Nitrogen and phosphorus in soil and groundwater following repeated nitrogen-based swine slurry applications to a tame grassland on coarse textured soil
Swine slurry is a source of nutrients to grasslands. However, accumulation of N or P can lead to their movement to groundwater. This thesis’ research was conducted using a tame pasture fertilized over six years with swine slurry at N-requirement rates, on a gravelly soil in south-eastern Manitoba. Objectives were to determine N and P soil surplus and accumulation over time, soil profile P fractions and soil surface P saturation, and nitrate and dissolved-P in shallow groundwater for treatment combinations of forage utilization (Hay and Grazed), slurry application in spring (Single), split in fall and spring (Split) and no slurry (Control), and Grassed areas of grazed paddocks and Bare areas where cattle congregated around water troughs. There were less surplus and accumulation of extractable-P in the soil surface for the Split compared to the Single treatment because of less P in fall- than in spring-applied manure. With Grazing, there were greater surpluses of N and P than with Haying, and surface accumulation of extractable-P over time was linear being 16.5 and 11.9 mg P kg-1 year-1at 0-5 cm for Grazed and Hay treatments, respectively. Labile inorganic-P fractions (water- and bicarbonate-extractable) increased in the Bare and in the Single Grassed treatments. In the Bare treatment, P-sorption capacity at 0-5 cm decreased compared to the Grassed treatment. The surface soil of the Bare areas had large concentrations of water-soluble-P up to 165 mg P kg-1 associated with an increase in P-saturation to 88 % of the sorption capacity, and nitrate was elevated in the soil profile. Concentrations of nitrate-N and dissolved-P in groundwater were below the environmental thresholds of 10 mg nitrate-N L-1 and 0.025 mg P L-1, respectively, in both the Hay and the Grazed Grassed treatments. In contrast, the Control and Single Bare treatments had nitrate concentrations always above the threshold, and the Bare areas in the Single paddocks had P concentrations of environmental concern in 2009, averaging 0.7 mg total dissolved P L-1. Nitrogen-based slurry applications did not cause leaching of N or P, but Bare areas in grazed pastures are at risk of N and P leaching.
nitrogen, phosphorus, soil, groundwater, swine slurry, leaching, grassland, pasture, chloride