Spatial variation of soil urease activity around irrigated date palms

The inhibition of soil urease activity by the accumulation of soluble salts in a coastal region of the Sultanate of Oman susceptible to salinization was investigated. Soil transects around six individual date paints (Phoenix dactylifera L.) growing on commercially managed farms were intensively sampled and assayed for soil urease activity. Activities ranged from 0.10 to 39.85 mu g NH4+ released g(-1) soil h(-1) at 37 degrees C. Soil collected from within 70 cm of the palms had a higher urease activity than soil from a distance > 1.5 m. Multiple I egression analysis showed that more than 75% of the variance in soil urease activity was accounted for by soil physicochemical properties, except in sandy soils low in organic carbon (C). The soil properties most closely associated with urease activity were soil organic C, salinity, and texture. A scattergram of urease activity against salinity showed high variability in activity at low salinities (<0.5 dS m(-1) in 1:5 soil to water extracts) and severe inhibition at salinity values >12 dS m(-1). In nonsaline soils, variation in urease activity was closely associated with variations in organic C and clay contents, whereas at saline conditions, only soil salinity was significantly associated with urease activity.