Aging effects on molybdate lability in soils

Aging reactions in soils can influence the lability and hence bioavailability of added metals in soils through their removal from labile pools into pools from which desorption is slow (non-labile pools). The aims of this study were to examine the effect of aging reactions on the lability of soluble molybdate (MoO42-) added into soils with varying physical and chemical properties and develop models to predict changes in the labile pool of MoO42- in soils with incubation time. Soils were spiked with soluble MoO42- at quantities sufficient to inhibit barley root growth by 10% (EC10) or 90% (EC90) and incubated for up to 18 months. The labile pool of MoO42- (E   value) was observed to decrease in soils with incubation time, particularly in soils with high clay content. A strong relationship was observed between measures of MoO42- lability in soils determined using E and L value techniques (R2 = 0.98) suggesting E   values provided a good measure of the potential plant available pool of MoO42- in soils. A regression model was developed that indicates clay content and incubation time were the most important factors affecting the labile pool of MoO42- in soils with time after addition (R2 = 0.70–0.75). The aging model developed suggests soluble MoO42- will be removed into non-labile pools more rapidly with time in neutral to alkaline clay soils than in acidic sandy soils. Labile MoO42- concentrations in molybdenum (Mo) contaminated soils was found to be <10% of the total Mo concentrations in soils.

► The labile pool of MoO42- in amended soils was observed to decrease with age in soils.
► A strong linear relationship was observed between E and L   value results.
► Labile MoO42- in Mo contaminated soils was <10% of the total Mo in soils.