Estimating soil ammonium adsorption using pedotransfer functions in an irrigation district of the North China Plain

Extensive use of chemical fertilizers in agriculture can induce high concentration of ammonium nitrogen(NH4^(+)_(-)N) in soil. Desorption and leaching of NH4^(+)_(-)N has led to pollution of natural waters. The adsorption of NH4^(+)_(-)N in soil plays an important role in the fate of the NH4^(+)_(-)N. Understanding the adsorption characteristics of NH4^(+)_(-)N is necessary to ascertain and predict its fate in the soil-water environment, and pedotransfer functions(PTFs) could be a convenient method for quantification of the adsorption parameters. Ammonium nitrogen adsorption capacity, isotherms, and their influencing factors were investigated for various soils in an irrigation district of the North China Plain. Fourteen agricultural soils with three types of texture(silt, silty loam, and sandy loam) were collected from topsoil to perform batch experiments. Silt and silty loam soils had higher NH4^(+)_(-)N adsorption capacity than sandy loam soils.Clay and silt contents significantly affected the adsorption capacity of NH4^(+)_(-)N in the different soils. The adsorption isotherms of NH4^(+)_(-)N in the 14 soils fit well using the Freundlich, Langmuir, and Temkin models. The models’ adsorption parameters were significantly related to soil properties including clay,silt, and organic carbon contents and Fe^(2+) and Fe^(3+) ion concentrations in the groundwater. The PTFs that relate soil and groundwater properties to soil NH4^(+)_(-)N adsorption isotherms were derived using multiple regressions where the coefficients were predicted using the Bayesian method. The PTFs of the three adsorption isotherm models were successfully verified and could be useful tools to help predict NH4^(+)_(-)N adsorption at a regional scale in irrigation districts.