Impact of controlled uptake long term ammonia nutritionenCULTAN fertilization with ammonium sulfate on field emissions of nitrous oxide

Agricultural soils have a great share on global nitrous oxide (N2O) emissions. The method of nitrogen fertilization is a manageable control parameter of N2O production in soil. Controlled uptake long-term ammonia nutrition (CULTAN) intends to aliment field growing crops mainly with ammonium instead of nitrate, aiming at a better N use efficiency and less N leaching by placing ammonium-based N fertilizer in highly concentrated depots in the soil. In this two years field study, we analyzed N2O flux rates and dynamics of mineral N in soils after injection of ammonium sulfate solution (CULTAN) and conventional surface application of the same fertilizer type (ammonium sulfate at a rate of 130 kg N ha-1) to winter wheat at two sites with different soil texture. Using 15N-ammonium as a tracer, we additionally measured fertilizer-derived emissions and fertilizer N uptake at one CULTAN plot. Grain yields were higher after CULTAN fertilization than after surface application of N fertilizer; significantly so in one year at each site. Neither N uptake nor N use efficiency were consistently different between fertilization methods. Nitrate accumulation in CULTAN treated plots occurred after fertilizer injection, showing that the concentrated NH4+ depots did not sufficiently inhibit nitrification. Total annual N2O emission ranged from 0.29 to 1.9 kg N ha-1 yr-1, with higher emissions from fertilized than unfertilized plots, but no significant difference between fertilizer application methods. N2O emission was higher at the loam than the sandy loam site, with twice as high annual emission at the loam site (1.2 + 0.5 kg N ha-1 yr-1) compared to the sandy loam site (0.6 + 0.2 kg N ha-1 yr-1) after CULTAN fertilization. Temporal N2O emission dynamics were influenced by weather conditions (i.e., thawing of soil) and irrigation and could partly be explained by changes in soil moisture and soil mineral N. With only 1–17% of total annual fluxes at the 15N CULTAN plot, fertilizer-derived N2O emissions were small, highlighting the dominance of soil N for N2O emission. In terms of N2O emission, CULTAN fertilization did thus not proof beneficial over surface application of the same fertilizer.