Long-term P fertilizer effects on pasture soil nitrous oxide and carbon dioxide emissions

Grazed pasture ecosystems are key contributors to anthropogenic nitrous oxide (N₂O) emissions. Within the grazed pasture, it is the deposition of ruminant urine, from the grazing animal, that creates the hot-spot for N₂O emissions. Grazed pastures also receive phosphorus (P) fertilizer to sustain plant nutrient needs. This in turn increases the soil microbial biomass pool and promotes dry matter production: this affects soil moisture dynamics and plant N uptake. How long-term P fertilizer affects N₂O emissions from pasture soils in situ is poorly understood. To address this an in situ study was performed on the Winchmore long-term fertilizer trial, in New Zealand. This has run for 71 years and the pasture has not been disturbed over this time. The trial consists of replicated (n = 4) field plots (0.09 ha) receiving either 0, 17, 23, or 34 kg P ha^-1 yr^-1 as single superphosphate. These field plots are rotationally grazed by sheep, with stocking rate adjusted according to feed on offer. Headspace chamber bases were installed in each field plot and synthetic ruminant urine or distilled water (control) were applied. Static chambers were placed on the chamber bases during gas flux measurement. Gas measurements were taken over a 76-day period to determine N₂O and CO₂ fluxes. Soil inorganic-N concentrations (0 – 7.5 cm) were also followed over time, along with soil sampling for determining soil chemical characteristics. Data from this study and their interpretation will be presented to assess the effect of long-term P fertilizer on grazed pasture N₂O and CO₂ fluxes.