Effects Of Fertilisation On Seasonal Dynamics Of Soil Nematodes Under Manchurian Ash And Davurian Larch Plantations

Soil nematodes play an important role in carbon and nutrient cycles in soil ecosystem. Total nematodes populations and trophic groups seasonal dynamics,soil layer distribution,and as well as the effects of nitrogen fertilisation on soil nematodes were investigated in ash (Fraxinus mandshurica) and larch(Larix gmelinii) plantations,which grow in the similar sites, during 2007 and 2008.The soil nematodes were collected from soil following Sieving-centrifugation -flotation method.The results showed that:The densities of soil nematodes and tropihc groups in ash were significatantly higher than that in larch(P＜0.05).Averaged two years estimates,730 to 1341 inds/100 g dry soil in ash, and 321 to 505 ind/100 g dry soil in larch.Among trophic groups,there were 629 to 1056(ash) and 266 to 353(larch),67 to 188(ash) and 41 to 107(larch),34 to 97(ash) and 16 to 45(larch) inds/100 g dry soil for plant-feeding nematodes(PF),bacterivores(BA) and omnivores-predators (OM-PR),respectively.The reasons may be the different humus types,root exudates and microorganism biomass carbon and notrigen in the two plantations.However,the majority of soil nematodes among trophic groups in two plantations were PF(78.7%to 86.2%in ash and 69.9%to 82.7%in larch),the secondary was BA(9.3%to 14.0%in ash and 12.9%to 21.1%in larch)and the minimum was OM-PR(4.2%to 7.3%in ash and 4.9%to 8.9%in larch).These results were caused by much more roots,to which PF feeding,in the two plantations.Across season during the two years,the maximum population of total nematodes and trophic groups occurred in autumn in ash and larch plantations,while the minimum were in summer or spring.These would attribute to seasonal dynamics of fine root biomass,fine root tissue nitrogen concentration and microorganism biomass.The densities of total nematodes and trophic groups were higher in topsoil and lower in deep soil,which significantly decreased from topsoil to deep soil across two years investigations.Forest soil physicochemical properties and fine roots biomass vertical distribution patterns were the major factors.The effects of N fertilisation on soil nematodes trophic groups were different.OM-PR decreased significantly 54.7%(ash) and 41.4%(larch)(P＜0.05).The reasons maybe ascribe to root biomass,bacterial biomass C and N and soil pH reduced,all of which were result from N fertilisation in the two plantations.In summary,soil nematodes in decidous forest are higher than that in coniferous,the majority of which are plant-feeding nematodes,soil nematodes trophic groups show significant seasonal dynamics and soil layer distribution patterns,individuals of omnivres-predators decreased result from N fertilisation.These results demonstrate that responses of soil nematodes could play an indicatory role in forest for environment change,and could provide scientific fundamental data linkage fine root growth and mortality.