Litterbag decomposition of residues from Bacillus thuringiensis (Bt) rice hybrids and the parental lines under multiple field conditions

Purpose

The cultivation of genetically modified (GM) crops has raised environmental concerns, since large amounts of plant materials remain in the field after harvesting. Specific proteins of GM crops might negatively impact soil ecosystem by changing residue decomposition dynamics. Particularly, the residue decomposition of crop-wild hybrids, which were formed through transgene escape to wild population, remains unexplored.

Materials and methods

We used litter bags to assess residue (leaves, stems and roots) decomposition dynamics of two stacked genes from Bacillus thuringiensis (Bt) Cry1Ac and the sck (a modified CpTI gene encoding a cowpea trypsin-inhibitor) (Bt/CpTI) rice lines (Kefeng-6 and Kefeng-8), a non-transgenic rice near isoline (Minghui86), wild rice (Oryza rufipogon) and Bt wild rice at three sites. The enzyme-linked immunosorbent assay (ELISA) was used to monitor the changes of the Cry1Ac protein in Bt rice residues.

Results and discussion

Mass remaining, total N and total C concentrations of rice residues declined over time and varied among plant tissues, with significant differences among cultivar, crop-wild hybrids and wild rice, but no differences between Bt and non-Bt rice cultivars. The initial concentration of Cry1Ac was higher in leaves and stems than in roots and was different between rice types. The degradation dynamics of Cry1Ac fitted best to a first-order kinetics model and correlated with the level of total nitrogen in residues but did not correlate with the mass decomposition rate. The predicted DT50 (50 % degradation time) of the protein ranged from 10.7 to 63.6 days, depending on plant types, parts and burial sites. By the end of the study (~170 days), the protein was present in low concentration in the remaining residues.

Conclusions

Our results suggest that the impacts of the stacked Bt/CpTI gene inserts on the decomposition dynamics of rice residues are insignificant.