Soil nematode community and soil properties in pear orchards as affected by frequent applications of pesticides

Abstract：【Objective】Pear is a fruit with high economic value, but diseases and insect pests always re-strict the development of the pear industry, the dominant pests are Psylla chinensis, mites, and aphids inthe northern pear orchard. When Psylla chinensis seriously occurs in pear orchards, farmers will spraypesticides more than 10 times. The technique of applying pesticides for 3 times is more popular for effec-tively controlling the main pests in northern orchards. Nematodes are ideal bioindicators for terrestrial eco-systems and their community structure can provide important insights regarding many aspects of the eco-system function. In addition, they are food-specific, and sensitive especially in their response to anthropo-genic disturbances and management changes, such as pollution, and agricultural management. So the ef-fects of pesticides with applications of only 3 times or many times on nematode communities and soil properties were examined to evaluate the soil pollution caused by overuse of pesticides in pear orchards.【Methods】Ten cores of samples were collected from the soil surface layer (0-20 cm) and sub-surface layer (>20-40 cm) , respectively. Nematodes were extracted from 100 g of fresh soil from each sample by us-ing the shallow dish method and were identified at the genus level by using a Leica microscope. Nema-todes were classified according to their feeding behavior as bacterivores, fungivores, herbivores, omni-vores or predators. The nematode community, using indices of biodiversity and ecological function in thesoil at 0-20 cm and >20-40 cm in depth in applications of many times (M) and only 3 times of pesticides (T) in pear orchards, were observed respectively.【Results】It is clear that the two management methodshad different impacts on all soil properties. Soil samples were alkaline in both soil layers through the twomanagement methods, with p H values between 7.90 and 8.27. The surface soil p H value in pesticides for3 times per orchard was significantly lower than applications of many times of pesticides per orchard (P<0.05) , and the content of available potassium (129.2 mg·kg^-1) increased significantly. While in the sub-surface soil, the content of ammonium nitrogen and organic matter decreased. The total number of nema-todes in applictionx of many times pesticides per orchard was significantly lower than pesticides for appli-cations of 3 times at the two soil layers. We identified seventeen nematode genera in the two managementmethods, and observed that the nematode community was significantly affected by the frequency of pesti-cide applications. Plectus and Rhabditis had strong pesticide tolerances, but Tylenchorhynchus respond-ed quickly to overuse of pesticides. At the depth of 0-20 cm, Plectus was eudominant genera in applic-tions of the many times pesticides orchards, but not found in applications of pesticides for 3 times; Tylen-chorhynchus was the dominant genera in M and eudominant in T. At the depth of >20-40 cm, Rhabditiswas the dominant genera in M and decreased to subdominance in T; Tylenchorhynchus had the samechange at 0-20 cm. There were no significant differences in the percentage of bacterivores and fungivoresbetween the two management methods at the two soil layers. The percentage of omniovores/predators inshallow soil and the herbivorous nematodes in the sub-surface soil with applications of pesticides for 3times in orchards were remarkably higher than in applications of the pesticides in the many times orchards.According to the analysis of ecological indices, at the soil layer of 0-20 cm, the nematode channel ratioand the indices of trophic diversity (TD) , species richness (SR) and maturity (MI) had no significant differ-ences between M and T. However, significant differences were observed in the indices (TD, SR and MI) be-tween the different management methods in the sub-surface soil. The indices of TD, SR and MI in applica-tions of pesticides for 3 times in orchards were significantly higher than in applications of pesticides formany times. The soil food webs of the tea plantation ecosystems were plotted along their respective enrich-ment index (SI) and structure index (EI) trajectories. For applications of many times and 3 times of pesticides, no significant differences were found in the nematode faunal profile, with surface layers located inquadrant D and sub-surface layers in quadrant A.【Conclusion】In applications of pesticides for 3 times inorchards, the soil nematode communities have been changed, the species richness of the soil nematodesand trophic diversity have been increased. Moreover, applications of pesticides for 3 times made the soilin the pear orchards tend to maturity. Therefore, the key genus and ecological function indices of the soilnematodes are good bioindicators for evaluating the soil pollution caused by overuse of pesticides.