Degradation Of Soil Lignin In Different Plant Communities And Its Microbiological Mechanism

Since the industrial revolution,global warming has intensified and profoundly changed the global carbon cycle.Increasing soil organic carbon sequestration and stability is an important measure to effectively alleviate climate warming.The degradation of soil lignin is the rate-limiting step of ecosystem organic carbon cycle,which can further reflect the stability and turnover state of soil organic carbon.Studying the distribution and degradation of soil lignin can further clarify the stability mechanism of soil plant-derived carbon,which has far-reaching significance for improving and stabilizing regional soil organic carbon.In this paper,three plant communities of Robinia pseudoacacia forest land,Caragana korshinskii forest land and abandoned land in Zhuanyaogou small watershed of Loess Plateau in western Shanxi were studied to study the effect of vegetation restoration on the distribution of soil lignin.The indoor simulation experiments were carried out at 15℃,25℃and35℃respectively to explore the degradation process and microbiological mechanism of soil lignin under warming.The main results are as follows:（1）The lignin content in the 0-10cm,10-20cm and 20-30cm soil layers of the three plant communities was distributed between 56.38-170.65 ug/g,51.86-128.55 ug/g,31.54-98.91 ug/g,and there was no significant difference in the vertical gradient（p>0.05）.Vegetation restoration had a significant effect on the content ratio of lignin S monomer to V monomer（S/V）and the acid aldehyde ratio of S monomer[（Ac/Al）s]in soil（p<0.05）.The soil C/N of the three plant communities was significantly higher in 0-10cm than in 20-30cm.The soil C/P and N/P of Robinia pseudoacacia and Caragana korshinskii were significantly higher in 0-10cm than in 10-20cm and20-30cm.The soil C/P of abandoned land was significantly higher in 0-10cm than in10-20cm and 20-30cm.There was no significant difference in N/P among the three soil layers.Soil C/N and C/P in the three soil layers showed no significant difference among the three plant communities.The soil N/P in the 0-10 cm soil layer was significantly higher than that in the abandoned land,and in the 10-20 cm soil layer was significantly higher than that in the abandoned land.There was no significant difference between the three plant communities in the 20-30 cm soil layer.（2）The indoor simulated soil lignin degradation test showed that the soil lignin content of the three plant communities decreased with the passage of culture time.The lignin content of Robinia pseudoacacia forest land decreased by 96.53%-98.84%,the lignin content of Caragana korshinskii forest land decreased by 97.78%-98.96%,and the lignin content of abandoned land decreased by 96.76%-98.11%.Under the three temperature conditions,the cumulative mineralization of soil organic carbon in the three plant communities showed an upward trend.The mineralization amount was significantly larger in the first 15 days,and the mineralization amount gradually decreased after 15 days.Under the same temperature conditions,the soil organic carbon mineralization rate of different plant communities decreased with the passage of culture time,which was manifested by the rapid decline rate in the early stage and the stabilization in the later stage.The mineralization rate of soil organic carbon under different temperature conditions was basically as follows:Robinia pseudoacacia forest land>Caragana korshinskii forest land>abandoned land.The Cp values of the potential mineralized carbon pool of organic carbon under different temperature conditions of the same plant community were significantly different（p<0.05）,and the plant community type also had a significant effect on the soil Cp value（p<0.05）.（3）In the warming simulation of soil lignin degradation,the activities of Li P,Mn P and La C in the three plant communities were basically higher than those in the Caragana forest land and abandoned land.The soil Li P,Mn P and La C activities of the three plant communities increased with increasing temperature at 15-35℃.The effects of temperature on the activities of Li P,Mn P and La C in soil were different.The response degree of three lignin degrading enzymes to temperature in R.pseudoacacia forest and abandoned land was La C>Li P>Mn P,while that in C.korshinskii forest was Mn P>Li P>La C..（4）In the simulation of soil lignin degradation by warming,the study of soil microbial diversity showed that the Chao1 index and Shannon index of soil bacterial community in three plant communities increased first and then decreased with the passage of time,and the Simpson index did not show obvious changes with the passage of culture time.The Chao1 index of soil fungal community showed a decreasing trend after 60 days of culture,while the Simpson index and Shannon index showed a flat or decreasing trend after 60 days of culture.The dominant bacterial phyla in the three plant communities included Proteobacteria,Actinobacteria,Acidobacteria and Chloroflexi.Under different temperature conditions,the relative abundance of Proteobacteria in the soil of the three plant communities decreased first and then increased or decreased with time.The relative abundance of Actinobacteria and Chloroflexi increased first and then decreased with time.The relative abundance of Acidobacteria decreased first and then increased with time.The dominant phyla of soil fungi in the three plant communities included Ascomycota and Basidiomycota.Under different temperature conditions,the relative abundance of Basidiomycetes in the soil of the three plant communities decreased or decreased first and then increased with the passage of culture time.The relative abundance of Ascomycota increased with time or increased first and then decreased.（5）The total amount of soil lignin was significantly positively correlated with SOC,TP,NO₃^--N content and N/P（p<0.05）,and was significantly positively correlated with TN and AK content（p<0.01）.S/V was significantly positively correlated with TN and AK content.There was a significant positive correlation between（Ac/Al）v and TP content.There was a significant positive correlation between the cumulative mineralization of soil organic carbon and the degradation rate of soil lignin in the three plant communities.The cumulative carbon mineralization at 15℃was significantly positively correlated with La C,the organic carbon mineralization rate at25℃was significantly negatively correlated with Li P and La C,and the cumulative carbon mineralization at 35℃was significantly positively correlated with Li P.The Alpha diversity of soil bacteria was negatively correlated with soil Li P,La C,carbon mineralization rate and lignin content,and positively correlated with cumulative carbon mineralization.The composition of soil bacterial community was mainly negatively correlated with cumulative carbon mineralization,and positively correlated with Li P,carbon mineralization rate and lignin content.There was no correlation between Alpha diversity of soil fungi and lignin degrading enzymes,carbon mineralization characteristics and lignin dynamic changes.The composition of soil fungal community was mainly negatively correlated with cumulative carbon mineralization,and positively correlated with Li P,La C,carbon mineralization rate and lignin content.Redundancy analysis of the relative abundance of dominant phylum and soil environmental factors with significant differences showed that the main soil factors affecting the composition of soil microbial community structure were Li P,Mn P,SOC and TN.In this paper,to explore the coupling relationship between soil lignin degradation and carbon mineralization,we found that vegetation restoration significantly affected the monomer composition of lignin in soil.The coupling process of soil lignin degradation and carbon mineralization was affected by soil physical and chemical properties,soil lignin degrading enzymes and other factors,and the change of soil bacterial community closely affected the content of soil lignin.These are conducive to revealing the stable mechanism of soil organic carbon cycle and turnover.