| Despite its significance in feeding the ever-growing human population,agricultural intensification with the increased use of inorganic nitrogen（N）fertilizers have been associated with soil biodiversity loss.Considering the important ecosystem processes and functions（e.g.,carbon and nutrient cycling）provided by soil biota,their loss have been directly or indirectly linked to the impairment of these functions.Therefore,soil biodiversity loss has become a major issue that has concerned the scientific community across the globe,calling for a shift in agricultural practices to an approach that preserve or restores the natural capital of the soil and at the same time sustains high levels of production.Due to its N fixing ability,legume intercropping have been shown to contribute to the reduction in the use of inorganic N fertilizers and agrichemicals that have significant impact on soil biodiversity.In China,although grain-legume intercropping such as maize/soybean,maize/faba bean,and maize/peanut have been widely practiced to this end,only recently has the intercropping of forage legume such as alfalfa with cereals such as maize received considerable attention because of the increasing demands for meat and milk production.Besides,although substantial amount of data exist on the yield advantage and nutrient use efficiency of legume-cereal intercropping system,little is known how these systems regulate soil nematode community structure and community composition,particularly nematode regulated carbon（C）and energy flow in the soil food web.N fertilization also known to greatly influence soil nematodes by regulating C inputs into the soil;however,the combined effect of intercropping and N fertilization on soil nematode community remains unclear.Furthermore,there is increasing recognition of the role of habitat spatial heterogeneity as an important contributor to the patterning of soil biota（e.g.,soil nematodes）in the soil food web,which is a major determinant of soil biological diversity and functions.Thus,quantifying the distribution and abundance of soil nematodes between the bulk and rhizosphere soil in the intercropping of maize with alfalfa would contribute to understanding the regulating factors that shape the spatial distribution and abundance of nematode community under different agricultural management practices and the consequences of such management practices on the soil food web.Hence,we analyzed the dynamic changes of soil nematode community between the bulk and rhizosphere soil compartments in the maize/alfalfa intercropping system.Here,we assessed how the spatial heterogeneity in soil physicochemical parameters and different organic acids and sugars shape the patterning of soil nematodes between the two soil compartments.The field experiment involving maize（Zea mays L.）and alfalfa（Medicago sativa L.）grown in a mixture or as a sole crops was conducted in 2×3 factorial completely randomized block design with four replications.The experiment consisted of two N levels,namely,no N fertilization and with N fertilization(225 kg N ha-1y-1)and three cropping modes:monoculture maize,monoculture alfalfa and maize intercropped with alfalfa.Total nematode（TNem）and trophic group abundances,nematode diversity and species richness,and ecological and food web indices including maturity（MI）,structure（SI）,enrichment（EI）,channel（CI）,and basal index（BI）and nematode metabolic footprints were measured.In addition,spatial patterning of soil nematode community between bulk and rhizosphere soils were compared and their relationships with soil properties and organic acids were studied.Major results are presented as follows:（1）Compared with monoculture treatments,the intercropping of maize with alfalfa significantly increased soil moisture content（SMC）,soil organic carbon（SOC）,and soil total nitrogen（TN）but reduced concentration of soil NO3?-N.（2）Both intercropping and N fertilization had positive effect on TNem abundance.Compared with monoculture treatments,TNem abundance under no N addition in 2017increased,on average,by 9.56%and 65.94%in the IMAM and IMAY treatments,respectively and increased by 37.44%and 63.71%in the IMAM and IMAY treatments,respectively under N addition.Similarly,in 2018,TNem abundance increased significantly in both the IMAM and IMAY treatments and both with and without N addition.（3）Nematode trophic groups responded differently to the effects of intercropping and N fertilization,with bacterivores being more abundant in the MA treatment and fungivores being more abundant in the MM treatment.Intercropping in the no N fertilization led to an abundance of enrichment opportunistic c-p1 nematodes,suggesting more resource availability in the intercropping system than that of monocultures.In contrast,general opportunistic c-p2and c-p3 nematodes become dominant under the N added condition and monoculture treatments,indicating a shift in the nematode community composition to disturbance tolerant nematode groups typical of high-input conventional management practices.On the other hand,we found greater abundance of omnivores and predators in the IMAM and IMAY treatments compared with the MA and MM treatments,but a reciprocated response for the plant-feeding nematodes which suggest either a suppressive effect of intercropping system on herbivore nematodes or a top-down control by omnivores and predators on plant-feeding nematode guild.N addition had a significant negative impact on omnivores and predatory nematodes but positive effect on herbivores.（4）We found greater nematode diversity and species richness in the IMAM treatment in2017 and in the IMAY treatment in 2018,but both generic diversity and species richness declined significantly following N addition.Compared with monoculture system,intercropping increased MI,SI,TNem biomass and nematode composite footprint,enrichment and structure footprints,and the footprints of omnivores-predators,particularly in the no N addition.The high MI,EI,SI,and metabolic footprints of omnivores-predators in the IMAM and IMAY treatments under no N addition indicated that the soil food web in the intercropping system under low soil N was mature,enriched,and stable and structured than those of monoculture system.（5）Spatial patterns of soil nematodes between the bulk and rhizosphere soil was distinct.TNem and omnivore-predator abundance and the MI,SI,H’and SR values were greater in the rhizosphere soil compared with that of the bulk soil.Similarly,soil total sugar and organic acids differed significantly between bulk and rhizosphere soil with the higher values being recorded in the rhizosphere soil.In addition,higher values of SMC,SOC,TN,and MBC were found in the rhizosphere soil.The presence of more roots,litters,and root exudation in the rhizosphere soil may have resulted in the observed differences in the SOC and TN between bulk and rhizosphere soil,which may have also accounted for the differences in soil nematode community composition between the two soil compartments.Overall,maize/alfalfa intercropping significantly influenced the community structure of soil nematodes and their spatial patterning.Intercropping system under low soil N improved soil parameters by increasing SMC,SOC,and TN and this was closely related to enhanced nematode community structure（i.e.,increased TNem biomass and nematode metabolic footprints）and soil food web complexity.The higher composite metabolic footprint of nematodes in the intercropping system,which is the measure of the overall energy flow channeled by nematodes in the soil food web,indicate soil nematodes may have stored a high amount of soil carbon under these systems.Besides,the higher TNem biomass,microbial biomass C and N in the intercropped system,particularly in the absence of N addition,is an indication of increased nematode activity,carbon and energy flow in these systems.Therefore,this study provides a promising way to improve soil health and ecosystem stability,revealing that alfalfa intercropping in low-N soil can improve resource flow in the soil food web by enhancing TNem biomass and metabolic footprints of nematodes. |
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