Pedotransfer Functions for Estimating Ammonium Adsorption in Soils

Urine depositions by grazing animals induce high concentration of ammonium (NH(4)(+)) in soils. The adsorption of NH(4)(+) in the soil plays an important role in the fate of the urine deposited N. However, adsorption isotherms for a wide range of soils are not readily available. Pedotransfer functions (PTFs) that relate easily measurable and widely available soil properties to the NH(4) adsorption capacity of soils might be a useful tool to help predict the fate of the urine if they are sufficiently accurate. To derive PTF's for NH(4) adsorption by soils, batch experiments were performed with eight different soils from New Zealand. The adsorption data for the different soils were fitted with the Freundlich isotherm and the model parameters related to soil properties including total C content, clay and sand fractions, and cation-exchange capacity (CEC). Clay fraction and CEC were found to be most related to the Freundlich model parameters, and were used to derive PTFs. The PTFs were validated using NH(4)(+) sorption data from the literature spanning a range of soils with different textures and CEC. Agreement between literature NH(4) adsorption data and those calculated using the PTFs were good, with R(2) of 0.7, provided that only soils with similar clay contents and CECs as those used for their development. While this is a potential limitation of the derived PTFs, the range of clay and CEC used are relevant for a wide range of soils.