|dc.description.abstract||The intensification of pastoral land use has increased with the use of fertilisers and the selection of highly productive pasture plant species to increase pasture production. When choosing a ryegrass cultivar infected with selected endophyte fungus, ploidy choices are vital for determining pasture production. Modern perennial ryegrass cultivars and White clover plant species mixtures and nitrogen fertiliser with seasonal climatic characteristics can influence the soil biodiversity and soil functions in a pastoral system. Pasture soils have unique systems with different soil faunal and microbial communities, including soil-dwelling nematodes and plant species composition. Nematodes have the potential to be indicators of soil processes and condition because of their abundance and ubiquity; they are simple multicellular animals, and highly diverse, taxonomically or functionally or both. They occupy multiple trophic positions in the soil food web and play a significant role in the decomposition of soil organic matter, the mineralisation of plant nutrients and nutrient cycling. The distribution of the nematode community can be influenced directly and indirectly by various management activities, including nitrogen fertiliser application and plant species, and the changing climatic conditions during seasons. The magnitude of these biotic and abiotic impacts was studied through analysis of the nematode community. Our study examined the impact on the nematode community of combinations of low and high rates of nitrogen application, clover presence and absence, and two different ranges of ryegrass cultivars, One50 (diploid AR37) and Base (tetraploid AR37), during two seasons. Sampling was carried out twice a year (autumn and spring) for two years (2013 and 2014) to investigate the distribution pattern between the seasons when host plants are actively growing and nematode activity is high. Soil samples were collected in the North Island of New Zealand from two sites in the Waikato and the Manawatu. Soil nematode assemblages are different between these chosen sites. Soil nematodes were extracted from soil samples, identified, enumerated, and assigned to the total nematode population, the plant parasitic group and trophic groups.
A total of 24 taxa of nematodes were found in this study, comprising 9 plant feeders, 3 fungal feeders, 8 bacterial feeders, 3 predators, and 1 omnivore. However, the range of taxon richness per sample collection period was 12 to 14. Overall, plant feeders (Tylenchids, Paratylenchus, Pratylenchus), fungal feeders (Aphelenchus, Aphelenchoides), and bacterial feeders (Cephalobids and Panagrolaimus) were the most common nematodes found in both regions. Meloidogyne abundance in the Waikato site was relatively high compared with the Manawatu. In contrast, Longidorous and Diplogaster were more abundant in Manawatu than Waikato. In both study sites, predators and omnivores were relatively low compared with plant, fungal and bacterial feeders; however, c-p 1 taxon of Panagrolaimdae and Rhabditide are most common in autumn.
Significant differences in the nematode community were observed with substantial impact changes (season), especially in the total nematode and Maturity Index (MI) measures. Interestingly, a relatively small difference in nitrogen fertiliser (N) input (in on-farm terms) resulted in a consistent, although not significant, decrease in MI at a high rate of N application. This reflects a proportional increase in resource opportunist nematodes (c-p 1 and 2), probably in response to the pulse of microbial growth post-fertilisation. Ryegrass cultivar/ploidy impacts were observed on the plant-feeding component of the nematode community (Plant Parasitic Index (PPI)). An interesting seasonal contrast between MI (greatest in spring) and PPI (greatest in autumn) shows how these two components of the nematode community respond to food resources and seasonal conditions with PPI relying directly on plant growth. We have used a range of different statistical properties to interpret the complex datasets generated from the soil community and environmental interactions. The analysis approaches taken here is often used for analysis of among-species relationships, but are also increasingly used for functional interpretation of ecosystem networks, including those generated by molecular approaches.
We conclude that the addition of even moderate rates of N to the soil has implications for soil functioning and sustainable use given that a more structured(i.e., greater MI) nematode community is a desirable indicator of soil health. The ability to graphically represent the range of the strength of interactions amongst the members of the nematode community adds new insights to these datasets and facilitates bringing together the results of morphological and molecular studies within a common framework of representation and hence understanding the function of ecosystem networks.||