Abstract
Phosphorus (P) is a limiting nutrient in freshwater systems that can cause harmful algal blooms even at low concentrations. Excessive application of both conventional fertilizers and manure for crop production has caused a build-up of legacy phosphorus stores in surface soils, which continue to export phosphorus from agricultural fields for decades after fertilizer application has ceased. In this thesis, soil P availability and cycling were evaluated in two unique systems: 1) P in soil amendments derived from dairy, and 2) P leached from a dryland agricultural field and tile drain in the Northwestern Wheat Growing Region. Isotopic tracing of phosphorus (P-δ18O) was used in the field study to understand temporal P leaching patterns through the soils. In the field study, P-δ18O signatures were used to identify key source areas of phosphorus transport in small watersheds and showed that seasonal climate drivers, not land management, controlled the turnover and transport of legacy phosphorus through the soil and into tile drains. To understand P cycling from dairy-derived nutrients, isotopic tracing, nuclear magnetic resonance (31P-NMR), x-ray absorbance near-edge structure spectroscopy (XANES), and chemical sequential extraction methods were used in a greenhouse study. Results show that P cycling and transformations, determined by isotopic and spectroscopic analysis, differed between dairy-derived and synthetic fertilizers, which could not be determined from chemical extractions alone. XANES analysis showed that soils were comprised mostly of Ca-P species (53.6 - 86.7%), but Ca-P species in soils amended with dairy-derived fertilizers were present in more soluble forms. Plant available P was similar between dairy-derived and synthetic fertilizer treatments within high and low levels, suggesting that the amendments generated from dairy wastes are a possible way to help recycle nutrients from dairy waste streams, helping to close the dairy bioeconomy in Idaho.