With increased cultivation of transgenic Bacillus thuringiensis (Bt) cotton in the saline alkaline soil of China, assessments of transgenic crop biosafety have focused on the effects of soil salinity on rhizosphere microbes and Bt protein residues. In 2013 and 2014, investigations were conducted on the rhizosphere microbial biomass, soil enzyme activities and Bt protein contents of the soil under transgenic Bt cotton (variety GK19) and its parental non-transgenic cotton (Simian 3) cultivated at various salinity levels (1.15, 6.00 and 11.46 dS m⁻¹). Under soil salinity stress, trace amounts of Bt proteins were observed in the Bt cotton GK19 rhizosphere soil, although the protein content increased with cotton growth and increased soil salinity levels. The populations of slight halophilic bacteria, phosphate solubilizing bacteria, ammonifying bacteria, nitrifying bacteria and denitrifying bacteria decreased with increased soil salinity in the Bt and non-Bt cotton rhizosphere soil, and the microbial biomass carbon, microbial respiration and soil catalase, urease and alkaline phosphatase activity also decreased. Correlation analyses showed that the increased Bt protein content in the Bt cotton rhizosphere soil may have been caused by the slower decomposition of soil microorganisms, which suggests that salinity was the main factor influencing the relevant activities of the soil microorganisms and indicates that Bt proteins had no clear adverse effects on the soil microorganisms. The results of this study may provide a theoretical basis for risk assessments of genetically modified cotton in saline alkaline soil.

Organic manure application is an important measure for high yield and good quality vegetable production, whereas organic manure is also a main source of residual antibiotic in soils. A 3-yr experiment was conducted on a fluvo-aguic soil in Tianjin of northern China. The objective of this study was to investigate the effects of different fertilization patterns on yield of six-season vegetables with celery and tomato rotation, and dynamic change of tetracyclines residues in the soil during the sixth growing season (tomato season). The field experiment comprised six treatments depending on the proportion of nitrogen of each type of fertilizer: 4/4 CN (CN, nitrogen in chemical fertilizer), 3/4 CN+1/4 MN (MN, nitrogen in pig manure), 2/4 CN+2/4 MN, 1/4 CN+3/4 MN, 2/4 CN+1/4 MN+1/4 SN (SN, nitrogen in corn straw), and CF (conventional fertilization, the amounts of nitrogen application were 943 and 912 kg N ha–1 for celery and tomato season, respectively). In addition to CF treatment, the amount of nitrogen application in other treatments was greatly reduced and equal (450 and 450 kg N ha–1 for celery and tomato season, respectively). Results showed that the combined application of 3/4 CN+1/4 MN achieved the highest yield and economic benefit in the first four seasons, but addition of straw (2/4 CN+1/4 MN+1/4 SN treatment) performed better in the subsequent two seasons, and the average yields of 2/4 CN+1/4 MN+1/4 SN treatment were respectively higher by 9.9 and 12.8% than those of 4/4 CN treatment, and by 5.6 and 10.5% than those of CF treatment. The residual chlortetracycline (CTC) in manure-amended soil for three consecutive years increased along with the increase of applied amount of pig manure. Under the same amount of pig manure application, content of CTC in straw-amended soil was obviously decreased compared with no straw-amended soil (3/4 CN+1/4 MN treatment), and averagely decreased by 41.9% for four sampling periods in the sixth season. Addition of crop straw facilitated the degradation of CTC in manure-amended soil. As a whole, the conventional fertilization was not the desirable pattern based on yield, economic benefit and environment, the optimal fertilization pattern with the highest yield and profit and the least soil chlortetracycline residue was the treatment of 2/4 CN+1/4 MN+1/4 SN under this experimental condition.