Abstract: Low nitrogen availability under salt stress is an important factor limiting crop productivity of coastal saline-alkali soil. Application of biochar and nitrification inhibitors can improve soil nitrogen availability. But there is relatively little research on the application of biochar and nitrification inhibitors in the Yellow River Delta coastal saline-alkali land. A field in situ experiment was conducted to explore the effects of biochar and DCD on nitrogen transformation and nitrogen uptake/utilization of soybean of coastal saline-alkali soil in the Yellow River Delta. There were five treatments: control (no nitrogen application, CK), nitrogen (N), nitrogen + biochar (NB), nitrogen + 2% dicyandiamide DCD (ND), nitrogen + biochar + 2% DCD (NBD). Results showed that: (1) Compared with N application, biochar and DCD decreased soil pH and electrical conductivity (EC), significantly inhibited soil nitrification, regulated the duration and level of soil nitrogen supply, and improved soil nitrogen sequestration by microorganisms in late stage of soybean. (2) The activity of soil nitrogen converting enzymes showed a trend of first increasing and then decreasing during the growing season of soybean. Compared with N treatment, both biochar and DCD additions increased the activity of nitrogen conversion enzymes. The highest activities of protease, urease, nitrate reductase and nitrite reductase were observed under NBD treatment, with an average increase of 33.8%, 28.3%, 26.0% and 18.8%, respectively. (3) Both biochar and DCD promoted nitrogen uptake and utilization of soybean, and increased its biomass and grain yield. The combined application of biochar and DCD had a synergistic effect, with more significant yield improvement. In the Yellow River Delta, combined application of biochar and DCD can improve soil nitrogen availability, and promote nitrogen uptake and utilization by plants, which provide a reference for crop yield improvement and efficient use of nitrogen fertilizer in salinized farmlands.

Key words: Yellow River Delta, nitrogen conversion enzyme, carbon and nitrogen coupling, crop yield, nitrogen utilization