Abstract
Land application of effluent can be used as irrigation for crops or forested land, providing plants with necessary water as well as essential nutrients. There are, however, potential negative consequences to land application of effluent, such as salt accumulation, nitrate leaching, and phosphorus overloading (Duan et al., 2010). A more complete understanding of the relationship between P mobility and soil properties needs to be developed to better manage P when wastewater is applied to forest soils. There are many different ways to model phosphorus status and sorption capacities in soils, such as degree of phosphorus saturation (DPS), which is calculated as the fraction of extractable soil P divided by the P sorption maximum. Water soluble phosphorus (WSP), is another common index that provides insight into phosphorus that could be easily desorbed and leached during rain or runoff events. Utilizing these two basic indices, DPS and WSP, a threshold can be determined. This study investigated the phosphorus sorption threshold for Northern Idaho Forest soils with land application of phosphorus-spiked water. After one six-month application cycle of three treatment levels (4.5 kg P/ha/yr, 15 kg P/ha/yr, and 30 kg P/ha/yr), there were no significant differences found in any major P pool in the soil. Experiments, ammonium oxalate in the dark (AOD) and water-soluble phosphorus (WSP) were run to investigate if a phosphorus sorption threshold occurred. No such threshold was found over the course of this study. The added phosphorus did not appear to alter the P status of the Northern Idaho forest soils. The lack of change is likely due to the short duration of this study, as well as the demonstrated ability of these forest soil to sorb P. This thesis investigated the potential of an experimentally calculated phosphorus sorption threshold for northern Idaho forests with the intent to inform wastewater treatment facilities and aid in calculations of lifespans for land application of wastewater.