Change Characteristic Of Soil Organic Matter In Mollisol Profile Under Different Ecosystems

Ecosystem conversion will change the amount and quality of soil organic matter?SOM?in the soil profile.These changes will finally affect the dynamic,distribution and chemical structure of SOM in soil profile.However,little is known about the difference and its controlling factors of SOM fraction in mollisol profile under different ecosystems.This paper based on the long term experiment in the typical area of mollisol,studied the amount of SOM,content of SOM in physical and chemical fractions in different layers?0-10?10-20?20-40?40-60?60-80?80-100?100-120?120-140?140-160?160-180 and 180-200cm?of soil profile under different ecosystems,which include grass land,forest land,cultivated land and bare land,analyzed the structure characteristics of SOM in soil profile use solid-state NMR techniques.Finally,reveal the dynamic of SOM in soil profile and its control mechanisms.These researches have significance not just for the stability mechanism of SOM but for global carbon cycling and rational use of black soil.The results were showed as following:?1?The influence of different ecosystem on soil organic carbon?SOC?reached to below 100 cm depth.SOC content down to a depth of 60 cm in grass land ecosystem was significantly higher than in arable land.The content of SOC at 10-120 cm layers in cultivated land was significantly lower than in grass land and forest land,even lower than the bare land.This may be caused by the intense priming effect in soil profile of cultivated land,which lead to the original SOC was decomposed by microbes.There were no significant differences in SOC content at 0-10 and 10-20 cm depth between forest land and cultivated land.?2?The organic carbon concentration of all soil density fractions in soil profile decreased with the increase of depth,this result reflected the nature of soil density fractions between different soil depth were different.In addition,with the increase of depth,organic carbon content of soil light fraction and its proportion in SOC reduced gradually,nevertheless,it was opposite for the heave fraction.The difference in carbon content of soil light fraction between different ecosystems decreased with the increase of soil depth,and the largest differences exist in 0-10 and 10-20 cm depth,the differences disappeared under 80 cm layer.The carbon content of free light fraction?f LF?at 0-80 cm depth and occluded light fraction?o LF?at 0-60 cm were higher in grass land than that in other three ecosystems,Therefore the grass land were favorable for the formation of soil light organic carbon.Due to the source of light fraction carbon was lack in bare land,its f LF carbon content in surface layer?0-10cm?was significantly lower than other ecosystems.For the carbon content of light fraction above 80 cm soil layer and its proportion in SOC,it was lower in cultivated land than grass land and forest land.In the coniferous forest of spruce,the litter quantity is less each year,and a majority of root biomass distributed in the top of soil,therefore,compare with cultivated land,light fraction carbon content in forest land was higher only above 40 cm depth,there were no significant difference of light fraction carbon content between cultivated land and forest land at the layers of 40-100 cm.As for the heavy fraction carbon,the carbon proportion in heavy fraction in cultivated land and bare land were significantly higher than that in grass land and forest land,this indicated that SOM in the two former ecosystems were more stable than that in the last two.At the layers of 20-100 cm,heavy fraction carbon content in cultivated land was generally lower than other treatments which may be caused by the priming effect.?3?Water soluble organic carbon?WSOC?content in 0-120 cm soil profile decreased with the increase of depth,below 120 cm,each layer of WSOC content tend to the same level.Under different ecosystems,WSOC content differences mainly appeare at 0-80 cm layers,and the largest differences exist in 0-10 cm,shown as grass land > forest land > cultivated land > bare land.Meanwhile,WSOC content in grass land and forest land at 0-60 cm layers were significantly higher than that in cultivated land and bare land.?4?Differences of fulvic acid?FA?and humic acid?HA?carbon content and HA/FA ratio under different ecosystems mainly appeare at 0-60 cm layers.In these layers,FA and HA in grass land were higher than that in other ecosystems.Except 10-20 cm layer,forest land has the lowest content and proportion of FA from 0 to 60 cm layers,indicated that the formation of FA in forest ecosystem was limited.In addition,at the layers of 0-60 cm,ratio of HA/FA were highest in forest land and lowest in grass land,demonstrated that the humification degree of SOM in forest land was higher than that in the other ecosystems,while it was opposite in grass land.Compare with FA and HA,a low degree impact of ecosystem types on humin?HM?content was found,and the impact mainly happened at surface layer,displayed as grass land > cultivated land > forest land > bare land.At the layers of 20-100 cm,carbon content of HM in cultivated land was generally lower than the other ecosystems.This could be explained by the priming effect in the profile of cultivated land.?5?The proportion of functional groups of SOM was different within the soil profile.In grass land,forest land and cultivated land,OCH₃/NCH and O-alkyl-C proportion of SOM decreased,while the aromatic C and carboxyl C increased with depth.Therefore,the stability degree of SOM increased with depth.Type of vegetation played an important role in the chemical structure of SOM.At 0-80 cm layers,alkyl C,OCH₃/NCH and O-alkyl-C proportion of SOM increased,while the aromatic C decreased when cultivated land were recovered to nature grass land.Nevertheless,after cultivated land were converted to spruce forest land,O-alkyl-C proportion of SOM increased at 0-60 cm layers,but there were no obvious change of aromatic C.Compared with grass land,at 0-60 cm layers,nonpolar alkyl C and O-alkyl-C proportions were lower,and aromatic C were higher in forest land,which demonstrated that the humification degree of SOM in forest land was higher than that in the grass land.Absence of vegetation in bare land weakened the structure difference of SOM between different soil depths.In conclusion,changes in ecosystem types have profound effects on quantity,composition and structure characteristics of SOM in mollisol profile,and the degree of these effect were vary with different soil depth.This revealed to us that carbon dynamics in topsoil and in subsoil may be controlled by different regulatory mechanisms.In this paper,a preliminary inference is that,in the field condition,the priming effect is a main control mechanism of SOM dynamics in profile.In future study,we need carry out simulation experiment,in order to clarify the influence of root and root exudates on SOM turnover and there regulate mechanisms.