Transgenic Bt-Cotton Affects Enzyme Activity and Nutrient Availability in a Sub-Tropical Inceptisol

In India, the area under transgenic crop cultivation has witnessed a phenomenal growth from 1.3 million hectares in 2005 to 3.8 million hectares in 2006, which is an increase of about 300 % in a year period (James 2006). Till March 2005, a total of 20 Bt-transgenic cotton hybrids have been released for commercial cultivation in northern, western and southern regions of India (Sharma et al. 2006). Although there is large-scale adoption of Bt-cotton by the farmers because of immediate financial gain, there is concern that transgenic Bt-crops (which release Bt-toxins into the environment) affect nutrient cycling in the agroecosystem. This toxin is produced in every major part of Bt-cotton plants (leaves, stems and roots) (Dong and Li 2007). During crop growth, soil micro-organisms come into direct contact with transgenic Cry endotoxin as it is released from Bt-crops in root exudates and from decomposing tissues (Motavalli et al. 2004). Thus, the Bt-toxin has the potential to enter the soil system throughout the Bt-cotton-growing season, through root release and root turnover processes (Motavalli et al. 2004). While Bt occurs naturally in soil, growth of transgenic Bt-crop causes a large increase in the amount of Cry endotoxin present in agricultural systems, e.g. roughly 0.25 g ha produced naturally (calculated from approximately 1000 Bacillus thuringiensis spores g soil) vs. 650 g ha in case of Bt-corn crop, excluding grain (Blackwood and Buyer 2004). Thus the transgenic plants, either through the products of introduced genes and modified rhizosphere chemistry or through altered crop residue quality, have the potential to significantly change the essential ecosystem functions such as nutrient mineralization, carbon turnover and plant growth (Dunfield and Germida 2004, Motavalli et al. 2004, O’Callaghan et al. 2005). As the rhizosphere (the zone directly surrounding and influenced by plant roots) contains a large