of seedling and maturing stages under HH1, XSD and KMD planting. Non-metric multidimensional scaling (nMDS) indicated that the two transgenic Bt rice had little effect on soil nematode assemblage, while there were significant differences of nematode community structure among different growing stages. Nematode total numbers had a significant positive relation with DON, nitrate nitrogen and BR (P<0.05). Herbivores, omnivorous and carnivorous had a significant negtive relation with DOC. With regard to nematode ecological indices, maturity index (MI), plant-parasites index (PPI) and structure index (S1) had a negative relationship with DOC and microbial biomass carbon (MBC), but a positive relation with BR and qCO2.Based on high-activity microsites (rhizosphere and detritusphere), hot-spots of greenhouse experiment was conducted to investigate the effect of Bt rice variety (HH1) and its parent line (MH63) on soil available resources, microbial properties and nematode assemblage. The main results from our field investigation were as follows:1) Significant difference was found among Bt protein concentration of different microsites at some sampling times under HH1planting. Bt protein concentration of detritusphere (D)(2011.09,2011.10and2012.06) had significant difference with that of rhizosphere (R).2) Compared with MH63, negative effect on DOC was found under HH1planting (P<0.05). Microsites had a significant effect on DON and ammonium nitrogen. HH1reduced DOC and DON of D soil at seedling (2011.07) stages. At seedling stage (2012.06), DOC and DON of D and R was significantly higher than that of DR. Nitrate nitrogen content of D and R at seedling stage (2012.06) was reduced by HH1. Mineral nitrogen of D was generally higher than that of R and DR at seedling and elongation stages (2012.06,2012.07). HH1increased MBC and MBN of DR at elongation and maturing stages respectively (2012.07,2012.10). At maturing stages (2011and2012), MBC and MBN of DR was significantly higher than that of D and R under MH63planting. HH1had significant effects on BR of R at seedling (2011.07) and elongating stages (2012.07)(P<0.05).3) As compared with the parent line, Bt rice increased nematode total numbers and the numbers of omnivorous/carnivorous at rice maturing stage (2011.10). The proprotion of herbivores in soil with rice grown was higher than that of soil with only residues amended. Repeated-measure ANOVA and non-metric multidimensional scaling (nMDS) indicated that transgenic Bt rice had no significant effect on soil nematode total numbers and community structure. The difference between transgenic Bt and non-Bt parent line we observed was mainly caused by microsites and their interactions, as well as seasonal variation. Nematode total numbers had a significant relation with DON and mineral nitrogen (P<0.05). Herbivores had a significant negative relation with nitrate nitrogen, and bacterial feeders had a significant relation with DOC and DON, ammonium nitrogen and MBC. With regard to nematode ecological indices, MI and SI had a negative relation with DON, ammonium nitrogen and Bt protein at a level of0.01, but a positive relation with MBC and BR(P<0.05).In conclusion, our results indicated that there’re no consistent negative effects of Bt rice cropping on soil microbial properties and nematode assemblage. The detected significant effects on specific sampling time and hot spots are more associated with the difference of rice growing stages and available resource. Hot spots of greenhouse experiment also indicated that effects of Bt rice were lower than that of rice growing stages and microsites. More attention should be paid to various temporal and spatial scales to evaluate and explore the ecological risk and mechanisms of Bt rice cropping on soil, respectively. Furthermore, it will be critical to evaluate the influnces of transgenic Bt crop by involving relatively complete analysis of soil food web and soil ecological functions under different environmental conditions. |
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