Control Of Root-knot Nematodes By Fumigation With Different Nitrogen Sources And Its Impact On Soil Microorganism Diversity

With the large-scale development of agricultural production,the year-round monocropping of economically important crops in controlled environments has led to severe occurrences of various soil-borne diseases,particularly widespread outbreaks of root-knot nematode diseases,resulting in significant crop yield reductions and constraining the sustainable development of controlled-environment crops.While soil fumigation with methyl bromide represents a technique that can effectively control root-knot nematode diseases,it is ecologically unfriendly and lacks safety.Therefore,there is an urgent need to explore alternative techniques for controlling root-knot nematode diseases.Nitrogen sources not only serve as essential nutrients for crop growth but also have certain control effects on root-knot nematode diseases.Nitrogen source fumigation is a cost-effective,ecologically compatible,and relatively efficient technique with great development potential.The present study selected three nitrogen sources,namely ammonium bicarbonate,ammonia water,and liquid ammonia,to fumigate the soil affected by cucumber root-knot nematode disease.The aim was to investigate the control efficacy of each treatment on cucumber root-knot nematode disease.For each nitrogen source,three different concentrations of low,medium,and high were designed based on the total nitrogen content.Specifically,these included low concentration of ammonium bicarbonate(ABL),medium concentration of ammonium bicarbonate(ABM),high concentration of ammonium bicarbonate(ABH),low concentration of ammonia water(AHL),medium concentration of ammonia water(AHM),high concentration of ammonia water(AHH),low concentration of liquid ammonia(LAL),medium concentration of liquid ammonia(LAM),and high concentration of liquid ammonia(LAH).Methyl bromide(MS)was set as the positive control,and untreated soil served as the blank control(CK).By investigating the control efficacy of nitrogen source fumigation on root-knot nematodes,the impact on soil physicochemical factors and soil microbial communities,this study aimed to explore the characteristics of different nitrogen sources and concentrations on nematode control efficacy and soil ecological processes.The goal was to develop a safe,low-cost,and easily scalable new technology for effective control of soil root-knot nematodes,providing technical support and reference for addressing soil continuous cropping obstacles and promoting sustainable agricultural production and development.The research results are as follows:1.The number of root-knot nematodes in the soil and the growth status of cucumber were assessed using the tray method and conventional techniques.The efficacy of different nitrogen sources for fumigation on root-knot nematodes was determined.Comparing with the control treatment of methyl bromide,when using ammonia as the fumigant material at a pure nitrogen dosage of 900 kg/hm~2,the highest efficacy in controlling root-knot nematodes was achieved,reaching 76%,significantly higher than other treatments.The efficacy of ABH(high-dose ammonium bicarbonate)and LAH(high-dose liquid ammonia)treatments were 73%and 63%,respectively.Under equal nitrogen concentration conditions,the order of effectiveness for controlling root-knot nematodes through nitrogen source fumigation is as follows:ammonia water treatment>ammonium bicarbonate>liquid ammonia treatment.Furthermore,Under conditions of equal nitrogen concentration,the order of effectiveness for nematode control through nitrogen fumigation of root-knot nematodes is as follows,from highest to lowest:ammonium hydroxide treatment>ammonium bicarbonate treatment>liquid ammonia treatment.Furthermore,when compared to the positive control and at a moderate nitrogen concentration,both ammonium hydroxide and liquid ammonia treatments significantly reduce the number of root-knot nematodes.At a pure nitrogen dosage of 600kg/hm~2,the order of plant growth in nitrogen source treatments,from highest to lowest,was as follows:ammonia>ammonium bicarbonate>liquid ammonia.The effects of different nitrogen source fumigation on crop yield indicated that at a pure nitrogen dosage of 600kg/hm~2,the cucumber yield in the ammonia and liquid ammonia treatments was higher than in the ammonium bicarbonate treatment.Moreover,at a high dosage of ammonia(600～900kg/hm~2),not only could it control root-knot nematode diseases,but it also promoted cucumber yield.2.Using the indophenol blue colorimetric method,ammonium nitrogen was determined,while the dual-wavelength method was employed to measure soil nitrate nitrogen.The conventional method was utilized to assess soil p H,electrical conductivity(EC),and soil urease activity.The variations in soil physicochemical factors during the process of nematode control through different nitrogen sources indicate the following characteristics:After fumigation,the measurement results of soil p H indicate that when the nitrogen source’s pure nitrogen dosage is high,the soil shows an alkaline nature.Compared to the blank control,the highest p H value recorded in the ABH treatment,reaching the value of 9.10.The content of ammonium nitrogen in the soil increased with the increase in nitrogen source dosage,and it was higher than the control treatment.Within the range of 300～600 kg/hm~2 of pure nitrogen dosage,the AHM treatment(medium ammonia water dosage)showed a soil ammonium nitrogen concentration of 1232.07 mg/kg,which was higher than the ABM treatment(medium ammonium bicarbonate dosage).The soil ammonium nitrogen concentration in the liquid ammonia treatment was significantly lower than that in the ammonia water treatment.Under the pure nitrogen dosage of 900 kg/hm~2,the ammonium nitrogen concentration produced by ammonium bicarbonate in the soil was significantly higher than that produced by ammonia water and liquid ammonia treatments.The soil urease activity in the ammonia water and liquid ammonia treatments decreased with the increase in dosage.Ammonium bicarbonate inhibited soil urease activity and was significantly lower than other treatments.The results of soil p H,EC,soil ammonium nitrogen,and nitrate nitrogen content at the time of cucumber harvest revealed that compared to post-fumigation,the soil ammonium nitrogen content,p H value,and EC value decreased.Soil urease activity increased with the increase in nitrogen source dosage,with significantly higher values observed in the high dosage ammonia water treatment,reaching 405.83/U.3.Using conventional microbial isolation methods,soil cultivable microorganisms were detected.The abundance of soil microorganisms(bacteria and fungi)was determined using high-throughput methods.The analysis of soil microbial changes during the process of nematode control through different nitrogen sources indicates the following characteristics:After nitrogen fumigation treatment,the results of cultivable soil microorganisms showed that nitrogen had a minimal impact on soil bacteria,actinomycetes,and fungi,while the population of soil Rhizoctonia decreased with increasing nitrogen dosage.The results of soil microbial alpha diversity show that compared to the control treatment(CK),there is no significant difference in bacterial richness with nitrogen treatments.However,the fungal diversity and richness are lowest in the treatment with high ammonia water dosage(AHH).Higher nitrogen dosage slightly reduces bacterial diversity,and the bacterial species diversity(Shannon index)in the AHH treatment is the lowest,reaching 3.78.The results of microbial beta diversity show that the number of soil bacterial operational taxonomic units(OTUs)decreases with increasing ammonia water and ammonium bicarbonate dosage,but without significant differences.The fungal community composition in the nitrogen treatment shows no significant difference.The heatmap sorting results at the phylum and genus levels of microorganisms indicate that nitrogen fumigation has a minor impact on the quantity and community changes of bacteria and fungi.Functional gene prediction results show that the response of bacterial functional genes to high ammonium bicarbonate dosage(ABH)is more significant,and it also has a higher response to fungal genes than high ammonia water dosage.After cucumber harvest,the application of a high dose of nitrogen source was found to decrease the population of cultivable actinomycetes in the soil,with no significant impact on fungi and bacteria.Beta diversity analysis revealed that the bacterial communities were similar among nitrogen treatments,as well as among the MS,CK,LAM,and LAL treatments for fungal community composition.Additionally,the AHH and ABH treatments showed similar fungal community compositions.Heatmap sorting of microbial phylum and genus levels indicated an increase in both bacterial and fungal populations,with no significant effect on the overall microbial community.Conclusion:Based on comprehensive analysis of the effectiveness of nematode control,plant growth conditions,soil physicochemical properties,and soil microbial diversity,it is determined that ammonium water is the optimal nitrogen source material,with a recommended application rate range of 600～900 kg/hm~2.