Response Of Soil Nematode Community Structure To Chemical Nitrogen Fertilizers In Fertilized Microsites

Nematodes are the most abundant metazoans in the soil occurring at multiple trophic levels,and response sensitively to the environmental variation.Studied on soil nematodes not only can reveal crucial information of soil ecological structure and function,but also provide unique insight into many aspects of soil processes.In the farmland ecosystem,factors such as organic fertilizer,chemical fertilizer and pesticide can indirectly affect the soil nematode community by mediated the soil environment.Many studies have shown that soil disturbance caused by application of the chemical nitrogen(N)fertilizer would strongly affect the nematode communities.To understand the effect of the chemical nitrogen fertilizer application on nematode communities systematically,soil nematode responses to chemical nitrogen fertilizer were investigated at the centimeter scale from the horizontal(soil was separated into two parts by a mesh bag with the inner section applied with chemical nitrogen fertilizer and outer layer of no fertilizer)and vertical directions in the present study.Different forms of nitrogen fertilizer and application levels were compared to study their effect on soil nematode abundance,community structure and ecological indices,for ammonium sulfate,potassium nitrate and urea were studied from the horizontal direction whereas urea and nitrification inhibitor DMPP were studied from the vertical directions.The purpose of this research was to study the characteristics of nematode communities response to different forms of nitrogen fertilizer,and their migration and distribution in horizontal or vertical directions,thus to provide a scientific evidence for improving the soil quality and fertilization management in the agroecosystem.The main results were addressed as follows:1.Ammonium sulfate application decreased the abundance of total nematodes and bacterivores in the inner soil.Nematode abundance was significantly decreased after 7 days,and the reduction was correlated with increasing N application.Mesorhabditis was the most sensitive genus response to ammonium sulfate application,which was significantly decreased during the incubation.Application of ammonium sulfate decreased the relative abundance of bacterivores but increased that of omnivores/predators at 21 and 28 days.Moreover,ammonium sulfate decreased NCR values in the inner soil,suggesting a shift from bacterial decomposition pathway to fungal decomposition pathway Redundancy analysis(RDA)indicated that pH is the most important factor affected nematode composition,and decreased the number of Mesorhabditis.2.Application of potassium nitrate significantly decreased the abundance of total nematodes and bacterivores in the inner soil during the incubation period.The abundance of fungivores was significantly decreased during the first 7 days,and the reduction was correlated with increasing N application.In the late incubation period,NCR values were significantly decreased in the inner soil of potassium nitrate treatment,suggesting a shift from bacterial decomposition pathway to fungal decomposition pathway.RDA indicated that soil nematode community was significantly affected by soil pH and electrical conductivity,which was mediated by potassium nitrate.3.Urea application decreased the abundance of total nematode and bacterivores in the inner soil,and the significant difference was observed during the 7 to 21 days.High urea application amount decreased total nematode abundance most significantly compared with the other application amounts.Mesorhabditis abundance was significantly decreased by urea addition during the first 14 days.Urea addition decreased the bacterivore abundance,and the reduction was correlated with increasing N application.Nematode diversity and stability was lower in the median and high application amount compared with the low application amount.The RDA analysis revealed that pH mediated by urea had the largest influence on soil nematode community.Different forms of nitrogen fertilizer significantly decreased the nematode abundance in the inner soil after the incubation of 7 days.Among the three fertilization types,potassium nitrate had the strongest effect on nematode community structure,and high N application amount inhibited the growth of nematode more efficiently than the median and low application amount.Compared with the ammonium sulfate,urea at the low and median application amounts had the lowest effect on nematode community structure,whereas high urea application amount inhibited the growth of soil nematodes.The RDA analysis revealed that soil pH mediated by the three fertilization type had the strongest effect on nematode community structure,and correlated positively with the abundance of Mesorhabditis.Moreover,soil NO3--N content and electrical conductivity correlated negatively with most of the nematode genera in the three fertilization types.4.Application of urea decreased the abundance of total nematodes and bacterivores in the microscale of fertilization zone(1 cm).Nematodes are easy to migrate in the soil,while the application of urea inhibits the movement of nematodes,and the migration ability is damaged.Mesorhabditis was the dominant genera during the incubation period,and Aphelenchoides was the dominant genera from 7 to 21 days.Median and high fertilizer amounts favored the growth and reproduction of Aphelenchoides,Mesorhabditis and Aphelenchoides had strong immigration ability.5.DMPP application decreased total nematode abundance in the fertilization zone(1 cm)on the day of 7.During the day of 21 to 35,DMPP application had higher total nematode abundance compared with urea and control,which indicated that the DMPP was beneficial to the soil organism.Total nematode abundance in the DMPP treatment was increasedin the 1-2 cm microscale during the later culture period,suggesting that DMPP stimulated the reproduction of soil nematodes.Urea treatment had lower total nematode abundance than the control treatment;DMPP application increased the relative abundance of bacterivore nematodes;and urea increased the relative abundance of fungivore nematodes.During the 21-35 days,the relative abundance of fungivore nematodes was decreased with the increasing distance to fertilization zone,which could be related to the concentration of NO3--N and NH4+-N.Urea and DMPP treatments decreased the relative abundance of herbivore nematodes from 1 to 5 cm.