Impact of urease and nitrification inhibitor on NH4+ and NO3- dynamic in soil after urea spring application under field conditions evaluated by soil extraction and soil solution sampling

The application of mineral nitrogen (N) fertilizers is one of the most important management tools to ensure and increase yield in agricultural systems. However, N fertilization can lead to various ecological problems such as nitrate ( NO3 - ) leaching or ammonia and nitrous oxide emissions. The application of N stabilizers (i.e., inhibitors) combined with urea fertilization offers an effective option to reduce or even prevent N losses due to their regulatory effect on ammonium ( NH4 + ) and NO3 - release into the soil. The present field experiment therefore aimed at soil N speciation dynamics after urea spring fertilization (225 kg N ha(-1)) in the presence of a urease inhibitor (UI), a nitrification inhibitor (NI), both inhibitors (UI+NI) or when no inhibitor was applied at all. The study focused on the distribution of N species among soil matrix and soil solution. Plant cultivation was completely omitted in order to avoid masking soil N turnover and speciation by plant N uptake and growth dynamics. Application of UI clearly delayed urea hydrolysis in the top soil, but a complete hydrolysis of urea took place within only 10 days after fertilization (DAF). Nitrification was significantly reduced by NI application, leading to higher NH4 + - N and lower NO3 - - N concentrations in treatments with NI. Due to sorption of NH4 + to the soil matrix a significantly larger fraction of NH4 + was always detected in the soil extracts compared to soil solution. However, while in soil extracts the impact of NI application was less apparent and delayed, in soil solution a quick response to NI application was observed as revealed by significantly increased soil solution concentrations of NH4 + . Because of the asymmetric soil phase distribution soil solution NH4 + was predominant over NO3 - only initially after fertilization even in inhibitor treatments (approximate to 8 to 10 DAF). Nevertheless, inhibitor application tended towards closer ratios of NH4 + to NO3 - concentration in soil solution and hence, might additionally affect concentration dependent processes like plant N uptake and root development. Despite cold spring conditions urea application along with UI and/or NI did not indicate a limited supply of plant available NH4 + and NO3 - .