Induction Mechanism Of Root Signaling Substances On Plant Growth Under Permafrost Degradation

Peatlands play an important role in regulating the carbon and nitrogen balance of the ecosystem in permafrost areas.As climate warming results in permafrost degradation,it affects the carbon and nitrogen cycle of the vegetation-soil system and the composition of the vegetation community in peatland,and thus affects the carbon and nitrogen cycle process of the regional ecosystem.The Great and Small Xing’an Mountains are located on the southern margin of the Eurasian permafrost zone,and permafrost degradation is very sensitive to the regional ecosystem.Climate change not only has direct effects on permafrost and vegetation growth,but the indirect effects of permafrost degradation on vegetation phenology can also explain the internal influencing mechanism between various parts of the ecosystem.The melting of permafrost increases the instability of the decomposition of organic matter in the soil in the peatlands,which will inevitably have a strong impact on the turnover of organic matter in the soil and the growth process of vegetation.Waterlogging stress is the first change that plants face after permafrost degradation.It is urgent to understand the internal mechanism of root signaling substances that regulate plant growth under permafrost degradation.Therefore,this study first analyzes the characteristics of permafrost degradation and its influence on vegetation phenology in the Great and Small Xing’an Mountains from 2000 to 2020.Secondly,three regions with different types of permafrost（discontinuous permafrost-Mohe,large island permafrost-Heihe and sporadic island permafrost-Yichun）were selected to study the effects of permafrost degradation on soil organic matter turnover,plant carbon and nitrogen allocation and transfer using isotopic tracer techniques,redundancy analysis and structural equation model.And how root signaling substances regulate plant growth under waterlogging stress.Finally,the turnover of soil organic matter and the mechanism of plant-soil carbon and nitrogen allocation in peatland were introduced into the UNCOM model to analyze the dynamic changes of peatland vegetation composition during 1980-2020,and to predict the peatland vegetation composition under different climate scenarios.The results are as follows:（1）Using the temperature at the top of the permafrost（TTOP）model to simulate the spatial distribution of permafrost areas in the Xing’an Mountains from 2000 to 2020,the areas of the three types of permafrost showed a decreasing trend.The mean annual surface temperature（MAST）increased significantly at a rate of 0.008℃year^-1from2000 to 2020,and the southern boundary of permafrost region moved north by 0.1–1degrees.The average NDVI value of the permafrost region increased significantly in8.34%of the regions.The significant correlations between NDVI and permafrost degradation,temperature,and precipitation were 92.06%（80.19%positively,11.87%negatively）,50.37%（42.72%positively,7.65%negatively）and 81.59%（36.25%positively,45.34%negatively）in the region of permafrost degradation,mainly distributed at the southern limit of the permafrost region.The significance test of phenology in the Xing’an Mountains showed that the end of the growing season（EOS）and the duration of the growing season（GLS）were significantly delayed and prolonged in the southern region of the permafrost of a sparse island.Sensitivity analysis showed that permafrost degradation was the main factor that affected the start of the growing season（SOS）and GLS.Excluding the effects of temperature,precipitation and sunshine,the regions with a significant positive correlation between permafrost degradation and SOS（20.96%）and GLS（28.55%）were located in continuous and discontinuous permafrost regions.The regions with a significant negative correlation between permafrost degradation and SOS（21.11%）and GLS（8.98%）were mainly distributed on the southern edge of the island permafrost region.In summary,NDVI and phenology changed significantly at the southern limit of the permafrost region,which was mainly attributed to permafrost degradation.（2）Theδ¹³C,δ¹⁵N,ε_δ13C,andε_soil¹⁵N values of different depths of permafrost were significantly different（P<0.05）.The foliage,roots and stems of C.schumidtii,V.uliginosum,and B.papyrifera under different degrees of permafrost degradation were significantly different（P<0.05）,except for theδ¹³C value of B.papyrifera roots.The nonstructural carbohydrate and proline of plants in sporadic island permafrost,large island permafrost and discontinuous permafrost peatlands increased first and then decreased with the labeling time,while the anaerobic metabolites decreased with the labeling time to resist the damage caused by anoxia and osmotic pressure.（3）The effects of permafrost degradation on the allocation and transfer of photosynthetic ¹³C in trees,shrubs,herb vegetation,and mosses were investigated using isotope tracing techniques,redundancy analysis,and structural equation modeling.Mohe had the largest ¹³C excess and more transfer of shrubs and herbs to the underground carbon pool.The photosynthetic recoveries of photosynthate-¹³C for Larix gemlini,Vaccinium uliginosum and Sphagnum were significantly different in different regions of permafrost using Tukey’s post hoc comparisons of means tests.The amount ofΔphotosynthate ¹³C transfer was larger in discontinuous permafrost than in large island permafrost and sporadic island permafrost.The allocation of photosynthetic ¹³C in the plant roots in the three permafrost regions was positively correlated with the total phosphorus content of the soil.Active layer thickness（ALT）was an important factor affecting the transfer ofΔphotosynthate-¹³C.The allocation and transfer of photosynthetic ¹³C to the subsurface were larger in the predominantly continuous and island permafrost area than in the sparse island and isolated patch permafrost regions.The allocation and transfer of photosynthetic ¹³C in the vegetation-soil system were affected by the physicochemical properties of the soils and the environmental changes after the degradation of the permafrost.（4）Most of the plant leaves in different permafrost regions had a lower N:P ratio and showed varying degrees of nitrogen limitation.Plants in discontinuous permafrost regions were better able to obtain nutrients from different soil depths and organic nitrogen sources.In autumn,when the thaw of the soil was the deepest in the active layer,a two-way analysis of variance of excess ¹⁵N using a linear mixed-effects model showed that all plants,except Sphagnum,obtained glutamic acid and urea from the newly thawed permafrost.In the sporadic permafrost region,the uptake of glutamic acid by plants could meet the plant growth needs.In the discontinuous permafrost region,the uptake of glutamic acid by some plants could not meet their growth needs.In competition with V.uliginosum,urea was selected as a nitrogen source,which was the key factor affecting changes in the plant community caused by degradation of the permafrost.（5）Compared to sporadic island permafrost and large island permafrost,discontinuous permafrost had a very high proportion of ¹³C allocated to the soil by the plant-soil system;in addition,discontinuous permafrost had the highest Sphagnum allocation among different plants.The nonstructural carbohydrate and proline content of plants in the sporadic island permafrost,large island permafrost,and discontinuous permafrost peatlands increased first and then decreased with labeling time.Anaerobic metabolites decreased with labeling time to resist damage caused by hypoxia and osmotic pressure.Root signaling substances regulate the allocation of carbon from plants to soil through indirect effects of climate change on soil physical and chemical properties.Plant communities are susceptible to transient changes in the soil environment in discontinuous permafrost.Climate change and soil physical properties indirectly affected plant–soil carbon allocation by affecting plant root signaling substances,and plant communities were susceptible to hormonal regulation in large island permafrost.Climate change indirectly affected plant root signaling substances in plant–soil carbon allocation by affecting soil physical properties.Larix gemlini was more susceptible to environmental changes than other plants.（6）Leaf uptake and fixation ofδ¹³C values through photosynthesis are significantly affected by soil moisture in different regions of permafrost.After 24 hours of labeling,Larix gemlini,Vaccinium uliginosum and Sphagnum leaves consumed more water during photosynthesis in sporadic island permafrost and large island permafrost than in discontinuous permafrost,while Carex schumidtii leaves consumed more water during photosynthesis in sporadic island permafrost and discontinuous permafrost.The consumption of ¹⁵N was higher in Larix gemlini and Carex schumidtii,while the consumption of ¹⁵N in Carex schumidtii and Sphagnum decreased with tagging time in sporadic island permafrost and large island permafrost.Larix gemlini and Vaccinium uliginosum initiated the anaerobic metabolism pathway.In different permafrost regions,plants consumed starch mainly through ethanol fermentation.In the discontinuous permafrost area,the plant antioxidant system is easily damaged and the main root signaling substances are more easily affected by environmental changes in the soil.Plants in different regions of the permafrost exhibit hypoxic resting strategies under flooding stress.More root signaling substances were involved in regulating plant growth in the peatland in the discontinuous permafrost area.The photosynthetic system and the anaerobic metabolism system of plants under water flooding stress were the first to respond to different areas of permafrost and then to maintain plant growth by consuming a small amount of starch and soluble sugar and to maintain osmotic pressure by increasing the content of proline in the late labeling period.Plant growth in the discontinuous permafrost region is most affected by climate change and permafrost degradation.（7）From 1980 to 2020,shrub biomass was higher than that of other types of plants,and the different regions of permafrost were sporadic island permafrost>large island permafrost>discontinuous permafrost.Based on the RCP scenario,biomass changes were simulated in five climate change scenarios during 2020-2060.The results showed that in the scenarios of T2.5P15,T5 P30,T8 P45 and T0 P45,the gramineous plants showed a significant increase trend,while other vegetation types had little change.In the T8 P0 scenario,the biomass of the shrubs increased significantly,while that of other plants decreased.The biomass of plants in different regions of permafrost is as follows:sporadic island permafrost>large island permafrost>discontinuous permafrost.Nutrients limit the growth of shrubs and trees in different climate change scenarios.The duration of the growing season,light,soil moisture,and nutrients in different regions of permafrost showed the same trend.Among the collapse sites of permafrost in different permafrost regions,discontinuous permafrost has the highest biomass of gramineae,followed by sporadic island permafrost and large island permafrost.