It is well known that crop monoculture can induce negative effects on soil ecosystems and crop productivity. However, little is known about how vegetable monoculture affects the soil nematode community structure and its relationship with vegetable yields. In this study, the composition, abundance, metabolic footprint, and ecological indices of soil nematodes are investigated in monocultures of pumpkin and melon. The relationships between nematode community structure and yields of pumpkin and melon were analyzed by linear regression. Both monoculture soils of pumpkin and melon suppressed the relative abundance of bacterivores but increased the relative abundance of plant parasites. Pumpkin monoculture soils decreased soil nematode diversity but increased the maturity index of plant parasites. Monoculture soils of pumpkin and melon decreased the metabolic footprint of lower- and higher-level trophic groups of the soil food web, respectively. Pumpkin and melon monoculture soils increased the food web indices channel index (CI) but decreased the enrichment index (EI) and the structure index (SI). The monoculture soils of pumpkin and melon led to a more fungal-dominated decomposition pathway and degraded soil food web conditions. The abundance of bacterivores and food web indices EI and SI were positively correlated with soil nutrients and pH, while the abundance of plant parasites and CI were negatively correlated with soil nutrients and pH. Paratylenchus was negatively correlated with pumpkin and melon yields and could be the potential plant parasites threatening pumpkin and melon productions. Redundancy analysis showed that monocultures of pumpkin and melon altered the soil nematode community via soil properties; total N, total P, alkeline-N, and pH were the main driving factors.

Long-term application of chemical fertilizer poses an environmental threat to belowground ecosystems. However, the impact of nitrogen (N) or phosphorus (P) fertilizers on soil biodiversity and the conditions of soil food web remains largely unknown. Soil nematodes are the most abundant multicellular soil animals and serve as excellent bioindicators of soil. Here, we investigated soil nematode communities and food web structure in a long-term experiment with different application rates of N and P fertilizers in northeast China. The application of N and P fertilizers increased the abundance of bacterivores but suppressed the abundance of omnivores and predators. The abundance of bacterivores exhibited an increasing trend, while that of omnivores and predators showed a decreasing trend with increasing rates of N and P fertilizers. Plant parasites displayed a decreasing trend in response to N fertilizer, but not to P fertilizer. N and P fertilizers also altered nematode functional guild composition, with N fertilizer increasing the abundance of Ba1, and P fertilizer increasing the abundance of Fu2 and Ba3. Nonmetric multidimensional scaling (NMDS) analysis revealed apparent successions of nematode communities from no fertilizer soils to high rates of N or P fertilizer soils at both the genus and functional guild levels. Furthermore, N and P fertilizers resulted in different nematode communities. In terms of nematode food web indices, N fertilizer increased the enrichment index (EI) but reduced the channel index (CI) and structure index (SI), whereas P fertilizer only reduced the SI value. High rates of N and P fertilizers increased the respired carbon of bacterivores but reduced the respired carbon of predators. Mantel tests revealed significant correlations between soil properties and the community composition of both fungivores and omnivores. Among all soil properties, available phosphorus (AP) had the greatest influence on the community structure of soil nematodes. Our findings indicate that N fertilizer has a powerful effect on nematode food web structure, while P fertilizer exerts a stronger effect on soil nematode community composition.

Maize is one of the most important crops in the world. Heilongjiang province has the largest maize area in China. Plant-parasitic nematodes are important agricultural pests, which cause huge economic losses every year and have attracted global attention. Potato rot nematode Ditylenchus destructor is a plant-parasitic nematode with a wide range of hosts and strong survival ability in different environments, which brings risks to agricultural production. In 2020, D. destructor was detected in seven maize fields in Heilongjiang province. Morphological identification and molecular approach were used to characterize the isolated D. destructor. The observed morphological and morphometric characteristics were highly similar and consistent with the existing description. The DNA sequencing on the D2/D3 region of the ribosomal DNA 28S and the phylogenetic analysis showed that D. destructor population obtained from maize and other isolates infesting carrot, sweet potato, and potato were in subclade I supported by a 96% bootstrap value. Additionally, the phylogenetic analysis of the ITS rRNA gene sequence further indicated that this D. destructor population from maize clustered in a clade I group and belonged to ITS rRNA haplotype C. An inoculation experiment revealed that D. destructor was pathogenic on the maize seedlings in pots and caused the disease symptoms in the stem base of maize seedlings. This is the first report of D. destructor causing stem rot of maize in Heilongjiang province, and contributes additional information on disease control and safe production of maize in the region.

In this study, the effects of different modifiers on rhizosphere soil microorganisms, their functions, and the soil properties of continuous tomato cropping were investigated. Nine amendments were selected to treat the soil from a 14-year continuous tomato cropping system. Tomato yield, soluble solids, soil physical and chemical properties, and soil enzyme activities were measured. Changes in soil microbial community structure and function were determined by metagenomic sequencing, and their correlation with environmental factors was analyzed. The results showed that among the nine amendments, the combination of farmyard manure + Bacillus subtilis + Trichoderma harzianum (T2) and plant-derived straw decomposed soil + Bacillus subtilis + Trichoderma harzianum (T3) had the most significant effects. The tomato yield, soil hydrolyzable nitrogen, available phosphorus, available potassium, organic matter, and total nitrogen contents and soil phosphatase activities were significantly increased under the T2 and T3 treatments. Compared with the CK treated with T2, the contents of yield, alkali-hydrolyzed nitrogen, available phosphorus, available potassium, organic matter, and total nitrogen were significantly increased by 34.46%, 41.84%, 52.44%, 45.01%, 24.5%, and 41.18%, respectively. The soil microbial community structure also changed significantly. The relative abundance of Proteobacteria, Chloroflexi, and Bacteroidota increased significantly. The relative abundance of Hyphomicrobium, Rhodomicrobium, and Rhodoplanes increased significantly compared with the control. The soil microbial function was mainly enriched in two pathways of amino acid metabolism and carbohydrate metabolism. Among them, T2 significantly enriched six community functions, such as bacterial chemotaxis. T3 significantly enriched three community functions, such as glutathione metabolism. A correlation analysis showed that soil hydrolyzable nitrogen, available phosphorus, pH, phosphatase, and catalase were the key factors affecting microbial community changes. The treatment of farm manure/plant-derived straw decomposed soil + Bacillus subtilis + Trichoderma harziensis improved the soil environment, increased crop yield, clarified the effects of different modifiers on the functional mechanisms of the soil microbial community, and provided a practical solution to the problem of soil degradation in agriculture monoculture.

Maize (Zea mays L.) is one of the most important crops worldwide. In September 2020, five maize plants and soil samples were received from a farmer. The samples were collected from a maize field in...