Effect Of Herbivory On The Priming Effect In Urban Forest

The rapid urbanization process makes extreme weather events occur frequently,which affects the generation and development of insect pests.Insect pests affect about one tenth of the total forest area in the world and are an important disturbing factor affecting the carbon cycle of terrestrial ecosystems.They can change the chemical properties of imported litter by canopy feeding,or affect the release of plant root exudation by changing plant photosynthetic rate and release of secondary metabolites,so as to have an important impact on forest soil carbon cycle.Soil priming effect(PE)refers to the phenomenon that the turnover of soil organic matter(SOM)changes after the input of exogenous carbon(such as plant litter and root exudation),which is an important process to regulate the carbon storage of terrestrial ecosystems.At present,although it has been reported that insect pests can stimulate soil microbial respiration and improve soil respiration rate,there are still few studies on the effects of insect pests on soil respiration,let alone the report on the response of PE to insect pests,which limits our understanding of soil carbon cycling process.Therefore,two experiments were designed in this study.One was to add litter of Pinus massoniana,damaged leaves and frass of Dendrolimus in the laboratory to study the influence of residues and frass caused by canopy feeding of pests on PE.The second is to simulate the effects of different degrees of Dendrolimus feeding on the root exudation release rate and rhizosphere priming effect(RPE).The main results are as follows:(1)The results indicated that the addition of leaf litter,damaged leaves,and frass significantly increased native soil organic carbon mineralization,producing a positive priming effect.Moreover,significant differences were observed between treatments.The accumulative priming effect induced by frass was the largest,followed by damaged leaves,and that of leaf litter was the smallest.Linear regression analysis revealed that the priming effect was positively correlated with total P,condensed tannin,total phenolic content,and the ratio of condensed tannin to P(condensed tannin/P),whereas the priming effect was negatively correlated with C/N,lignin/N,C/P,and lignin/P in the early stage of incubation.We observed a significant negative correlation between the priming effect and lignin content in the later stage of incubation.(2)Different degree of insect infestation significantly affected the intensity the rate of root exudation release and RPE.At the early stage of the experiment,there was no significant difference in the release rate of root exudation among different herbivory treatments,and at the late stage of the experiment,no herbivory treatment was significantly higher than other treatments.With the extension of test time,the direction of RPE changed from positive priming to negative priming for 100% herbivory treatment,and from positive to negative for all other treatments.At the early stage of the experiment,the RPE of 100%herbivory treatment was significantly higher than that of other treatments,and at the late stage,the RPE of 100% herbivory treatment was the lowest.In addition,RPE was positively correlated with the root exudation release rate.In conclusion,damaged leaves and frass by Dendrolimus increased the magnitude of the positive priming effect,which was influenced by different factors at different incubation stages.It would strengthen the understanding in the effects of insect canopy herbivory on the carbon cycle of forest soils,and improve the accuracy of the assessment of its effects on the forest ecosystem carbon sink.Different degrees of pest treatment of Dendrolimus will help to further understand the driving role of pest-induced root exudation in RPE,and provide data support for soil carbon pool management of Pinus massoniana forest.