Field evaluation of nitrogen volatilization loss during fertigation through center pivots

Nitrogen volatilization loss is an important consideration for sprinkler fertigation. To evaluate the effects of fertilizer types, N concentrations and meteorological factors on nitrogen volatilization loss during the fertigation process, urea (nitrogen mass concentration of 0.025-0.15%), ammonium sulfate (0.025-0.15%), mono-ammonium phosphate (0.01-0.02%), ammonium bicarbonate (0.015-0.025%) and nitrogen-potassium compound fertilizer (0.025-0.15%) were tested through a center pivot system in the North China Plain. A total of 135 tests were conducted under varying environments with temperatures ranging from 14 to 31 celcius, relative humidity ranging from 9% to 90% and wind speeds ranging from 0.65 to 3.85 m s(-1). The results showed that fertilizer types and nitrogen concentrations had little influence on the uniformity coefficient (CU) of nitrogen concentration obtained above the ground, which remained at a high level with an average of 98%. The average loss rates of nitrogen volatilization for urea, ammonium sulfate, nitrogen-potassium compound fertilizer, mono-ammonium phosphate and ammonium bicarbonate were 6.7%, 9.2%, 10.7%, 12.3% and 14.0%, respectively. Temperature, relative humidity and wind speed influenced nitrogen loss, while temperature was the most important factor. The effects of N concentrations on nitrogen loss were dependent on fertilizer type. For urea and nitrogen-potassium compound fertilizers, the nitrogen loss increased with increasing N concentrations up to 0.05%; however, a decreasing trend in the nitrogen loss rate was observed when the fertilizer concentration exceeded 0.05%. The nitrogen loss rate of ammonium sulfate decreased significantly with increasing N concentrations, while the loss of monoammonium phosphate and ammonium bicarbonate increased significantly with increasing N concentrations. A machine learning model was constructed with fertilizer types, N concen-trations, temperature and wind speed as input variables to predict nitrogen loss. The model was verified with acceptable accuracy and can be used to estimate nitrogen volatilization loss during sprinkler fertigation. Our study suggested that the fertilizer type and concentrations should be carefully selected while considering meteorological factors to minimize nitrogen volatilization losses during sprinkler irrigation.