Determining the effects of soil moisture on ion bioavailability using PRS™-probes

Background/Question/Methods

Ion exchange resins (IERs) are commonly used to determine nutrient bioavailability in soils. Changes in soil moisture strongly influence ion bioavailability, but their influence on IER measurements and interpretation is not well understood. We used Plant Root Simulator (PRS™)-probes (ion exchange membranes in plastic supports) to examine the effects of burial time and moisture treatments under controlled laboratory conditions. PRS™-probes were inserted into a sieved silt loam soil for 0.125, 1, 3 and 7 days under permanent wilting point, field capacity and saturated conditions. After one week of burial, PRS™-probes were removed from each treatment, washed, and then reinserted into soil of each of the 3 moisture treatments for 1 or 7 days of additional burial. After removal from soil, PRS™-probes were eluted and analysed for NO₃^--N, NH₄⁺-N, P, K, Ca, Mg, S, and Mn.

Results/Conclusions

Increases in PRS™ ion levels with increasing moisture and burial time were common for some ions, but this trend was not ubiquitous for all ions. After 0.125 and 1 day of burial, PRS™ ion levels of NO₃^--N, K, Ca, and S were positively related to soil moisture. However, only Ca and S levels maintained a positive relationship to soil moisture throughout all burials periods. Adsorption of NO₃^--N after 1 day was greater at field capacity than at saturation, which was attributed to greater mineralization and possibly lower denitrification in soil at field capacity. Adsorption rates of NO₃^--N in soil at field capacity remained high through day 7, presumably because little denitrification occurred. No ions exhibited a continued increase in PRS™ ion levels over the full 14 days of burial for all moisture levels. Most PRS™ ion levels continued to increase from 0.125 to 1 day of burial, but the net rate of ion adsorption after 1 day had declined for most ions and ceased for others. Ion desorption from the membranes also occurred for some ions. These results demonstrate strong ion-dependent impacts of changes in soil moisture on soil ion bioavailability as measured by PRS™-probes.