| The problems of poor ecological stability,declining soil fertility and low productivity in artificial pure forests in China are becoming more and more prominent.Building mixed forest and adjusting the stand structure of pure forest are considered as effective ways to solve these problems.However,the research on the theory and technology of mixed forest construction is still relatively weak,and it is not clear whether the construction of mixed coniferous and broad-leaved forest can solve the fundamental problem of soil fertility decline in the nutrient cycle of plantation.The Loess Plateau occupies an important position in the construction of ecological civilization and the strategy of"double carbon".The widely distributed Robinia pseudoacacia plantation plays an important role in vegetation restoration and ecological environment improvement.However,the effect of litter decomposition on soil fertility restoration is still unclear,and there are few studies on the decomposition mechanism of litter by microorganisms.Based on the litter decomposition and nutrient return of mixed forest,this study selected three common stands（pure Robinia pseudoacacia forest,pure Platycladus orientalis forest,mixed forest of Robinia pseudoacacia and Platycladus orientalis）in the gully region of the Loess Plateau.By studying the dynamic changes of litter accumulation and litterfall,the litter storage condition in the three stands was clarified,and the litter decomposition characteristics of three kinds of stands were studied by in-situ decomposition experiment.With the high-throughput sequencing technology,the change characteristics of litter microorganisms were explored.Under four treatments（no microorganisms,only litter microorganisms,only soil microorganisms,litter and soil microorganisms）,the contribution of litter and soil microorganisms on litter decomposition was evaluated.By measuring nutrient content of litter and soil,the ability of nutrient release and return of litter in pure forest and mixed forest and their effects on soil nutrients were compared.The main conclusions are as follows:1.The dynamic changes of litterfall and litter decomposition rateThere were significant differences in the litter accumulation and litterfall amount among the three stands,and the seasonal dynamics of leaf,branch,and fruit litter in each component showed a"single peak",and the highest peak appeared in October.The highest peak of flower was different due to the biological characteristics,and the proportion of leaf litter in each component was the largest.The annual litter accumulation of the three stands was as follows:Platycladus orientalis(677.56 g m-2)>Robinia pseudoacacia(640.99 g m-2)>mixed forest(606.45 g m-2),while the annual litterfall was Robinia pseudoacacia(840.13 g m-2)>mixed forest(723.06 g m-2)>Platycladus orientalis(618.52 g m-2).The litter of Platycladus orientalis forest accumulated year by year because the annual litter accumulation was larger than that of litterfall,while the litter of Robinia pseudoacacia forest and mixed forest was the opposite.In the process of decomposition,the litter decomposition rate of the three stands increased rapidly in the early stage and increased slowly in the later stage,and the decomposition rate was mainly affected by the contents of lignocellulose,total carbon,carbon nitrogen ratio（C/N）,lignin/N,total calcium,total manganese and total magnesium.The high spatial heterogeneity and rich nutrients in the litter of the mixed forest led to the highest decomposition rate,while the higher contents of total carbon,total phenol,lignin and lignin/N led to the lowest decomposition rate in the litter of Platycladus orientalis.The nutrient cycling of mixed forest was faster than that of Robinia pseudoacacia and Platycladus orientalis pure forest.Olson model predicted that it takes 5.4 years for mixed forest,5.9 years for Robinia pseudoacacia forest and 6.3 years for Platycladus orientalis.2.Characteristics of main organic matter,enzyme activity and microorganism during litter decompositionLignocellulose content,enzyme activity and microbial community composition and diversity were significantly affected by stand type and decomposition time,in which enzyme activity and fungal diversity increased at first and then decreased at 0-540 days.In the initial decomposition stage,the diversity of bacteria and fungi in the litter of the mixed forest increased gradually,and the enzyme activities of cellulose and hemicellulose were higher,which promoted the decomposition of cellulose and hemicellulose.After 270 days,the species and number of bacteria and fungi in the mixed forest were higher than those in the pure forest of Robinia pseudoacacia and Platycladus orientalis,and the activities of endoglucanase,exoglucanase and xylanase in the mixed forest were also higher than those in the pure forest of Robinia pseudoacacia and Platycladus orientalis.In the three stands,in the early stage of decomposition（0-90 days）,bacteria were mainly Sphingomonas,Hymenobacter,Pseudomonas,Pedobacter and Spirosoma,and the fungi involved in the decomposition of cellulose and hemicellulose were mainly Nothophoma,Cladosporium,Septoria and Alfaria.In the later stage of decomposition,the bacteria were mainly Amycolatopsis and Streptomyces,and the fungi involved in lignin decomposition were mainly Helicodendron,Lecanicillium,Chalara and Trechispora.3.Decomposition of litter by microorganisms.Microorganisms not only directly affect the decomposition rate of litter,but also indirectly affect the litter decomposition rate by affecting the activity of related enzymes firstly then affecting organic matter.Among the three stands,the contribution of microorganisms to litter decomposition rate was positive,and the order from large to small was the group of litter and soil microbial microorganisms（LM+SM）,the group of litter microorganisms（LM）and soil microorganisms（SM）.Litter and soil microbial microorganisms played a positive role in litter decomposition rate,and litter microorganisms had a higher litter decomposition effect than soil microorganisms.They existed and worked together on the decomposition of litter,and coordinated to improve the litter decomposition rate.In indoor LM+SM and SM groups,the direct contribution of bacteria to litter decomposition rate was higher than that of fungi,and its role in litter decomposition was greater than that of fungi.4.Nutrient release and return of litter and its effect on soil nutrientsExcept for carbon,the release patterns of nitrogen（N）,phosphorus（P）,potassium（K）,calcium（Ca）,magnesium（Mg）and manganese（Mn）in the litter of the three stands showed significant differences,and the release rate of macroelements was C>N>K>P,and the release amount of C and N was higher than that of P and K.The decomposition of litter finally increased the content of carbon and nitrogen in soil.Compared with pure Robinia pseudoacacia and Platycladus orientalis forest,mixed forest accelerated the release rate of total carbon,total phosphorus and total potassium in litter,shorten the turnover time of total carbon and total potassium,and increase the return rate of total carbon in litter.Overall,compared with pure forest,mixed forest improved the rate of litter decomposition and nutrient release,shorten the nutrient turnover time,and accelerated the nutrient cycle in the ecosystem.Litter and soil microorganism cooperated on the decomposition of litter to improve the litter decomposition rate.The results of this study not only provide a new idea for the theory and technology of mixed forest afforestation and the study of nutrient cycling,but also provide a new insight into the decomposition mechanism of microorganisms on litter,which has important guiding significance and reference value for the sustainable development of Robinia pseudoacacia plantation on the Loess Plateau. |
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