This study aimed to clarify the regulatory effects of the reduced nitrogen combined with controlled water, biochar, and the nitrification inhibitor dicyandiamide (DCD) on nitrogen losses within a facility soil-tomato system, so as to provide a scientific basis for efficient nitrogen use and sustainable green development in facility vegetable production.

A field plot experiment was conducted using facility tomato as the research object, with two irrigation treatments including traditional drip irrigation (TD) and reduced drip irrigation (CD). Based on this, eight nitrogen treatments were designed, including no nitrogen (TDN0, and CDN0), traditional nitrogen (TDN1, and CDN1), reduced nitrogen (TDN2, and CDN2), and reduced nitrogen combined with biochar and dicyandiamide (DCD) (TDN2+BD, and CDN2+BD). During the topdressing period, soil N₂O emission and NH₃ volatilization, NO₃^--N accumulation in the 0-100 cm soil profile, and tomato nitrogen uptake, yield, and fruit quality indicators were systematically monitored to quantitatively evaluate the synergistic regulatory effects of biochar and DCD under reduced nitrogen and controlled water conditions.

Nitrogen application significantly increased soil N₂O emissions and NH₃ volatilization, with peak fluxes occurring 2-3 days after topdressing. Nitrogen reduction effectively suppressed gaseous nitrogen losses, and the combined application of biochar and the nitrification inhibitor DCD further enhanced the mitigation effect on N₂O emissions. Although biochar and DCD posed a potential risk of increasing NH₃ volatilization, this adverse effect was substantially alleviated by reduced nitrogen and controlled water conditions. Compared with conventional nitrogen application, nitrogen reduction significantly decreased total gaseous nitrogen losses by 45.7%-56.6% and reduced NO₃^--N accumulation in the 0-100 cm soil profile by 13.7%-16.2%. When biochar and DCD were applied on the basis of nitrogen reduction, total gaseous nitrogen losses further declined by 49.4%-59.0%, while the reduction in NO₃^--N accumulation expanded to 27.4%-30.0%, with a clear suppression of deep leaching. Both irrigation methods enhanced the synergistic mitigation effects of water nitrogen regulation. However, controlled drip irrigation demonstrated superior performance compared with conventional drip irrigation. Under controlled irrigation, the CDN2+BD treatment reduced total nitrogen losses by 30.0% compared with CDN1, with decreasing the net nitrogen loss rate from 3.3% to 2.5%. This treatment also significantly inhibited deep NO₃^--N accumulation and increased NO₃^--N retention in the 20-40 cm soil layer by 27.1%. In addition, biochar and DCD increased the apparent nitrogen use efficiency under controlled irrigation to 28.9%; TDN2+BD and CDN2+BD improved nitrogen use efficiency by 53.0% and 30.2%, respectively, compared with their corresponding nitrogen-reduction treatments (TDN2 and CDN2) (P<0.05). Furthermore, the controlled irrigation with biochar and DCD (CDN2+BD) achieved the highest tomato yield (115.93 t·hm^-2), while significantly enhancing fruit vitamin C content and improving soluble protein, soluble sugar, and total soluble solid levels.

Reduced nitrogen application combined with controlled water, biochar, and the nitrification inhibitor dicyandiamide (DCD) significantly decreased gaseous and leaching nitrogen losses, optimized soil nitrogen distribution, and improved nitrogen use efficiency and tomato quality, representing an optimal technical approach for nitrogen reduction and efficiency enhancement in facility vegetable production.

This paper aimed to explore the comprehensive effects of biochar, dicyandiamide (DCD) and their combined application on the greenhouse gas (N₂O, CO₂ and CH₄) emissions from facility soil, so as to provide a scientific basis for reducing the greenhouse gas emissions and green development of facility vegetable production system.

In this study, the facility vegetable production system was used as the research object, and a total of six treatments were set up, including no nitrogen application (CK), traditional nitrogen application (CN), recommended nitrogen application (RN), recommended nitrogen application+biochar(RNB), recommended nitrogen application + DCD (RND), and recommended nitrogen application+biochar+ DCD(RNBD). A pot experiment method was applied to analyze the effects of soil greenhouse gas emissions, and the difference in greenhouse gas intensity (GHGI) and global warming potential (GWP) under different treatments.

Compared with the CN treatment, the rape yield decreased by 2.9%-29.3% under the recommend nitrogen treatments (RN, RN, RND and RNBD). However, under the same nitrogen application rate, the rape yield increased by 34.4% in the treatment of recommend nitrogen combined with biochar and DCD (RNBD), indicating that biochar and DCD showed a synergistic effect on rape yield increase (P<0.05). The recommend nitrogen treatments reduced the soil N₂O emissions by 29.4%-76.5% in comparation with the CN treatment, especially the RND treatment showed the best effect. However, the recommend nitrogen treatments showed little effect on soil CO₂ and CH₄ emissions. Compared with the CN treatment, the total GWP under the recommended nitrogen treatments decreased by 4.3%-51.2%, and the RND treatment showed the best emission-reduction effect. In terms of GHGI, the difference among the recommended nitrogen treatments was not significant (P>0.05), and the RND treatment also showed the best emission-reduction effect.

Under the same nitrogen application rate, the application of biochar alone or DCD alone had little effect on rape yield, but the combination of biochar and DCD could significantly increase the rape yield. Additionally, the combination of biochar and DCD could reduce the cumulative greenhouse gas emissions and GWP, but it was not superior to single application of DCD in the facility vegetable field.