Alkaline phosphatase activity in soil layers from 0–10 cm (a), 10–20 cm (b), and 20–30 cm (c) and 30–40 cm (d) under different treatments during the winter wheat and summer maize seasons of 2012.

<p>Within one growth stage, bars with different lowercase letters are significantly different at <i>P</i> < 0.05 according to the LSD test. 3RT, 3 years of RT to a depth of 10–15 cm; 3PT, 3 years of PT to a depth of 30–35 cm; PT+2RT, 1 year of PT followed by 2 years of RT; and 2PT+RT, 2 years of PT followed by 1 year of RT.