| Litter decomposition is one of the core processes of nutrient cycling in forest ecosystems,as well as the main material source for soil texture improvement and nutrient enhancement.It also plays an important functional role in the restoration of artificial forest vegetation in fragile ecological areas.The chemical quality of litter,including its nutrients,chemical composition,metal elements,and stoichiometric ratio,is an essential factor determining its difficulty in decomposition,speed of transformation,and amount of nutrient release.It also indirectly affects the accumulation and transformation of soil nutrients and vegetation restoration.Therefore,in order to reveal the impact mechanism of chemical quality decay of litter decomposition on soil organic matter accumulation in different sand fixing vegetation,this study focuses on the restoration of 26 and 47 year old tree and shrub sand fixing forests in the desertification area of Maowusu sandy land.Litter and soil were collected and simulated experiments were designed according to the original site ratio to simulate the process of litter decomposition quality loss.Based on the dynamic changes of dry weight,organic carbon,total nitrogen,soluble organic carbon,soluble nitrogen,manganese,magnesium,cellulose,hemicellulose and lignin of litter in the process of litter decomposition,the decay characteristics of basic chemical quality in the process of litter decomposition were analyzed.The cumulative characteristics of soil organic matter were analyzed from 0-20 cm soil organic carbon,total nitrogen,soluble carbon and nitrogen,microbial biomass carbon and nitrogen,and light recombination carbon and nitrogen indicators.Furthermore,the structural equation model was used to reveal the impact mechanism of chemical mass decay of litter decomposition of two types of sand fixing vegetation on soil organic matter accumulation,which can provide scientific basis for understanding the nutrient cycling mechanism of artificial forest vegetation in this area.The main research results obtained are as follows:(1)After 180 days of indoor decomposition,the dry weight loss rate of tree litter was higher than that of shrub litter in two types of sand fixing vegetation.The dry weight loss rates of tree litter after 26 and 47 years of restoration were 17.33% and 19.00%,respectively,while the dry weight loss rates of shrub litter after 26 and 47 years of restoration were 13.50%and 14.33%,respectively.Using the Olson model to fit the relationship between different residual ratios of litter and decomposition time,it was found that the decomposition rate of the four types of litter showed that the decomposition rate of trees was greater than that of shrubs.(2)During the decomposition process of litter,the chemical quality of tree and shrub litter that had been restored for 26 and 47 years showed continuous release with the passage of decomposition time.After 30 to 60 days of decomposition,it showed rapid release,and then the release rate slowed down.By 180 days of litter decomposition,the average cumulative indexes of organic carbon,total nitrogen,soluble organic carbon,soluble nitrogen,manganese,magnesium,cellulose,hemicellulose and lignin in litter were 80.74%,57.03%,55.85%,42.72%,59.38%,70.66%,65.97%,72.36% and 77.67%,respectively.Especially,the content of manganese and magnesium in litter exhibits a characteristic of first enrichment and then release.Through differential analysis of different indicators,it was found that total nitrogen,soluble carbon nitrogenand cellulose are more sensitive to the decomposition of litter.(3)The decomposition of litter can significantly increase the composition of soil organic matter.The trend of changes in soil organic matter components during the cultivation process in the litter decomposition chamber shows that the accumulation rate of organic matter is the highest within 30-60 days of litter decomposition,and the accumulation of soil organic matter is slow within 60-180 days.The decomposition of litter mainly affects the accumulation of soil organic matter at 0-5cm and 5-10 cm,but has no significant impact on soil organic matter at 10-20 cm.After 180 days of decomposition of litter,the change rates of organic carbon and total nitrogen in 0-5cm tree soil were 17.95%and 32.05%,respectively.The change rates of soluble carbon and nitrogen,microbial biomass carbon and nitrogen,light carbon and nitrogen,and recombinant carbon and nitrogen in tree soil were 58.54%,51.98%,39.01%,110.72%,29.79%,69.54%,47.25%,and 28.29%,respectively.The change rates of organic carbon and total nitrogen in 0-5cm shrub soil were 9.28% and 31.54%,respectively.The change rates of soluble carbon and nitrogen,microbial biomass carbon and nitrogen,light carbon and nitrogen,and recombinant carbon and nitrogen in tree soil were 57.49%,62.45%,47.86%,106.53%,21.28%,78.57%,48.41%,and 30.19%,respectively.In summary,the change rate of total organic matter in soil is lower than that of each component,while the change rate of total organic matter in tree soil is higher than that in shrub soil.There is a significant positive correlation between the dry weight loss rate during the decomposition process of four types of litter and the change rate of soil soluble carbon and nitrogen and microbial carbon and nitrogen content.(4)By constructing a structural equation model,the impact path of litter chemical quality on soil organic matter composition during indoor cultivation of tree and shrub litter with different restoration years was simulated.On the whole,cellulose,hemicellulose and lignin indirectly affect the accumulation of soil organic matter through their effects on the trace elements and carbon and nitrogen of litter during litter decomposition.In addition,cellulose also has a direct effect on the accumulation of soil organic matter.Litter trace elements indirectly affect the accumulation of soil organic matter by affecting litter carbon and nitrogen nutrients.The response of soil soluble carbon and nitrogen,microbial carbon and nitrogen,and light component carbon and nitrogen to litter decomposition is more sensitive.In summary,the decomposition chemical quality decay of tree and shrub litter with different restoration years has a positive impact on the accumulation of soil organic matter.The chemical quality of litter exhibits rapid release after 30 to 60 days of decomposition,followed by a slower release rate.Research has found that total nitrogen,soluble carbon nitrogen,manganese,and cellulose are more sensitive to the decomposition of litter.During the decomposition process of litter,both the composition of litter and trace elements have an indirect impact on soil organic matter components through pathways that affect the carbon and nitrogen content of litter.Soil soluble carbon and nitrogen,microbial carbon and nitrogen,and light carbon and nitrogen are more sensitive to the response of litter decomposition. |
