The Environmental Biogeochemistry Laboratory (EBL) at UD focuses research on the biogeochemical processes that regulate the source, sink, transformation and internal cycling of phosphorus in terrestrial and coastal environments; redox controlled and coupled biogeochemical cycling of iron and phosphorus, and mineralization at chemical and biological interfaces.
Phosphorus: an essential nutrient turned into a pollutant
Phosphorus (P) is one of the most important nutrients for all life forms irrespective of physiological uptake mechanism or metabolic pathway. It is also one of the most scarce nutrients in terms of its demand in both terrestrial and aquatic environments due to typically low concentrations of dissolved inorganic phosphate (micro- to submicro- molar range). While this is true for most natural environments, many agricultural lands, sewage contaminated aquifers and water bodies near densely populated cities often have elevated concentrations of dissolved P. Because of the low stoichiometric need for P compared to other major nutrients (ca. 106C: 16N: 1P; Redfield ratio), small amounts of P addition can cause severe impacts on water quality in receiving catchments or groundwater aquifers and could promote disastrous consequences such as eutrophication.
Understanding of nutrient-soil interactions, however, is not straightforward for phosphate particularly because a) the amount of dissolved phosphate is several orders of magnitude smaller than the pool of rapidly cycling phosphate as well as total phosphate in soil and sediments (Frossard et al., 1995); and b) both biotic and abiotic reactions occur side by side to dissolve and precipitate phosphate at time scales varying from a few seconds to several years, and with redistribution of phosphate from one phase/pool to another (Fardeau, 1996; Jaisi et al., 2011).
Phosphate oxygen isotope ratios as a tracer for P cycling
Phosphorus is unique among elements in that it has only one stable isotope, 31P, leaving variations in the O isotope ratios of phosphate (δ18Op) as the only candidate for a potential stable isotopic environmental tracer where P is involved and cycled (Blake et al., 1997; Paytan et al., 2002). Fundamental properties of O isotope ratios of phosphate are: a) in abiotic systems, there is negligible O-isotope exchange between PO4 and H2O (or other oxyanions) at low temperatures (< 80 oC) and near-neutral pH values such as in modern geochemical processes on/near earth surfaces. This means phosphate can preserve its original isotopic signature of the environment in which it is formed and therefore enables identification of P sources; and b) in biological systems, O-isotope exchange is rapid between phosphate and water and temperature-dependent equilibrium O-isotope fractionation is attained (Longinelli and Nuti,1973; Blake et al., 2005)
Temperature-dependence of PO4-H2O fractionations in microbial cells, enzymes and in biogenic apatite (Longinelli and Nuti, 1973;Blake et al., 2005).
Therefore δ18Op values of phosphate in biotic systems provide information about bioavailability, extent of biological uptake of specific P phases and its cycling between phases. Furthermore, unlike bioelements such as S and C that are characterized by kinetic fractionations, microbial metabolism of P compounds is characterized by overall equilibrium of O-isotope fractionations between phosphate and water.