Effect of nitrogen fertilization management on mineral nitrogen content in soil and winter wheat productivity

In recent years farmers must use integrated crop growing principles. One of the most important principle is to balance usage of mineral elements in crop cultivation, especially nitrogen management. Excessive and unbalanced usage of nitrogen fertilizer reduces nitrogen use efficiency and increases nitrate leaching in surface and groundwater. The dynamics of nitrogen forms in soil at different depths and different plant growth stages are studied to increase the productivity of winter wheat, promoting nitrogen uptake in plants and reducing nitrogen leaching during the vegetation period. Field experiments were carried out at the Research and Training Farm Vecauce of the Latvia University of Life Science and Technologies from 2012 till 2015. Researched factors were nitrogen (N) fertilizer rate: 0 – control, 85, 153, 187, and N rate determined by chlorophylmeter (Yara N-tester) 180 (2012/2013), 150 (2013/2014), 205 (2014/2015) N kg ha-1, nitrogen and sulphur (S) fertilizer rate – N175+S21 kg ha-1, and conditions of the growing seasons: 2012/2013, 2013/2014 and 2014/2015. The content of nitrate (NO3–N) nitrogen and ammonium (NH4–N) nitrogen was determined in the soil layers 0–20 cm, 20–40 cm and 40–60 cm at the growth stages (GS) 30–32, 49–51, 69 and 90–92. All trial years the amount of nitrate nitrogen and ammonium nitrogen in soil decreased during vegetation, but increased with increasing fertilization dose. Nitrate nitrogen content was significantly influenced by year in 0–40 cm soil layer (P < 0.01) and by nitrogen fertilizer in the 20–40 cm soil layer. Ammonium nitrogen content had significant influence only on nitrogen fertilizer at 20–40 cm soil layer (P < 0.05). Average grain yields did not show significant correlation with the nitrate nitrogen and ammonium nitrogen in different soil layers and plant growth stages, except nitrate nitrogen content in soil layer 40–60 cm at GS 30–32 and ammonium nitrogen content in soil layer 40–60 cm at GS 69 and GS 90–92.