Identification of phosphorus sources in the East Creek using isotope signatures of the residual phosphorus pool
Date
2021
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Publisher
University of Delaware
Abstract
Phosphorus (P) is often the limiting nutrient contributing to phytoplankton blooms and deteriorating water quality. It is imperative to identify key sources of P in watersheds, especially in areas where P has accumulated in the soil and created a potential source of long-term releases into water bodies. The existing limitation of analytical methods for tracing P in the environment has posed a challenge to discern specific sources. In this research, the recalcitrant P pool (extracted using sequential extraction methods but largely biologically unavailable) and residual P pools (remaining P after sequential extraction methods) were extracted from potential P source soils and sink sediments in the East Creek watershed near Crisfield, MD. Methods of extracting residual soil P were successfully developed and tested for isotopic integrity; reagents were found not to compromise original isotope values. Residual P pools of East Creek soils and sediments were extracted in the following order: 10 M HNO3, 10 M NaOH, and aqua regia. Phosphate oxygen isotopes (δ18OP) of residual soil P revealed a distinct range of isotope values among different land uses (i.e., P sources), suggesting the usefulness of isotope determination using the new residual P extraction methods to discriminate different P sources. Carbon and nitrogen isotopes of residual and residual-residual soil pools also showed distinct isotope signatures among source sites and between source soils and sink sediments. Sources of P in the upstream sediment were found to be derived from wetlands, streambanks, and agricultural soils. Upstream wetlands are thin buffer zones between the creek and agricultural lands, serving as depositional environments for the nutrients lost from nearby soils, which under favorable hydrodynamic conditions can undergo resuspension and erosion. Downstream wetland sources, however, contributed less to sediment. Forested soils were shown to contribute the most to downstream residual sediments. Results from 18OP, 15N, and 13C values of residual soil pools were generally consistent with that of the Bayesian elemental fingerprinting model. In summary, combined isotopes and elemental fingerprinting methods are more promising tools for source tracking of phosphorus in watersheds.
Description
Keywords
Non-point pollution, Phosphorus, Residual, Soil, Source tracing, Stable isotope chemistry