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Application of biochar changes the soil microenvironment and affects the structure of the soil microbial community. The objective of this field study was to determine the effects of cotton straw biochar (0, 2.25, or 4.5 t ha(-1)) on soil microbial community composition and activity in drip-irrigated desert soil planted to cotton. Specially, we measured (i) the activity of five soil enzymes involved in C and N cycling, (ii) microbial functional diversity, and (iii) phospholipid fatty acid (PLFA) concentrations. The 4.5 t ha(-1) treatment increased microbial biomass C by 32%, microbial biomass N by 58%, and basal respiration by 13% compared with the control. The 2.25 and 4.5 t ha(-1) treatments both increased the activities of three key enzymes related to C cycling. The 4.5 t ha(-1) treatment increased enzyme activities related to N cycling. C substrate utilization and substrate diversity were both greater in the biochar amended treatments than in the control. Application of 4.5 t ha(-1) biochar increased total PLFA by 27% compared with the control, and shifted the microbial community toward bacteria (both Gram-positive and Gram-negative bacteria) and actinomycetes. In addition, there was high positive correlation between C substrate utilization and the biomass of actinomycetes and total bacteria. In conclusion, addition of 4.5 t ha(-1) cotton straw biochar shifted the microbial community composition in the drip-irrigated desert soil and increased soil microbial biomass, C substrate utilization, and enzyme activity related to C and N transformation. (C) 2015 Elsevier Masson SAS. All rights reserved.