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Effects Of Litterfall On The Dynamics Of Extracted Soil Humic Substances In A Subalpine Forest

Posted on:2021-03-25Degree:MasterType:Thesis
Country:ChinaCandidate:X Y WeiFull Text:PDF
GTID:2480306506455684Subject:Forest cultivation
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Humus is the main component of soil organic matter and is important for soil fertility and nutrient cycling.As a basic carrier of nutrients and carbon,forest litterfall plays an essential role in the formation of soil humus.On the one hand,the input of active components of litterfall can stimulate the activity of soil microorganisms and enzymes,leading to the degradation of soil humic substances;on the other hand,the formation of humus during the decomposition of litterfall is also the main source of soil humic substances.Meanwhile,different kinds of litterfall have different chemical compositions and qualities that might determine the development processes of humus in soil.However,the rhythmic production of litterfall due to climate changes and seasonal changes may also alter its decomposition rate,thereby affecting the quality and content of soil humic substances.The relationship between litterfall and soil humic substances still lacks a unified understanding,and further research is urgently needed.As an important ecosystem in southwestern China,the subalpine forests in western Sichuan plays an important role in water conservation as well as in regulating the climate and conserving water and soil.Due to extreme climate and frequent geological disasters,the mineral soil layer of the subalpine forest soil is very shallow.Therefore,forest productivity is more dependent on the organic layer of the soil,which is rich in humus.Based on the early research of subalpine forest soil humification,we selected the typical coniferous forest,mixed forest and broadleaved forest in a subalpine forest in Wanglang National Nature Reserve of western Sichuan as research objects.We conducted in situ incubation experiment in these forests to determine the effects of litter input on soil humic substances at different periods.We assessed the litter production,the content of soil total extracted humus,humic acid and fulvic acid,and the optical characteristics of humus of three forests in winter,early growing season and late growing season,and then analyzed the soil humus dynamics,the formation and stabilization of humic substances,the controlling factors for soil humification and the potential effects of litterfall on the formation of humic substances in this subalpine forest.We found that:(1)After two years of incubation,the content of soil total extracted humus in the three forests with continuous litter input was 41.48-45.65 g·kg-1,which was higher than the control soil by 8.60%-19.26%.This indicated that long-term continuous input of litterfall promoted the synthesis of soil humus,and the promotion effect of broadleaved litterfall was the most significant.The short-term input of litter in each key period had no significant effect on the content of soil humus in coniferous forest,but promoted the synthesis of soil humus in mixed forest and broadleaved forest.It suggested that broad leaf litter contained more nutrients than coniferous litter,which was more conducive to the synthesis of soil humus.(2)After two years of incubation,the content of soil humic acid in the three forests with continuous litter input was 22.70-26.99 g·kg-1,of which the content of soil humic acid in coniferous forest was higher than that of control soil by 33.46%,but the soil humic acid content in the mixed and broadleaved forests was 4.39%-25.52%lower than the control soil.The content of soil fulvic acid in the three forests with continuous litter input was 18.66-19.39 g·kg-1.Contrary to the content of humic acid,soil fulvic acid content in coniferous forest was 10.84%lower than that in control soil,while soil fulvic acid content in mixed forest and broadleaved forest was 11.23%-16.67%higher than control soil.This indicated that the impact of different litter on soil humus was different,which might be determined by the difficulty of decomposition of litter.The input of litterfall which was decomposition slowly,and contained much hard residues was beneficial to the synthesis of soil humic acid.(3)At the end of incubation,the input of litter increased soil humification in mixed forest,while soil humification in coniferous and broadleaved forest decreased.This showed that different types of litter had different effects on the degree of soil humification.The single litter decomposed slowly,and the content of lignin was high,which was not conducive to the growth and reproduction of soil biological communities and inhibited the process of soil humification.Litter in mixed forest decomposed quickly and promote the soil humification process.At the same time,we found that the humic substances types of the three forests were all younger Rp types,and the humus in the soil was still active.(4)Soil total extracted humus and fulvic acid degraded in winter and early growing season,but accumulated in late growing season.Different from soil humus,the soil humic acid content of the three forests degraded in winter but accumulated during the growing season.It suggested that the process of soil humification was not a continuous accumulation of humic substances,but a dynamic process of continuous degradation and accumulation.In addition,the HA/FA ratios in the three forest was less than 1 in winter and increased and greater than 1 during the growing season.This indicated that humic acid was a relatively active part of the soil humic substances.The degradation rate of humic acid was greater than that of fulvic acid,and the synthesis rate of humic acid was greater than that of fulvic acid during the growing season.(5)The spectroscopic characteristics of humic substances found that during the two years of incubation,the?log K values and E4/E6 values of soil humic substances in the three forests did not changed significantly,but the A600/C values gradually increased,and the humic substances were type Rp during the incubation period.It showed that the soil humic substances in the three forests were in a dynamic change process.The formation of new humus and the decomposition of old humus continued,and the soil humus was always in a relatively active state.In conclusion,the short-term input of litter depended on the rapid input of available carbon sources to affect the accumulation of soil humus,while the long-term cumulative input depended on the decomposition of refractory substances to promote the synthesis of soil humus.In the short term incubation,the input of broad leaf litter was more conducive to the synthesis of soil humic substances,and it mainly occured in winter and early growing season.In the long term incubation,the input of mixed litter was more favorable to the synthesis of soil fulvic acid,while the input of single litter was more favorable for the synthesis of humic acid.The results also showed that litterfall with fast decomposition speed and high nutrient content could promote the soil humification process and increased the degree of soil humification.These results were significant for maintaining soil fertility of subalpine forests and sustainable forest management.
Keywords/Search Tags:humic substances, humic acid, fulvic acid, spectroscopic characteristics, litter, subalpine forest
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