An experiment was conducted in spring 2024 to investigate the effects of biochar, biogas slurry, and dicyandiamide (DCD) on N₂O emissions from soil in protected tomato cultivation. Five treatments were applied: conventional fertilization (CK1), biogas slurry alone (CK2), biochar combined with biogas slurry (T1), DCD combined with biogas slurry (T2), and the combination of biochar, biogas slurry, and DCD (T3). The study aimed to assess the response of the soil physicochemical properties and nitrifying ammonia-oxidizing microorganisms in the tomato root zone to these treatments and to determine their impact on soil N₂O emissions. The results showed that adding biochar and biogas slurry increased the soil pH, organic matter content, and levels of nitrate-N and ammonium-N, without affecting ammonia-oxidizing archaea (AOA) but inhibiting ammonia-oxidizing bacteria (AOB). The inclusion of DCD raised the soil pH and ammonium-N levels, enhanced AOA growth, did not alter organic matter content, and significantly reduced nitrate-N levels and AOB activity. Compared to CK1, treatments CK2, T1, T2, and T3 decreased the average N₂O emission flux by 5.83%, 8.24%, 15.27%, and 16.16%, respectively. The application of biochar, biogas slurry, and DCD enhanced the physicochemical properties of the root zone soil and notably reduced N₂O emissions in protected tomato cultivation, with T3 showing the most effective results. The biochar and biogas slurry used in this study, both derived from agricultural waste, promote sustainable agricultural development and enhance economic benefits. However, this study only considered the short-term effects of biochar, biogas slurry, and DCD, necessitating further research to explore their long-term impacts and mechanisms.

To explore fertilization strategies that achieve both high yield and emission reduction in greenhouse tomato production, a two-season experiment was conducted in autumn 2023 and spring 2024 under equal nitrogen input. Seven treatments were established: conventional fertilization (CK1), biogas slurry alone (CK2), 0.5% biochar + biogas slurry (T1), 2% biochar + biogas slurry (T2), dicyandiamide + biogas slurry (T3), 0.5% biochar + biogas slurry + dicyandiamide (T4), and 2% biochar + biogas slurry + dicyandiamide (T5). The effects of each treatment on tomato root traits, yield, irrigation water use efficiency (IWUE), partial factor productivity of nitrogen (PFPN), and soil N₂O emissions were systematically evaluated. An analytic hierarchy process (AHP) was applied for comprehensive assessment. The results showed that fertilization treatments significantly affected tomato root traits (p < 0.05), with T5 exhibiting the best performance in root length, average diameter, total surface area, total volume, and root activity, all significantly higher than CK1. T5 also achieved the highest yield in both seasons, with increases of 8.13% (autumn 2023) and 10.19% (spring 2024) over CK1. Moreover, T5 showed superior IWUE (475.38 kg ha⁻¹ mm⁻¹) and PFPN (405.92 kg kg⁻¹). In terms of environmental performance, T5 significantly reduced soil N₂O flux, with the largest reduction reaching 16.16%, particularly during the peak emission stages in the flowering and fruit-setting periods. The AHP-based comprehensive evaluation confirmed that T5 had the highest overall weight with satisfactory matrix consistency. In conclusion, compared with conventional fertilization, the integrated T5 treatment increased tomato yield by up to 10.19% and reduced cumulative N₂O emissions by up to 16.16%, highlighting its potential as a feasible fertilization pathway and technical reference for low-carbon and sustainable agriculture.

In intensive greenhouse agriculture, irrational fertilization practices can exacerbate carbon emissions and impair ecosystem service functions. To address this issue, biogas slurry and biochar were introduced as waste-derived substitutes for mineral fertilizer, and the effects of different fertilization strategies on the net ecosystem carbon balance (N_ECB) and ecosystem service value (ESV) of greenhouse tomato (Solanum lycopersicum L.) production systems over two growing seasons (spring–summer and autumn–winter) were systematically evaluated. When economic return was prioritized, the treatment with 25% biogas slurry substituting for mineral fertilizer (BS25) performed best, with ESVs of 641,606.83 and 629,987.37 CNY ha⁻¹ in the spring–summer and autumn–winter seasons, respectively; the treatment with 50% biogas slurry substitution (BS50) ranked second, and both treatments were significantly superior to the others (p < 0.05). When the objective was to enhance carbon sink capacity while maintaining high yield, the treatment with 75% biogas slurry combined with biochar substituting for mineral fertilizer (BS75 + C) showed the best overall performance, with N_ECB values of 6.30 and 6.34 t ha⁻¹ in the two respective seasons, while also demonstrating clear advantages in soil organic matter accumulation and atmospheric regulation. Based on the VIKOR model with AHP-CRITIC combined weighting, BS75 + C was identified as the optimal option. However, the most suitable fertilization strategy depends on management objectives: BS25 is recommended when maximizing short-term economic return is the primary goal, whereas BS75 + C is preferable for enhancing carbon sink capacity and ecological benefits. Considering both ecosystem service value and comprehensive performance, BS50 and BS75 + C are recommended as sustainable fertilization strategies for greenhouse tomato production.