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The effect of water absorbing soil amendments on soil microbiological properties (soil enzyme activity and soil microbial biomass) was investigated in a field experiment under potato production in a semi-arid region in northern China in 2010-2012. Treatments included two different synthetic water absorbing amendments (potassium polyacrylate-PAA, polyacrylamide-PAM) and one natural amendment (humic acid-HA), both as single amendments, and combined amendments (natural combined with a synthetic) and a no amendment control. Soil amendments had a highly significant effect (P <= 0.01) on soil enzyme activity (catalase, invertase, urease and phosphatase) and soil microbial biomass (carbon, nitrogen and phosphorus). The PAM + HA amendment treatment achieved the greatest effect on soil microbiological properties of all of the amendment treatments, both in all soil layers between 0 and 40 cm, and at all four measurement periods during the growing season. Soil amendments improved the catalase, invertase, urease and phosphate by 4.6-39.8%, 4.4-27.7%, 3.7-40.4% and 0.9-29.8% respectively and increased soil microbial biomass carbon, nitrogen and phosphorus by 2.5-27.3%, 2.4-28.1% and 3.5-31.6% respectively in all three years. Water absorbing soil amendments improved soil quality by increasing soil moisture content and its microbiological status, as reflected in the values of microbial biomass and enzymatic activity. (C) 2016 Published by Elsevier Masson SAS.