Solar greenhouse vegetable fields have been found to be hotspots of nitrous oxide (N₂O) emissions in China, mainly due to excessive manure application and irrigation. Pulses of N₂O emissions have been commonly reported by field monitoring works conducted in greenhouse fields, though their significance regarding total N₂O emissions and the driving mechanism behind them remain poorly understood. N₂O fluxes were monitored in situ using a static opaque chamber method in a typical greenhouse vegetable field. Then, laboratory incubations were conducted under different soil moisture and manure application gradients to monitor nitrous oxide emissions and related soil properties, using a robotized incubation system. Field monitoring showed that the occurrence of clear N₂O emission bursts closely followed fertilization and irrigation events, accounting for 76.7% of the annual N₂O efflux. The soil N₂O flux increased exponentially with the water-filled pore space (WFPS), causing extremely high N₂O emissions when the WFPS was higher than 60%. During the lab incubation, emission bursts led to N₂O peaks within 40 h, synchronously changing with the transit soil NO₂⁻. An integrated analysis of the variations in the gas emission and soil properties indicated that the denitrification of transit NO₂⁻ accumulation was the major explanation for N₂O emission bursts in the greenhouse filed. Nitrous oxide emission bursts constituted the major portion of the N₂O emissions in the Chinese greenhouse soils. Nitrite (NO₂⁻) denitrification triggered by fertilization and irrigation was responsible for these N₂O emission pulses. Our results clarified the significance and biogeochemical mechanisms of N₂O burst emissions; this knowledge could help us to devise and enact sounder N₂O mitigation measures, which would be conducive to sustainable development in vegetable greenhouse fields.

Denitrification, as an important part of the soil nitrogen cycle, is widely considered to be a major source of nitrous oxide (N₂O). Both biotic and abiotic denitrification processes contribute significantly to soil N₂O emission, especially under acidic conditions. Increasing soil pH was found to suppress N₂O emissions from denitrification, while the underlying mechanism remains uncertain. In this study, we incubated fresh forest soil anaerobically after increasing soil pH and adding nitrate (NO₃⁻) under both sterilized and unsterilized conditions. The dynamic changes of NO₃⁻, nitrite (NO₂⁻), N₂O and dinitrogen (N₂) were monitored continuously during the 15 days of incubation. The results showed that nitrate reduction rates increased with soil pH in both sterilized and unsterilized soils, with the former having higher rates. The obvious production and consumption of nitrite were found at pH 7.1, rather than at pH 5.5, especially in sterilized soils. In both sterilized and unsterilized soils, accumulative emission of N₂O and N₂O-N/(N₂O+N₂)-N product ratios decreased significantly with increasing pH, while N₂ showed the opposite trend. In sterilized soils, N₂O was the dominant end gas product, accounting for 40.88% and 29.42% of the added nitrate at pH 5.5 and 7.1, respectively. In unsterilized soils, N₂ was the only final gas product at pH 7.1 (59.34% of the added nitrate), whereas N₂O dominated at pH 5.5 (26.67% of the added nitrate). Our results here showed that increasing soil pH promoted the conversion of N₂O to N₂ under both sterilized and unsterilized conditions, and highlighted the potential importance of abiotic denitrification on N₂O emission.

Nitrogen (N) availability is generally a limiting factor in highly acidic soil, which could be improved by amending these soils with alkaline materials. Soil extracellular enzyme activity (EEA) plays an important role in N transformation; a current knowledge gap is how this occurs in acidic soils amended with alkaline material. The present 45-day incubation experiment was designed to examine the effects of different amounts of alkaline materials (urea and/or calcium–silicon–magnesium–potassium fertilizer (CSMP)) on N transformation. The results show that soil pH significantly increased after the CSMP amendment (1.2 units) and increased soil net N mineralization (R_min), net nitrification (R_nit) rates, and net ammonification (R_amo) rates. CSMP amendment changed the different soil EEA but with differing or opposing effects, e.g., R_nit was positively correlated with the activities of L-leucine aminopeptidase, β-xylosidase, α-glucosidase, and N-acetyl-β-glucosaminidase but negatively correlated with β-1, 4-glucosidase and β-cellobiosidase. A machine learning analysis indicated that the best predictor for R_min and R_amo was soil pH, and for R_nit, it was nitrate. The results of the present study improve our understanding of N availability in acidic soils amended with materials to control soil pH. Such knowledge could lead to more bespoke nutrient management planning at the field scale, leading to better agronomic and environmental outcomes.

Cucumber (Cucumis sativus L.) is an important vegetable. China's annual output exceeds 50 million tons, accounting for more than 40% of the world total. In Nov 2021, a wilt disease was observed on cucumber plants in Beiluo Town, Shouguang, Weifang City, Shandong Province, China (36°88'N, 118°74'E). The incidence across the greenhouses (≈0.13 ha) was approximately 25%. Leaves of plants in the field turned yellow and wilted. Finally, the whole plant withered and died. The base of the main vine stem shrank and the epidermis cracked longitudinally. Tissue fragments from infected stem bases (5 mm × 5 mm) were surface disinfected with 0.1% mercuric chloride solution for 30 s, followed by 75% ethanol for 30 s, and then washed three times with ddH₂O. These sections were then placed onto PDA and incubated at 25 ± 2°C for five days. Twelve strains (similar colony morphology) belonging to fungal species were obtained from 10 disease samples. On PDA, the colonies appeared off-white or light-orange, occasionally with radiating wrinkles, a damp surface, and defined margins. The conidiophores were solitary, unbranched or rarely irregularly branched, transparent, smooth-surfaced, and thin-walled. Sporulation cells (bottle-stem type autotrophic hyphae) extended with a transparent, smooth surface, a slightly wider base, and a pointed top. Typically, they were solitary upright bottle-necks, 6.3-24.3 μm × 1.4-3.3 μm (n = 30). The conidia aggregated into a head-shaped, oval, shaped, or round shape with narrow ends and a wide middle, transparent, smooth-surfaced, with thin walls and 0-1 septa, containing oil globules. The size of conidia ranged from 4.0-8.9 μm × 2.0-4.0 μm. These morphological characteristics were similar to those of Plectosphaerella sp. At the same time, the internal transcriptional spacer (ITS), and the large subunit (LSU) of the 28S rRNA gene were amplified and sequenced using the primer pairs ITS1/ITS4 and LR5/LROR (Giraldo et al., 2019). BLASTn analysis, the obtained sequences of the ITS amplicon (OR994322), and the LSU domain amplicon (PP092994) showed 99.80% (507/508 bp) and 97.21% (872/897 bp) identity to sequences of P. cucumerina isolates OP881967.1 and MK143394.1, respectively, in the NCBI database. A multilocus phylogenetic tree constructed by MEGA 11 software showed that the representative isolates were assigned to the group of P. cucumerina. To confirm their pathogenicity, 10 ml of spore suspension (105 spores/ml) were poured onto the rhizomes of seedlings (1-2 leaf stage; 20 plants per strain). Another 20 seedlings treated with sterile distilled water were used as the control. Each trial was performed three times. Seedlings inoculated with P. cucumerina began to show symptoms on the 5th day, which continued until the 21st day after inoculation. The symptoms were similar to those observed in the diseased plants in the field. The control seedlings showed no symptoms. P. cucumerina was re-isolated from inoculated diseased seedlings with 100% homology, whereas no fungal growth was observed from the control seedlings. The results indicate that P. cucumerina was the pathogen of cucumber blight. So far, More than 10 species of plants, including Brassica oleracea and Foeniculum vulgare, are known hosts of P. cucumerina in China (Li et al.2017, Su, et al. 2017). This is the first report of P. cucumerina causing wilt on C. sativus L. (Lei et al., 2017). Our study laid a critical foundation for the prevention and control of this disease on cucumbers in China.