Responses Of Soil Nematodes To Plant Community Changes And Above/belowground Resource Inputs In Ailao Mountains, Yunnan

Over the last century the vegetation has changed dramatically in the world. The aboveground vegetation changes could influence belowground biotas which in turn affect the soil ecosystem function and the plant growth. However, the impact of plant community changes on soil biodiversity and the underlying mechanisms are still not fully understood. Among soil biota, nematodes are the most abundant metazoan taxon, closely linked with plants and soil ecological processes, and possess attributes that are useful to reflect the consequential changes belowground in the course of plant community changes. Ailao Mountains in Yunnan Province possess a large subtropical evergreen broad-leaved forest in China. Since 1950, part of the virgin forests of Ailao Mountains has been replaced by secondary forests and shrub-grassland under human disturbance. In this study, the responses of soil nematode community to plant community changes due to human disturbance and natural succession were studied. To understand the mechanisms of how plants affect soil biota from a plant physiological perspective, litter exclusion, girdling and root trenching experiments were carried out to investigate the impacts of above/belowground resource inputs on soil nematode communities. In addition, four new species of soil free-living nematodes from Ailao Mountains were recorded.To understand the responses of soil biota to vegetation restorations in ever-green broad-leaved forests, the soil nematode communities were studied in shrub-grasslands, secondary forests and virgin forests along a plant secondary succession continuum. Total nematode density did not change significantly during the plant successional processes. The total nematode diversity, Plant Parasite Index (PPI), relative abundance of herbivores and bacterivores were significantly different among shrub-grasslands, secondary forests and virgin forests. Shrub-grassland had the lowest nematode diversity, smallest bacterivores proportions, highest PPI value and largest herbivore proportions. Nematode generic richness, Maturity Index (MI), trophic diversity and relative abundance of fungivores were lowest in shrub-grassland, but the differences between secondary forests and virgin forests were not significant. The differences in soil nematode community structures were greatest between the shrub-grasslands and the virgin forests, and smallest between the secondary forests and the virgin forests. There were significant differences in soil grain structure, pH, organic matter and total nitrogen between shrub-grassland and virgin forest, but no significant differences between secondary forest and virgin forest were detected. These results suggest that the soil physiochemical characteristics have recovered when secondary forest stage is reached; however, the recovery of soil biota such as nematodes is relatively slow.To assess the effect of orginal vegetation type on soil nematode communities during the land use change processes, a study were carried out along two transects where contrasting natural vegetation types (grass versus shrub) were replaced by tea monocultures. Soil nematode communities were compared between two transects at three chronosequence stages: natural vegetations, 3-year-old tea plantations and 7-year-old tea plantations. Nematode density, generic richness, diversity, and relative abundance of bacterivores and fungivores were higher at each stage of GT transect (from grassland to tea plantation) than at the equivalent stage of ST transect (from shrubland to tea plantation), whereas lower values of Maturity index and Nematode Channel Ratio occurred at each stage of GT transect relative to ST transect. The difference in nematode community structure was greatest between the two 3-year-old tea plantations, although the greatest difference in environmental conditions was observed between two original vegetation habitats. These results suggest that the original vegetation types may affect the following nematode community development, but the effects can decrease over time.To understand the impact of plant canopy photosynthate inputs on soil nematode communities, a large-scale tree girdling experiment was conducted in a evergreen broad-leaved forest of Ailao Mountains. The tree girdling treatment significantly reduced total nematode densities in both mineral and humus soil layers and fungal feeding nematode densities in the humus layer. It shows that the termination of belowground photosynthate-C allocation achieved by tree girdling can affect soil nematodes. There were no differences in bacterivore, herbivore and omnivore-predator densities between the girdled and control plots in both soil layers, which suggests that different trophic groups vary in their responses to girdling treatment.To examine the effects of plant roots on soil biota, a trenching experiment was conducted to study the impacts of root exclusion on soil microbial biomass and nematode communities. There were no significant differences in soil bacterial and fungal biomass, total nematode density and diversity, herbivore density, bacterivore density and omnivore-predator densities between the trenched and control plots in both mineral and humus soil layers. Trenching treatments significantly reduced fungivore density and diversity and enhanced nematode channel ratio in both soil layers. It provides direct evidences that eliminating the interactions between plant and soil achieved by root trenching can affect fungal feeding nematodes and result in a declined energy flow through fungal based pathways.A litter exclusion experiment was carried out to determine the effect of aboveground resource inputs from plants on soil microbial biomass and nematode communities. The litter exclusion treatment enhanced omnivore-predator diversity but did not affect the density and diversity of total nematodes and other trophic groups. This study suggests that the effects of aboveground resource inputs on soil nematodes in evergreen broad-leaved forests could be relatively weak.Tylencholaimus orientalis sp. n., Tylencholaimus sinensis sp. n., Proleptonchus sinensis sp. n. and Dorylaimoides alpinus sp. n. collected from soils of Ailao Mountains are reported as new to science.