| At present,there are few studies on the effects of litter removal andaddition on soil nutrients and soil microbial community structure and function in planted forests.In particular,there is a lack of comparative studies on over-growth pure planted forests and mixed forests,which need to be further discussed.This study used a 60-years old Pinus massoniana pure plantation and its un-even mixed plantations with Castanopsis hystrix as study objects,and used the field investigation,biochemical analysis and comprehensive comparison methods to explore the effects of litter removal and addition on soil nutrients and soil microbial community structure and function.Our results revealed the different response of the pure and mixed plantations to the litter removal and addition.The results are of great significance for the near naturalization and adaptive management of P.massoniana planted forest.The main results are as follows.(1)When litter was treated for half a year(dry season),litter removal and addition significantly increased soil organic carbon content in the pure plantations by 34.19%and 29.37%,respectively(p<0.05),while mixed plantations showed a decrease in soil organic carbon by 14.07%and 10.67%,respectively(p>0.05).After one year of litter treatment(wet season),the soil organic carbon content in the pure plantations decreased by 7.37%(p>0.05)after litter removal,while that of mixed plantations decreased significantly(19.40%)(p<0.01).The addition of litter significantly increased the soil organic carbon content of pure and mixed plantations by 24.8%and 17.74%respectively(p<0.01).Three-way ANOVA showed that different treatments,seasons,stands and their interactions had significant or extremely significant effects on soil organic carbon,total nitrogen,total phosphorus,total potassium,available nitrogen,available phosphorus and available potassium.(2)In the dry season,in pure plantations,litter addition significantly reduced the amount of soil bacteria and total PLFAs,but significantly increased the amount of fungal PLFAs(p<0.05),while the removal of litter had no significant effect on the amount of PLFAs of various groups(p>0.05).In the mixed plantations,the amount of fungi,actinomycetes and total PLFAs was significantly increased by litter removal and addition,and the amount of bacterial PLFAs was significantly increased by adding litter(p<0.05).In the wet season,litter removal and addition had no significant effect on soil bacteria,fungi,actinomycetes,arbuscular mycorrhizal fungi and total PLFAs in the two stands(p>0.05).Three-way ANOVA showed that the amount of soil bacteria,fungi,actinomycetes,arbuscular mycorrhizal fungi and total PLFAs in the control,addition and removal of litter in the two stands were significantly higher in the dry season than in the wet season(p<0.01),and that in the mixed plantation was significantly higher than that in the pure forest(p<0.05).(3)In dry season and in pure plantations,litter addition can significantly improve Simpson index and Pielou's evenness index of soil microbial PLFAs.In mixed plantations,litter treatment had no significant effect on Simpson index and Pielou's evenness index.In wet season,no matter pure plantations or mixed plantations,litter removal and addition had no significant effect on Simpson index and Pielou's evenness index(p>0.05).In both stands,no matter in dry season or wet season,litter treatment has no significant influence on soil microbial Biolog diversity index(p>0.05).Three-way ANOVA of variance showed that the interaction of stands in different seasons had no significant influence on the diversity of PLFA and Biolog.(4)In the dry season,litter addition of increased the activity of the?-1,4-glucosidase and phenolic oxidase in the soil of the two stands by 5.57%,26.98%,28.18%and 72.42%,respectively(p<0.05),while the activity of N-acetyl-?-glucosidase decreased by 9.23%and 17.23%,respectively.Removal of litters significantly increased the activities of-1,4-glucosidase,acid phosphatase and peroxidase in pure forest soil by 15.15%,20.96%and 10.06%(p<0.05),but reduced the activities of polyphenol oxidase by 31.97%.Similarly,removal of litter significantly increased the activities of N-acetyl?-glucosaminase,P-1,4-glucosidase,acid phosphatase,phenol oxidase and urease in mixed forest soil by 50.46%,38.62%,26.28%,111.52%and 12.84%,while the activities of peroxidase significantly decreased by 32.34%.In wet season,the addition of litter significantly increased the activity of N-acetyl-?-glucosidase in pure forest soil by 25.28%,but reduced the activity of soil?-1,4-glucosidase by 12.78%(p<0.05).The activities of ?-1,4-glucosidase and acid phosphatase in pure forest soil were significantly reduced by 15.18%and 19.40%after removal of litter.In the wet season,the addition of litter significantly increased the activities of N-acetyl-?-aminoglucosidase,acid phosphatase,phenol oxidase,peroxidase and urease in the mixed forest soil,respectively,by 13.40%,21.20%,33.49%,36.20%and 17.95%(p<0.05).The removal of litter significantly reduced the activities of N-acetyl-?-glucosaminase,acid phosphatase and urease in mixed forest soil by 20.06%,20.75%and 17.15%,respectively(p<0.05).In this study,it was also found that after one year of litter treatment,soil microorganisms mainly used carbon sources to change from acids to lipids and acids.(5)Correlation analysis showed that in addition to the soil effective nitrogen and total phosphorus,soil organic carbon,total nitrogen,available potassium,phosphorus,potassium and total potassium bacteria,fungi,actinomycetes,arbuscular mycorrhizal fungi,gram-positive bacteria and gram-negative bacteria and total PLFA community were significantly or extremely significantly negatively correlated(p<0.05 or p<0.01),while soil total potassium,effective phosphorus,available nitrogen,total nitrogen,total phosphorus,effective phosphorus and the content of the 7 kinds of bacteria and total PLFA were significantly or extremely significantly positive correlation(p<0.05 or p<0.01).Soil organic carbon,total nitrogen,available potassium,and available potassium:total potassium was positively correlated with N-acetyl-(3-glucosidase and urease activity,but negatively correlated with?-1,4-glucosidase activity.On the contrary,available P,available N:total N and available P:total P were negatively correlated with N-acetyl-?-glucosidase and urease activity,but positively correlated with?-1,4-glucosidase activity.Soil organic carbon,total nitrogen,available potassium,available potassium:total potassium showed significant or extremely significant negative correlation with 5 kinds of carbon sources(except alcohols),while total potassium,available phosphorus,available nitrogen:total nitrogen,available phosphorus:total phosphorus showed significant or extremely significant positive correlation with the 5 kinds of carbon sources(except alcohols).(6)The results showed that there were significant differences in the short-term responses of soil nutrients,soil microbial community structure and function to litter removal and addition in the pure and mixed plantations,which supported our hypothesis.This phenomenon is closely related to vegetation type,climate,soil and human activities.Field investigation showed that the understory vegetation of pure plantations were rich in diversity and deep litter,while the mixed plantations had high canopy density,understory shrubs,herbs and their scarcity,and litter was deep,but its structure was more complex.The effect of litter removal and addition on pure and mixed plantations cannot be generalized,but there is complexity and variability,which is determined by the complexity of forest biodiversity and its relationship with soil microorganisms. |
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