Application of the Ntrace tool to investigate long-term P fertilizer effects on gross N transformations in a pasture soil

The Ntrace tool uses soil 15N analysis of soil N pool dynamics to establish soil gross nitrogen (N) transformations. An understanding of what initiates and controls soil N transformation dynamics is critical when considering the environmental impacts (e.g. nitrate leaching and nitrous oxide emissions) and fate of N in pasture soils. Phosphorus (P) is an essential nutrient required by soil microbes that facilitate both soil N and soil organic matter transformations. A laboratory mesocosm study was performed to examine the effect of long-term superphosphate use on soil gross N and soil organic matter transformations. Sheep grazed pasture soils, undisturbed for 71 years, receiving either nil fertilizer (control) or 188 kg ha-1 year-1 of single superphosphate (17 kg P ha-1 yr-1) were collected at a depth of 0 – 7.5 cm. Soils were sieved (4 mm) and placed in kilner jars, whereupon they received treatments that consisted of: 15NH414NO3 + natural abundance 13C perennial ryegrass (Lolium perenne) root material; 14NH415NO3 + natural abundance 13C perennial ryegrass root material; or 14NH414NO3 + 13C enriched ground perennial ryegrass root material. Over 7 days measurements included soil inorganic-N concentration and 15N enrichments, nitrous oxide emissions, microbial biomass, and soil organic C concentrations. Data and initial conclusions from this experiment will be presented with respect to the long-term P fertilizer effects on soil gross N and soil organic matter transformations.