Stabilization Mechanisms Of Soil Organic Carbon In Chinese Fir Plantations

In recent years,global warming and other environmental problems issued by the rising CO2 concentration have drawn attentions to domestic and overseas scientists.As the bigest carbon sink in terrestrial ecosystem,carbon stock in soils exceed the total of air and plant biomass.The change in soil carbon stock may lead to the great fluctuation in atmosphere CO2 concentration.Therefore,it is greatly significant to keep carbon stable in soils for alleviating global warming.Many research results showed that the mechanisms influencing stabilization of soil organic carbon included biochemical recalcitrance of soil organic matter(SOM),soil mineral adsorpting SOM and physical protection by aggregates,however the roles in formation and stabilization processes of soil organic matter are different in different texture and mineral charateristic soils.Due to the complicated factors that influence the input of organic carbon and its turnover rate,it is not clear for the stabilization mechanisms of organic carbon in different climate zones,soil parent materials,and soil texture.It is very difficult to study the stabilization mechanism of soil organic carbon in plantation ecosystem owing to the frequent anthropogenic interference,such as soil preparation,fertilization,thinning,and logging.Thus,it is important to find out the stabilization mechanism of soil organic carbon in plantation ecosystem for the fatal research direction of recent the world of science.Chinese fir(Cunninghamia lanceolata(Lamb.)Hook)which is widely planted in subtropic area of China,is a unique evergreen needle species,with a high productivity,high quality of timber and rapid reproductive.According to assessment,the planting area is almost near to 12×104 hm2 which is accounting for 30%of plantation area in China,which indicated that Chinese fir plantation have a huga potential of C sequestration.However,the traditional regime of Chinese fir lead to severe soil erosion and soil fertility degradation and the carbon stock is declining with the successive rotation of Chinese fir plantations.Face with the increasingly serious problem of climate change,how to find a balance between the demand timber and soil C sequestrat ion is becoming a hot research.Nevertheless,process and mechanism of soil sequestration was seldom understood,which restrict the potential of soil C sequestration and the accurate assessment of the effect of afforestation on global climate change.The factors of influence on soil C sequestion and the mechanism of carbon stabilization should be studied to make suitable policies of forest regime and soil management which are significant to improve soil C sink.In view of this,from the aspect of three mechanisms of organic carbon stabilization in soil(Biochemical recalcitrance,physical protection by aggregate,mineral-proected SOM),two soils derived from different parent materials were chosed for studing the role of three stabilization mechanisms on soil C sequestration in Chinese fir plantations.In order to evaluate the effect of mechanism biochemical C stabilization on soils C sequestration in Chinese fir plantations,the change and distribution of carbon fractions and biochemical stability in soils was studied with the development of Chinese fir and soil depth.The quantity and structure feature of mineral-adsorbed SOM in two soils from different parent materials were studied using to the techniques of stable isotope and pyrolysis GC-MS to discuss the SOC stabilization of typical minerals.Finally,the influence of litter on the formation process of aggregates,the redistribution of exogenous new carbon and microbial community composition on different sizes of aggregates were studied to discuss the mechanism of aggregates formation impact on the stabilization of organic carbon.It is to offer the theory for organic carbon stabilization and,also provide practical significance and guiding value for making soil management measure to improve the protential carbon sink in subtropics red soils area.There are main results as follow.(1)The recalcitrant index of carbon in two soils were 18.63-32.11%and 31.99-40.39%,respectively.That indicated soils in Chinese fir plantations have 18%organic carbon have resisitance to be decomposed by microorganisms.There were very significantly correlation between soil total carbon,labile carbon and recalcitrant carbon pool,which suggested that the size of the pool of labile carbon,recalcitrant carbon is related to the accumulation of soil carbon and the process of soil organic carbon is influenced by the quality of soil organic matter.(2)The composition of organic matter changed with the changing of density fractions.With the increasing density of soil particle,the concentration of polysaccharide,aromatic compound,and lignin decreased and the nitrogenous compound increased.the fractions of 2.4-2.5,2.5-2.65 and>2.65 g cm-3 were 13C enriched and 2.5-2.6 have the most 13C enriched in 3 fractions,which showed the heavier fractions tended to sorb the decomposed organic matter.The organic matter in heavy fractions was not only stablized by minerals but also was stable by itself.However,the source of organic matter in lighter fractions was recent.(3)41%-49%soil carbon was protected in heavy fractions(D>1.9 g cm-3)in soils from granity and 41%?46%soil carbon was sorbed with minerals in soils from siltstone,which suggested that 41?49%were protected by soil minerals in red soils of southern Chinese fir plantations.Although the size of organic matter which is protected by soil minerals was similar in two soils,the role of minerals type on stabilization of carbon was different.The protection of poor-crystallized aluminosilicates for soil organic matter in soils from granity was most poor and the proportion of C and N in the fraction of 2.2-2.5 g cm-3 was highest,suggesting that phyllosillicates had a stabilization role on organic matter in soils from granity.In soils from siltstone,the sorbed carbon in primary minerals was about 18?33%and 26?41%in nitrogen.Clearly,the stabilization of primary mineral was dominating in soils from siltstione.(4)The proportion of>2000 0?m macroaggregates increased sharply and the proportion of 250-2000 ?m,53-250 ?m and<53 ?m decreased in the 7th day,which indicated that the three fractions were incorporated into macroaggregates in 7 days.And the MWD in two soils significantly increased in 7th day,suggesting further soil stucture improved after incubating 7 days.The proportion of macroaggregates significantly higher than control after fresh litter was added into soils from siltstone in 7 days and the silt plus clay fraction was lower than control.Finally,after 30days,there was no discrepancy in each fraction between litter treatment and control,which suggest that soil have the function of recovering its stucture relying on decomposition of soil organic matter and activities of microorganisms and the process could be accelerated by adding litter into soil.(5)The 5'3C measurement confirmed that 16%and 28%of litter carbon were into 250-2000?m in two soils after 7th incubation days.The litter carbon enriched in 250-2000?m fraction may cause to the rising total carbon in this fraction.We also observed that the significant increase in total carbon of silt plus clay particles of soils from granity in the first incubation day.The increased total carbon mostly in a way of dissolved organic matter which was sorbed onto silt plus clay particles that had the stronger capacity of adsorption in soils from granity.(6)The PLFA and fungal biomass in whole soil and aggregates were stimulated to grow by litter input,however the relative content of bacterial and gram-positive bacterium were surpressed for growth to increase the ratio of fungi:bacterial.The PCA results showed the single PLFA have different distributions in each soils and suggested the composition of PLFA were different in both soils.In the soils from siltstone,the dominated PLFA were 18:2 ?6c,18:1 co9c and 17:1 ?7c which represented for fungal and gram-negative bacterium in the litter treatment and the dominated PLFA were branched fattic acids standing for gram-positive bacterium.The PCA results of soils from granity showed the abundance of gram-negative bacterium and fungi were higher in litter treatment and the dominated PLFA in control were saturated fattic acid standing for gram-positive bacterium and actinomycetes,which indicated that the fungal was stimulated to grow and the high distribution of gram-negative bacterium in aggregates when fresh litter was added to soils.The PCA results showed in the condition of soils from siltstone,>2000 ?m macroaggregates contained the fattic acids for gram-negative bacterium and actinomycetes.The specific PLFAs used for indication of fungal and gram-negative bacterium in 250-2000 ?m fraction.The main microorganism in 53-250?m microaggregates was gram-positive bacterium which was indicated for branched unsaturated fatty acids.The gram-positive bacterium were in<53 ?m silt plus clay fraction.In the condition of soils from granity,the high variation was in the composition of PLFA in different aggregates.Overall,three stabilization mechanisms play a important part in stabilizaing soil organic carbon in Chinese fir plantations.Soils in Chinese fir plantations have 18%organic carbon have resisitance to be decomposed by microorganisms.There were very significantly correlation between soil total carbon,labile carbon and recalcitrant carbon pool and positive correlation between the index of biochemical stabilization and carbon stock,which suggested that the process of soil organic carbon is influenced by the quality of soil organic matter.41-49%were protected by soil minerals in red soils of southern Chinese fir plantations,but the role of soil mineral type in the stabilization and adsorption of organic carbon was different depending on soil types.In the soils of Chinese fir plantations,primary minerals and phyllosillicates were important for stabilizing organic carbon.However,different mineral types adsorb selectivity the type of organic carbon.Primary minerals and phyllosillicates preferentially adsorb stable organic matter with higher ?13C and composition of organic matter,and free light fraction of organic matter was mostly instable organic matter,which means the organic carbon adsorped by minerals is stable by itself,and is stablized by minerals.The soil aggregates was restored through soil cultivation and the exogenous input of organic matter accelerated the formation of soil aggregates.There were two ways to provide physical protection fro organic carbon through the formation of soil aggregates.The exogenous and free organic matter was occluded into soil macroaggregates.In the other hand,the dissolved organic matter released from the decomposition of exogenous organic matter adsorped on soil particle(silt+clay)to formate stable organo-mineral microaggregate fractions which was transferred into the interior of soil macroaggregates.The result was consistent with the model of organic carbon physical protection by soil aggregates.The input of plant residue promoted the growth of fungi and gram-negative bacteria.Gram-negative bacteria and actinomycetes mainly distributed in macroaggregates(>2000 ?m ? 250-2000 ?m)which of habitat facilitate the growth of fungi and gram-negative bacteria.The growth of fungi and its hypha promote the stablization of soil aggregates.