Douglas J. Soldat

Synthesis and Detection of Oxygen-18 Labeled Phosphate

Phosphorus (P) has only one stable isotope and therefore tracking P dynamics in ecosystems and inferring sources of P loading to water bodies have been difficult. Researchers have recently employed the natural abundance of the ratio of 18O/16O of phosphate to elucidate P dynamics. In addition, phosphate highly enriched in oxygen-18 also has potential to be an effective tool for tracking specific sources of P in the environment, but has so far been used sparingly, possibly due to unavailability of oxygen-18 labeled phosphate (OLP) and uncertainty in synthesis and detection. One objective of this research was to develop a simple procedure to synthesize highly enriched OLP. Synthesized OLP is made up of a collection of species that contain between zero and four oxygen-18 atoms and, as a result, the second objective of this research was to develop a method to detect and quantify each OLP species. OLP was synthesized by reacting either PCl5 or POCl3 with water enriched with 97 atom % oxygen-18 in ambient atmosphere under a fume hood. Unlike previous reports, we observed no loss of oxygen-18 enrichment during synthesis. Electrospray ionization mass spectrometertry (ESI-MS) was used to detect and quantify each species present in OLP. OLP synthesized from POCl3 contained 1.2% P18O16O3, 18.2% P18O2 16O2, 67.7% P18O3 16O, and 12.9% P18O4, and OLP synthesized from PCl5 contained 0.7% P16O4, 9.3% P18O3 16O, and 90.0% P18O4. We found that OLP can be synthesized using a simple procedure in ambient atmosphere without the loss of oxygen-18 enrichment and ESI-MS is an effective tool to detect and quantify OLP that sheds light on the dynamics of synthesis in ways that standard detection methods cannot.

Evidence, Regulation, and Consequences of Nitrogen-Driven Nutrient Demand by Turfgrass

Nutrient uptake is strongly influenced by plant growth rate. Accelerated growth leads to nutrient levels incapable of sustaining the optimal growth rate, resulting in shoot to root signaling for increased nutrient absorption. The factors controlling nutrient demand in turfgrass and its consequences have not been investigated. The objectives of this research were to verify that turfgrass exhibits the principal characteristics of demand-driven nutrient uptake and to identify the primary factor controlling nutrient demand via regulation of growth rates. Kentucky bluegrass clipping production increased linearly up to annual fertilizer N rates of 600 kg ha−1 and to 1000 kg N ha−1 for creeping bentgrass. At the typical annual N fertilization rates of 150 to 300 kg ha−1 for the two grasses, N supply was the primary determinant of turfgrass growth rate, plant nutrient demand, and nutrient uptake. Nitrogen uptake accounted for over 88% of uptake of all other nutrients. Uptake of P and K were strongly related to tissue N content irrespective of soil test levels. Variations in turfgrass species and cultivar nutrient requirements and nutrient use efficiencies were found to be directly related to differences in growth rates and, by inference, to differences in nutrient demand.

Preferential Soil Sorption of Oxygen-18-Labeled Phosphate

Oxygen-18-labeled phosphate (OLP) and natural abundance 18O have been used as tools for elucidating the dynamics of phosphorus (P) in soils, yet much remains poorly understood. The objective of this research was to determine the extent of preferential soil sorption across the range of species contained within OLP. A variety of soils were shaken with water containing 65.5 mg L−1 OLP-P for a 24-h period. Following shaking, the OLP species remaining in the solution were determined. Increasing the oxygen-18 atoms in the phosphate molecule by one resulted in a 1.8% increase in the amount of that OLP species sorbed to the soil, and this increase in sorption was uniform across soils. A strong correlation (r2 = 0.94) was found between the amount of phosphate sorbed and the Mehlich 3 P saturation ratio of the soil. These results will be useful for studies using natural abundance and enriched 18O-phosphate in soils.

Soil Phosphorus Levels and Stratification as Affected by Fertilizer and Compost Applications

Little information exists that describes how soil P levels and vertical distribution throughout the soil profile are influenced by fertilization practices and the addition of composted manures. Two field studies were designed to provide more information on how adding P fertilizer or compost influences the concentration and distribution of P in turfgrass soils. Application of P fertilizer at rates of 19, 38, or 72 kg P 2 O 5 per ha/year over a period of 4 or 5 years increased soil P in the upper 0 to 5 cm of soil by a factor of 2.7 to 3.3. Applying P at a rate of 10 kg P 2 O 5 per ha did not increase soil P in the upper 0 to 5 cm of soil. With one exception, soil P levels at depths of 5 to 10 or 10 to 15 cm were not increased by fertilizer applications over a period of 4 or 5 years. In contrast, adding composted poultry or dairy manures to plots at rates of 12 to 24 mm/year resulted in 8 to 333-fold increases in soil P in the upper 5 cm of soil. Soil P levels also increased substantially in deeper layers as a result of poultry compost application, but not for dairy. These findings indicate that common fertilization practices have a much smaller influence on soil P levels compared to composted manures. The benefits of using composted manures must be weighed against the potentially negative environmental impacts that could result from a large increase in soil P layer where runoff occurs.