3.2 



2.8H 



h 



X 



Von. 



J 2.0 



£l.6 



> 



ul.2 

 UJ 



20.8H 



0.4 

 0.0 



1.40 



1.5 3. 



DRYING PERIOD (months) 



Figure 1. Effect of soil drying on urease activity. 



lowest for the samples dried for 3 months, and an intermediate rate for the undried samples. 

 With an extended drying period, many changes occur in the soil system. Ponnamperuma (1972) 

 and DeLaune et al. (1976) showed that there was a significant increase of oxygen flux within the 

 interstitial lattice during the development of aerobic conditions. This result would have occurred 

 immediately with drying when the moisture was drastically lowered from 437% (undried) to 21% 

 (3 months) (Table 3). It would appear, however, that aerobiosis as a result of soil drying in 

 brackish marsh soils did not favor urease activity. At the low moisture level of 21%, the lack of 

 free water in the soil apparently limited the contact between urea and soil urease while at the 

 higher water content of 437%, urea hydrolysis is reportedly less dependent on soil moisture 

 (DeLaune and Patrick 1970; Vlek and Carter 1983). Reports on the effects of soil submergence 

 and air drying on urease activity are divergent for wetland soil system. The actual processes are 

 not completely understood. Zantua and Bremner (1977) found that air drying of field-moist soils 

 had no effect on urease activity. Bremner and Mulvaney (1978) noted that there was a difficulty 

 in accounting for the effect of water content on urease activity in soils. Overrein (1963) reported 

 that oxygen had a significant effect on the rate of hydrolysis of urea added to Indian soils. 

 Conversely, Zantua and Bremner (1977) found that oxygen had no effect on the results obtained 

 from the assay of urease activity in Iowa soils. The findings reported here differ from those of 

 Savant et al. (1985), who disclosed that the order of urease activity of the wetland soil system was: 

 oxidized > reduced > flooded. Data on the influence of soil drying on some soil properties 

 measured to evaluate their relationship to urease activity are presented in Figures 2, 3, and 4. The 



89 



