Effects of biuret addition on soil nitrogen transformations and Douglas fir seedling growth
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This project was carried out to evaluate the feasibility of using of biuret as a plant growth and/or soil microbial activity regulator at low concentrations, and as a slow-release N fertiliser at high concentrations. Biuret stimulated Douglas-fir seedling growth at low concentrations, but inhibited it at high concentrations. The stimulation of biuret on seedling growth in sand and solution culture was associated with increased net photosynthetic rate and nutrient uptake and when grown in the soil it was also related to the improvement of soil N availability and therefore the uptake of N by seedlings. Provenance 93 (relatively fast growing) was more responsive to lower concentrations of biuret, but more sensitive to the toxicity of biuret at high concentrations than provenance 98 (relatively slow growing). Yellow-tip (chlorosis) in needles was a typical symptom of biuret toxicity. Applied nitrogen had a much greater effect on Douglas fir seedling growth than biuret. Provenance 93 grew faster at lower N supply due to its greater ability to absorb more nutrients and further to distribute more into the shoot. Biuret at lower concentrations had a positive real priming effect on net mineralisation of native soil N in both soils. The causes for the positive priming effects were related to the stimulation of microbial growth and activity at an early stage of the incubation and/or the death of microbes at a later stage, which was biuret-concentration-dependent. Biuret additions increased gross rates of soil N mineralisation and nitrification and affected the turnover rates of mineral N pools. The biuret-¹⁵N tracing technique further confirmed that biuret addition stimulated the mineralization of native soil N by increasing the turnover rate constant (k value). The potential of biuret as a slow-release N source at high concentrations (especially in the Burnham soil) was related to the slow mineralisation, nitrification and more immobilisation of biuret-N, which reduced the potential of N losses by volatilisation and denitrification.