Effects Of Organic Amendment Types On Nematode Communities In Different Aggregates Of Farmland Red Soil In Dryland

Biodiversity is an important basis for maintaining ecosystem functions,and unreasonable agricultural measures will lead to soil biological degradation.Red soil in dryland of farmland in South China is facing the challenge of soil degradation.Using surplus agricultural organic waste is an important measure to repair degraded soil and promote biodiversity and ecosystem function.Previous studies mainly focused on the effects of organic matter application on soil biological communities.In fact,the types of organic materials can also affect soil biological communities by affecting the quantity and quality of resources and the physical structure of soil.The change of soil nematode community can sensitively indicate the health status of soil,especially respond sensitively to the management measures of organic materials.In view of this,this study is based on the upland red soil farmland with rotation of Ipomoea batatas L and Brassica napus L,taking the agricultural organic materials widely used at present as the representative,and further selecting straw and biochar,pig compost and vermicompost to represent plant-derived and animal-derived organic amendments respectively.In addition,in order to fully reflect the influence of organic materials on soil nematodes through soil resources and physical structure,dry screening method was used to classify aggregates(>2 mm,1-2 mm,0.25-1 mm and <0.25 mm)and analyze soil physical and chemical properties and microbial properties,so as to fully understand the change mechanism of nematode community in microhabitat scale mediated by organic amendments.In order to show the results more clearly,this study includes analyzing the effects of plant-derived and animal-derived organic materials respectively,so as to reflect the difference between high-quality organic materials with simple structure(such as straw and conventional compost)and low-quality organic materials with complex structure(such as biomass charcoal and earthworm compost made from straw and conventional compost).The first part explores the influence of chemical fertilizer reduction and plant-derived organic materials,and adopts the interaction of chemical fertilizer reduction(2 levels: NPK of total chemical fertilizer and 60%NPK of reduced chemical fertilizer)and biochar(3 levels: no organic amendments control,straw and biochar),with a total of six treatments.Secondly,the effects of vermicompost were compared under the condition of reducing fertilizer application by 60%NPK.There were only three treatments: no organic amendment control,pig compost and vermicompost.The main results are as follows:(1)Reduced application of chemical fertilizer,straw,biomass charcoal and aggregates all significantly affected nematode communities,such as quantity and trophic groups(p <0.05).Both straw and biochar increased the total number of nematodes in soil.With the increase of soil aggregate particle size,the number of nematodes on the aggregate also increased on the whole,with >1 mm particle size and <1 mm particle size as the most obvious limits of the change in number.The response of nematode community to organic materials is related to sampling season.Compared with straw,in rape season,the total number of nematodes with >0.25 mm aggregates decreased under the application of biochar.In sweet potato season,the total number of nematodes in >1 mm aggregates decreased under the application of biomass charcoal,which indicated that biochar reduced the number of nematodes in large aggregates.(2)Both straw and biochar increased the number of microbivores and predatoryomnivores in soil,and the promotion effect of biochar was lower than that of straw.Similarly,compared with straw application,the number of bacterivores and fungivores decreased on aggregates of >2 mm and <0.25 mm,and the number of predatory-omnivores decreased on aggregates of 1-2 mm,and the proportion of fungivores and bacterivores nematodes increased on aggregates of 1-2 mm,which indicated that the nutrition group of nematodes of this size changed towards the fungal energy channel under the action of biochar.(3)Structural equation model shows that,on the one hand,organic materials from plants can control the nematode community structure by affecting the size of soil aggregates,their resources and microbial community structure;on the other hand,organic materials can also influence the microbial community structure and then the nematode community structure by affecting soil p H or resources.Compared with straw,there was no negative effect on nematode community structure under biochar.(4)Like plant-derived organic amendments,the particle size and interaction of animalderived amendments and aggregates significantly affect most soil biological indicators,such as the total number of soil nematodes,trophic groups and soil microbial communities.Among them,1mm particle size is also the biggest change limit of nematode community on different aggregates.Compared with conventional compost,the number of bacterivores and fungivores in vermicompost decreased obviously,while the number of predatory-omnivores s caught on aggregates >0.25 mm was higher.The change trend of soil microbial total,bacterial total and fungal total is the same as that of nematodes,and the trend is more obvious.(5)Structural equation model shows that,on the one hand,animal-derived organic materials affect nematode community structure by affecting aggregate size,organic carbon content,bacteria and fungi,on the other hand,organic materials also affect nematode community structure by affecting soil p H or organic carbon and then bacteria and fungi.Compared with pig composting,the size of aggregates has a direct effect on nematode community structure under vermicompost.(6)Nematode faunal analysis showed that the structure index and enrichment index of soil nematodes decreased with the decrease of aggregate size in both animal and plant organic materials,especially in the control treatment,the food web had poor stability and suffered more interference.To sum up,the size and interaction of organic materials and aggregates from both plant and animal sources significantly affected the nematode community,and straw and conventional compost mainly promoted the number of microbivores,while biochar and vermicompost increased the number of predatory-omnivores.