Urease Activity and Electrical Conductivity 



Changes in urease activity in response to different salinity levels is shown in Figure 6. The 

 highest urease activity of 0.82 mg/kg/hr was obtained from the sample at salinity level of 0.1 S/m 

 and the least was observed at salinity level of 0.8 S/m with a mean of 0.53 mg/kg/hr. It is likely 

 that high salinity can partially or totally block the mechanism of urea hydrolysis because of the 

 increased energy expenditures required for the hydrolysis of urea to ammonium-urea as expected 

 from the increasing ionic strength of the soils (Tanji 1969; Ponnamperuma 1972; DeLaune et al. 

 1976). It should be recalled that urease activity is negatively related to the increasing concentration 

 of Na (Equation 1). Because of the increased Na and other salt components, it is concluded that 

 these soils have established protective mechanisms that lead to the destruction or inactivation of 

 enzymes (Zantua and Bremner 1975; Zantua and Bremner 1976; Zantua et al. 1977). 



This depressing effect of increasing levels of EC on urease activity is in agreement with the 

 work of Sankhanyan and Shukla (1976) and Myers and McGarity (1968), who found that the rate 

 of hydrolysis of urea was slower in soil samples with high EC. 



SUMMARY AND CONCLUSIONS 



The results are summarized as follows: 



1. Na, Fe, Mg, and Ca toxicity may develop during long-term drying and detrimentally affect 

 native vegetation and revegetation establishment. 



2. Addition of gypsum did not result in a significant increase of urease activity in dried soils. 



3. General reduction of urease activity was observed as EC increased. 



These results reaffirm the importance of N fertilization and water management for the 

 restoration of suitable marsh vegetation. Because N transformation decreased with increasing 

 salinity and because of the probable increase of N losses via denitrification during soil drying, N 

 fertilization should be included in the revegetation plans for brackish and saline marsh soils. 



In an open area like the marsh where there are few growing plants, the fate of fertilizer N 

 added to the soil as urea is likely controlled by soil urease. Therefore, draining and drying the 

 soil to a maximum (20% MC) should not be practiced because of the reduced urease activity that 

 may eventually lead to lower N availability and fertilizer efficiency. Soil drying also altered the 

 physicochemical properties which were found to be detrimental to the overall productivity of the 

 area. 



94 



