[Effects of drying-rewetting alternation on urease activity in Chongming east intertidal flat: results of a simulation study].

Sediment and sea water samples were taken from Chongming east intertidal flat, a typical coastal wetland in China, to define the responses of urease activity to periodic alternation of drying and rewetting. Sediment cores were incubated under 30, 35 and 40 degrees C with simulated spring tides (approximately twice a month) and semi-diurnal tides (two almost equal high tides and two low tides in a lunar day). Urease activities in sediments were determined during the incubations as well as dissolved organic nitrogen (DON) and ammonium nitrogen (NH+4-N) contents. The apparent temperature sensitivity index (Q'10) of urease activity was calculated based on the observations of urease activities under 30, 35 and 40 degrees C. Urease activities rapidly increased following the rewetting of dried sediments (1%-3% ) and dropped with decreasing sediment moisture during simulated spring tides. Repeated drying and rewetting cycles lowered the sensitivity of urease to the changes of moisture. In contrast, semi-diurnal tides slightly affected urease activities, in which the enzyme maintained relatively stable activities with (0.067 +/- 0. 018 ) , (0. 143 +/- 0. 027 ) and ( 0. 028 +/- 0. 011) g (10 g.h)-1 under 30, 35 and 40 degrees C , respectively. Urease activities during spring tides were found to be much lower than those of semi-diurnal tides under both 30 and 35 degrees C , implying that repeated drying and rewetting cycles possibly lessened urease activities in sediments. In addition, the highest Q'10 of urease activity occurred happened between 35 degrees C and 40 degrees C during the spring tides, but happened between 30 degrees C and 35 degrees C during semi-diurnal tides. This indicated that drying and rewetting events helped to increase the temperature regime, where the Q'10 of urease activity achieved to the highest. Moreover, NH+4 -N contents depended on the urease activities in sediments during spring tides under 30 degrees C and 35 degrees C. However, No significant relationship between urease activities and DON contents was found. In contrast, changes in DON and NH+4 -N contents hardly affected urease activities during semi-diurnal tides.