Direct and Indirect Effects on Soil Nematode Communities Differ Between Facilitative and Allelopathic Plants

Plants are expected to affect soil nematode communities. However, comparative studies on the direct and indirect ways dominant plants influence soil nematode communities are rare. In this study, we compared the effects of a dominant allelopathic plant, Ligularia virgaurea, and a dominant facilitative plant, Dasiphora fruticosa, on soil nematode richness and community composition in an alpine meadow of the Tibetan plateau. Our result indicated that 1) D. fruticosa significantly increased nematode richness whereas L. virgaurea had no significant effect; 2) D. fruticosa had no significant impact on bacterial and fungal richness, but L. virgaurea increased fungal richness; 3) D. fruticosa had strong positive direct, and weak positive indirect, effects on nematode richness, mainly mediated by a marginal decrease in fungal richness. By contrast, L. virgaurea had no significant direct effect on soil nematode richness but had strong indirect effects, mainly mediated by changes in soil pH and soil organic carbon content; 4) L. virgaurea influenced soil nematode community composition predominantly through direct effects but also indirectly through soil organic carbon. By contrast, D. fruticosa affected nematode communities through changes in understory plant communities, soil physiochemical, and microbial communities. Both facilitative and allelopathic plants thus influence soil nematode richness and community composition but seemingly in different ways. These highlight the importance of plants in determining soil community diversity and provide new insight to disentangle the complex above- and belowground linkages.

Use of universal primers for the 18S ribosomal RNA gene and whole soil DNAs to reveal the taxonomic structures of soil nematodes by high-throughput amplicon sequencing

Nematodes are abundant metazoans that play crucial roles in nutrient recycle in the pedosphere. Although high-throughput amplicon sequencing is a powerful tool for the taxonomic profiling of soil nematodes, polymerase chain reaction (PCR) primers for amplification of the 18S ribosomal RNA (SSU) gene and preparation of template DNAs have not been sufficiently evaluated. We investigated nematode community structure in copse soil using four nematode-specific (regions 1–4) and two universal (regions U1 and U2) primer sets for the SSU gene regions with two DNAs prepared from copse-derived mixed nematodes and whole soil. The major nematode-derived sequence variants (SVs) identified in each region was detected in both template DNAs. Order level taxonomy and feeding type of identified nematode-derived SVs were distantly related between the two DNA preparations, and the region U2 was closely related to region 4 in the non-metric multidimensional scaling (NMDS) based on Bray-Curtis dissimilarity. Thus, the universal primers for region U2 could be used to analyze soil nematode communities. We further applied this method to analyze the nematodes living in two sampling sites of a sweet potato-cultivated field, where the plants were differently growing. The structure of nematode-derived SVs from the two sites was distantly related in the principal coordinate analysis (PCoA) with weighted unifrac distances, suggesting their distinct soil environments. The resultant ecophysiological status of the nematode communities in the copse and field on the basis of feeding behavior and maturity indices was fairly consistent with those of the copse- and the cultivated house garden-derived nematodes in prior studies. These findings will be useful for the DNA metabarcoding of soil eukaryotes, including nematodes, using soil DNAs.

Soil Microbial and Nematode Community Response to the Field Application of Recycled Bio-Based Fertilisers in Irish Grassland

Phosphorus (P) is an essential plant nutrient routinely applied to soils as an agricultural fertiliser, frequently in non-renewable, inorganic forms. Finite reserves and growing demand for agricultural phosphorus mean alternative P resources need to be explored. Recycling-derived fertilisers (RDF) recovered from specific waste streams, using nutrient recovery technologies, have the potential to replace conventional phosphorus fertilisers used in agriculture. Healthy functioning soil microbial and nematode communities are essential players in maintaining soil health and nutrient status. Thus, it is important to assess the responses of these communities to RDF application. We compared soil microbial and nematode communities of conventional fertiliser and RDF treated soil, in the form of struvite and ash, using next-generation sequencing (NGS) technologies in a phosphate-fertiliser replacement value (P-FRV) field trial. Bacterial and nematode communities displayed significant changes under the different P fertilisation treatments, while fungal communities were relatively unaffected. Bacterial diversity was higher among RDF treatments than conventional treatments, while nematode diversity was reduced by one ash treatment. Available potassium and phosphate were the main drivers of bacterial community changes when analysed by canonical correspondence analysis (CCA), while available phosphate alone was the driver of nematode community shifts. Of the RDF, struvite products yielded the highest crop biomass, maintained microbial diversity and were associated with the least disturbed nematode communities.