Modeling Through-Soil Transport of Phosphorus to Surface Waters from Livestock Agriculture at the Field and Catchment Scale

Research Division, Scottish Agricultural College (SAC), Bush Estate, Penicuik, Edinburgh EH26 0PH, UK.
Science of The Total Environment (Impact Factor: 4.1). 06/2005; 344(1-3):185-99. DOI: 10.1016/j.scitotenv.2005.02.015
Source: PubMed


A model of phosphorus (P) losses in a small dairy farm catchment has been set up based on a linkage of weather-driven field-scale simulations using an adaptation of the MACRO model. Phosphorus deposition, both in faeces from grazing livestock in summer and in slurry spread in winter, has been represented. MACRO simulations with both forms of P deposition had been calibrated and tested at the individual field scale in previous studies. The main contaminant transport mechanism considered at both field and catchment scales is P sorbed onto mobile colloidal faeces particles, which move through the soil by macropore flow. Phosphorus moves readily through soil to field drains under wet conditions when macropores are water-filled, but in dry soil the P carrying colloids become trapped so losses remain at a low level. In the catchment study, a dairy farm is assumed to be composed of fields linked by a linear system of ditches which discharge into a single river channel. Results from linked simulations showed reasonable fits to values of catchment outflow P concentrations measured at infrequent intervals. High simulated outflow P concentrations occurred at similar times of year to high measured values, with some high loss periods during the summer grazing season and some during the winter when slurry would have been spread. However, there was a lack of information about a number parameters that would be required to carry out a more exact calibration and provide a rigorous test of the modelling procedure. It was nevertheless concluded that through soil flow of colloid sorbed P by macropore flow represents a highly plausible mechanism by which P is transported to river systems in livestock farming catchments. This represents an alternative to surface runoff transport, a mechanism to which high P losses from livestock farming areas have often been attributed. The occurrence of high simulated levels of loss under wet conditions indicates environmental benefits from avoiding slurry spreading on wet soil or during rain, and from some forms of grazing management.

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    • "Previous monitoring campaigns in the same watershed, but focusing on the composition of dissolved organic matter using ultraviolet spectrometry, carbon isotopes and molecular biomarkers highlighted the release of microbial-derived organic compounds in the soil solution at the same time of the year (Jeanneau et al., 2014; Lambert et al., 2013). Hence, the most probable mechanism causing P release when the water table rose at the beginning of the hydrological season was mobilization of a P pool of microbial origin, limited in size, which can migrate under water saturated conditions (McGechan et al., 2005). Further work Fig. 4. Water table level, molybdate-reactive P (MRP) and Fe 2þ concentrations in soil solutions of riparian wetland A. Solid circles: 10e15 cm depth; empty circles: 50e55 cm depth. "
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    • "Concentrations of P in river water may also reflect the legacy effect of past management practices (Schulte et al., 2010), notably past applications of fertilisers. Furthermore, cattle are associated with both point and diffuse sources of P (McGechan et al., 2005), either through direct addition of their faeces and urine or indirectly as a result of their destruction of riverbanks and re-suspension of river sediments (Jarvie et al., 2010; Miller et al., 2010; Smith et al., 2013). Arable land use in the catchments was not identified as a significant influence on P concentrations, despite a strong correlation found in a study of river catchments in England and Wales (Evans, 2002). "
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