Community structure and ecological niches of free-living terrestrial nematodes in north-eastern Vietnam.

Nematodes are highly abundant and globally the most diverse group of soil fauna. Due to the lack of taxonomists, only a handful of studies exist on communities of free-living nematodes in tropical areas worldwide. The true species richness and functional composition of nematode communities in Vietnam are virtually unknown. Changes in soil functioning affect nematode community composition long before adverse effects on chemical and physical soil parameters are measurable. Consequently, nematodes are excellent indicators of soil function and ecosystem services. We investigated the nematode fauna in soils of three Nature Reserves in Cao Bang, Lang Son and Cat Ba provinces in north-eastern Vietnam in May, 2013. First, we identified the nematodes (if possible) until species level to estimate the numbers and diversity of known and undescribed taxa. Then, based on the different trophic groups, nematode functional guilds were determined. These data were used to calculate different ecological indices in order to describe how changes in land use affected the functioning of soils in a gradient from primary forest, secondary forests, slash and burn agriculture and intensive agricultural management. In total, more than 14,000 specimens were identified. These nematodes belonged to 105 genera, 42 families and 9 orders, of which 48 genera and 13 families belonged solely to the order Dorylaimida. The Order Dorylaimida is taxonomically particularly challenging because it is highly diverse, but species show only little differences in morphological characters. More than 1/3 of the identified species of Dorylaims were new to science. Twelve new species were described comprising three species of the genus Aporcelaimoides: A. brevistylum, A. minor, A. silvaticum; three species of the genus Aporcelinus: A. falcicaudatus, A. paramamillatus, A. paraseychellensis, and six species of the genus Sectonema: S. birrucephalum, S. buccociliatum, S. caobangense, S. ciliatum, S. tropicum and S. vietnamense. Three species were recorded for the first time from Vietnam: Axonchium thoubalicum, Belondira murtazai and Oxybelondira paraperplexa. One species Cephalobus topali was re-described and transferred to the genus Acrobeloides. Also molecular data confirmed that the diversity of dorylaims is little known, only 5 sequences obtained from Genbank could be assigned to the sequenced nematodes with a similarity higher than 95%. The nematode assemblages and diversity were significantly different between different regions and among four land use intensities. Nematode assemblages in primary forests were composed mainly of large and long-lived predators and omnivores (46-74%), compared to a successive increase of short-lived bacterivores (5-14%), plant parasites (11-24%) and fungivores (3-15%) with land-use intensification. Our data indicated that nematode community analysis was a useful tool to predict functional changes in soil ecosystems. Different ecological indices (i.e. the Maturity Index (MI), the Plant-Parasite Index (PPI), the Basal Index, Enrichment index, Structure index and Channel Index) have been developed in temperate ecosystems to describe shifts in soil functioning and ecosystem services based on changes in the composition of nematode trophic groups. We tested whether this framework could be applied to tropical soils. In particular the Maturity indices were hardly applicable in tropical soils. Further it turned out that comparable change in land use led to very different transformations of soil conditions. For example the soil C/N ratios did not change as expected with increasing land-use and were not correlated with the composition of the soil microflora. Overall, our studies substantially contribute to our understanding of the functioning and the causes of degradation of tropical soils and to the knowledge of the biodiversity of free-living nematodes. This study found clear evidence corroborating the idea that the diversity and species richness of nematodes from tropical regions is higher than temperate regions. In conclusions, my studies of free-living nematodes communities of Vietnam showed: 1) The diversity of nematodes in tropical soils has been vastly underestimated; 2) Land-use intensity had strong impacts on nematode communities; 3) Changes in nematode communities along gradients of within increasing land use intensity could not be simply explained by abiotic soil conditions.