Modelling through-soil transport of phosphorus to surface waters from livestock agriculture at the field and catchment scale

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

Soil Dynamic Models: Predicting the Behavior of Fertilizers in the Soil

This chapter provides an overview of the most dominant processes for the situation where organic biobased derivatives are added to the soil, with emphasis on decomposition, production of mineral nutrients, and losses to the environment. Organic compounds in the soil (organic matter, organic fertilizers, biobased derivatives, soil biomass) are subject to microbiological conversion reaction. Decomposition of organic matter usually occurs relatively quickly as long as enough molecular oxygen is present. Two major models can be distinguished in modeling mineralization of organic matter: multi‐pool models with a constant decomposition rate factor and models with a time‐dependent decomposition rate factor. Leaching is the loss of nutrients via draining of water as a result of excess rainfall or irrigation. It is of environmental concern as it contributes to the contamination of the groundwater and surface‐water systems. The chapter presents an overview of processes for some selected soil dynamic models.<br/