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The activities of three extracellular hydrolytic enzymes, soil invertase, urease, and alkaline phosphatase (AlP), were measured across seasons and with the experimental addition of nitrogen (N) in the soil of the Gurbantunggut Desert, Northwest China. Seasonal fluctuations in hydrolytic enzyme activities were not correlated with seasonal variations in soil temperature, water content, pH, conductance, and organic carbon. Invertase and AlP activities increased with low rates of N addition, peaked at a N addition rate of 3.0 g N m(-2) y(-1), and then decreased at higher N addition rates. Urease activity decreased with increasing N addition. Higher organic matter content in the upper depths of soil resulted in higher hydrolytic enzyme activity at depths of 0-5 cm in soil samples and hydrolytic enzyme activity at that depth was more sensitive to N addition and seasonal environmental factors than that at depths of 5-10 cm in soil samples.