| Quaternary geology is a science to study the development and evolution of the Quaternary environment,such as crustal movement,climate change and biological succession.The hypergene processes such as soil development and biological succession are one of the basic geological processes in geosciences.With the increasing of global temperature,the mountain glaciers in the world generally show a shrinking trend.In the glacial retreat area,all kinds of chemical substances migrating,transforming and accumulating among the earth's layers,which is the earth critical zone of human activities and geological environment.Hailuogou glacier retreated obviously since the end of the Little Ice Age.After glacial retreat,soil formation is continuous,and a complete and continuous vegetation primary succession sequence is formed from the original bare land to the climax community,which provides a natural experimental site for us to study the complex biogeochemical processes in the earth critical zone.We set up seven sampling sites in Hailuogou glacier retreat area(0,12,30,40,52,80 and 120 years of glacier retreat)to explore the relationship between soil development and vegetation succession,and the biogeochemical process of main mineral elements in soil-plant.The main research results are as follows:(1)In the 120–year time series of Hailuogou glacier retreat area,the early weathering of minerals was dominated by carbonate minerals,after 52 years of glacier retreat,the weathering of silicate minerals gradually strengthened.The grain size composition of soil A and C layers were mainly composed of sand,accounting for more than 80%of the soil grain size,which was related to the mineral composition of soil in this area was mainly quartz.With the development of soil,the thickness and organic carbon content of O and A layers increased significantly,but the p H value decreased significantly,it has changed from alkaline soil at the end of glacier to acidic soil after 120 years of glacier retreat.Except for N,the contents of other elements in surface soil(O and A layers)were decreased with the soil development,and the principal component analysis showed that the sources and characteristics of N were different from other elements.The soil forming rate of A layer was 188.9 t/km~2/y,and it was the fastest in the broad-leaves forest stage during the 40–52 years of glacial retreat.According to the important values of species and biomass of dominant species(Hippophae rhamnoides,Salix spp.,Populus purdomii and Abies fabri)in different glacial retreat periods,the vegetation primary succession experienced from herb and shrub stage with H.rhamnoides as dominant species to broad-leaved forest stage with P.purdomii as dominant species,and finally formed climax community of coniferous forest dominated by A.fabri.Through RDA analysis of the effects of soil environmental factors on vegetation primary succession,it can be seen that the p H value,SOC and N contents of surface soil have significant effects on the important values of dominant species,and the effects of SOC and N will increase with the vegetation primary succession.(2)The contents of main mineral elements in each soil layer varied greatly with the soil development.Except for N,the contents of P,K,Ca and Mg in surface soil were lower than those in C layer.The contents of main mineral elements in surface soil were Ca>N>K>Mg>P,and in C layer were Ca>K>Mg>P>N.In vegetation,the contents of main mineral elements in different organs of dominant species were highest in leaves and lowest in stems,and the contents of N,P,K and Mg in the leaves of dominant species decreased with succession;the contents of N,P,K and Mg in stems first decreased and then increased;the contents of N,P,K and Mg in branches and roots fluctuated;the contents of Ca in all organs of dominant species were highest in broad-leaved forest stage.In understory vegetation,N content was highest in shrub,herb and ground cover(moss),while P and Mg contents were lower in herbs and ground cover.In the process of migration of P,K,Ca and Mg from parent material to surface soil,P showed a certain loss,while K,Ca and Mg showed pure migration,without obvious loss.In the process of migration and enrichment of mineral elements from surface soil to vegetation,N was significantly enriched in organs of H.rhamnoides,Salix spp.and P.purdomii,and the ability of the above-ground organs to absorb from the soil were higher than that of the underground organs in the dominant species;the accumulation of P in the branches and leaves of dominant species indicated that the loss of P in soil was partly due to its transfer to plants;K was enriched in leaves and branches of the dominant species,especially K enrichment coefficient in leaves increased with the growth of plants;while Ca and Mg elements were not significantly enriched in organs of the dominant species.(3)The storage of main mineral elements in soil and vegetation increased with the increasing glacial retreat year.With the development of ecosystem,N storage in surface soil was higher than that of vegetation,and became the main N pool of ecosystem;the P and K storage proportion of vegetation in the ecosystem increased gradually,and exceeded the surface soil after 80 years of glacier retreated;the storage of Ca and Mg in surface soil was much higher than that of vegetation in each stage of glacier retreat.The annual accumulation of main mineral elements in the ecosystem were Ca>N>K>Mg>P,and the N and P showed positive accumulation in the whole sequence;however,K,Ca,Mg showed negative increase in the mixed stage of broad-leaved forest transition to coniferous forest.The annual accumulation of main mineral elements in surface soil,living vegetation and the whole ecosystem were highest in broad-leaved forest stage;the annual accumulation of N in the whole ecosystem and surface soil were slowest in the herb and shrub stage,while the annual accumulation of other mineral elements was the lowest in the coniferous forest stage.In addition,the annual accumulation of N in the ecosystem was significantly positively correlated with P,while the annual accumulation of K,Ca and Mg were significantly positively correlated.(4)In the whole succession sequence,the utilization efficiency of P and K were higher.However,the circulation intensity and fluidity of N,Ca and Mg were higher in vegetation,which consumed less N,Ca and Mg in soil,and it was conducive to the accumulation of N,Ca and Mg in soil.In addition,the main mineral elements absorbed by vegetation in the herb and shrub stage and broad-leaved forest stage were large,and the cycle rates were fast,but the utilization efficiency was low;while the main mineral elements absorbed by vegetation in the coniferous forest stage were small,the cycle rates was slow,but the utilization efficiency was high.Furthermore,there was a significant positive correlation between the soil development rate and the cycling rate of N.The results showed that the relative accumulation rate of P in dominant species was higher than that in N,while the relative accumulation rates of N in surface soil and understory vegetation were higher than that in P.Therefore,the N:P ratio of surface soil increased logarithmically with the increasing glacial retreat year,while the N:P ratio of dominant species decreased with the increasing glacial retreat year.From the threshold of N:P ratio,the N:P ratio of vegetation was kept at a low level(<14)in the whole succession sequence,indicating that the growth of vegetation was mainly affected by N element.In conclusion,the effects of soil environmental factors on the succession of dominant species,the recycling characteristics of main mineral elements in the ecosystem and the N:P ratio of vegetation all indicated that N was the main driving factor affecting the occurrence and development of supergene process in Hailuogou glacier retreat area. |
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