An assessment of the implications of timing and soil nitrogen dynamics during and after summer drought on Waikato Allophanic soils

Abstract

Recommendations for farming when coming out of a drought are to apply fertiliser nitrogen (N), but when a drought breaks late in the season (i.e. mid to late autumn), the release of soil N has the potential to compound fertiliser N loads at a time when pasture N demand is low. Using the APSIM model, based on measurements made on a Waikato dairy farm and using data from the drought of 2007/8, an accumulation of 20 kg NO3-N/ha in the top 50 cm of soil post-drought was predicted, and therefore any fertiliser N applied during the drought would have added to the available N pool. Where a total of 70 kg N/ha fertiliser was applied during the drought, 35 kg NO3-N/ha was leached during the following winter. A glasshouse trial was subsequently run to observe N dynamics under controlled conditions. The pot trial confirmed that a substantial amount of available N was released (40-60 kg N/ha; 0-15 cm) upon rewetting after a "severe" drought, which may be sufficient to promote growth. The ability of pasture to grow and utilise soil/fertiliser N and the subsequent risk of N leaching will depend on how late in the season drought soils are returned to field capacity and when drainage commences. Keywords: pasture, nitrogen, nitrate leaching, nitrogen fertiliser, drought, ASPIM