Using Stable Water Isotopes to Characterize Pathways of Subsurface P Loss in a Ditch-Drained Field.

Phosphorus (P) loss by shallow subsurface flowpaths is a major concern in low-lying agricultural watersheds with artificial drainage and elevated legacy soil P. While lateral subsurface flow is hypothesized as the dominant pathway of P loss from ditch-drained agricultural fields, the flow components of subsurface drainage remain poorly understood. In this study, we will evaluate the relative contributions of macropore and matrix flow to subsurface leaching of P. We hypothesize that subsurface drainage consists of two isotopically distinct hydrologic pools, which can be used to partition different flow components of subsurface P leaching. These two hydrological pools are described as “mobile water”, which bypasses “old” water by preferential transport along macropores, and “bound water”, which is stored in pore spaces and transported predominantly by matrix flow. The bound water pool is assumed to reflect the isotopic composition of soil water and analyzed seasonally by direct vapor equilibrium (DVE-LS). The mobile water pool is assumed to reflect the isotope value of precipitation and measured weekly. Additional subsurface hydrological components, which may reflect mixtures of the bound-mobile water end members, will also be evaluated, including soil water sampled from suction lysimeters and shallow groundwater sampled from piezometers. Pre-programed automatic (Sigma®) water samplers will collect baseflow (time-paced, 12-72 hr) and stormflow (flow-paced) components of ditch drainage. In addition, 1 m soil cores will be collocated with lysimeters, divided into 10 cm sections, dried, and sieved for a routine soil analysis. Isotopic samples will be analyzed on a cavity-ring down spectrometer. Resulting water isotopic ratios will be used in mixing models coupled with water and soil P data to better understand the hydrology of subsurface leaching of P in ditch-drained fields. Our results are intended help modify regional P Indices by improving conceptual representations of subsurface P loss in artificially drained agroecosystems.