Factors affecting stability and efficiency of ion exchange resins in studies of soil nitrogen transformation

Mixtures of cation and anion exchange resins are used as part of the resin core technique to determine nitrogen transformation in forest soils as they adsorb the NH4-N and NO3-N from soil solution percolating through the incu bated soil cores. In the field, the exchange resins may be subjected to a v ariety of conditions, involving drying, rehydration, freezing, and thawing. This paper examines how these processes affect adsorption of NH4-N and NO3 -N and the stability of the resins. Lab tests were performed on the anion r esin Amberlite IRA-93, the cation resin Amberlite IR-120, a mixture of IRA- 93 and IR-120, and the commercially-mixed bed resin Amberlite MB 1. The bac kground content of NO3-N and NH4-N on the resins was large and highly varia ble between different batches of resins in spite of a 2 M NaCI pre-rinse. T he IR-120 cation resin that was subjected to 48 hours air-drying contained significantly less NH4-N than the moist resins, while the drying of the IRA -93 anion resin caused a significant release of NO3-N from resins with no N addition. Although the variation was large, the mixed bed resin MB 1 indic ated a release of NH4-N, which supports results from long term in situ depl oyments. A reduced adsorption of NO3-N was found on the IRA-93 anion resins and the MB1 mixed bed resins that were dried prior to N addition while the dry IR-120 cation resins adsorbed significantly less NH4-N than the contro l resin. No effect of freezing and thawing efficiency was observed on resin stability or N adsorption efficiency. Sufficient blanks that have been sub jected to similar moisture changes are necessary in N limited systems with low levels of available NH4-N and NO3-N.