The Characters Of SOM Evolvement And Their Accumulative Machanisms For Robinia Pseudoacacia Forests In Zhifanggou Drainage Basin

Soils have the potential for C release or sequestration,can be affected by vegetationcover and land management. Many studies found the transformation of cultivated soil toforest ecosystem can change soil carbon sequestering. However, soil carbon stock change inforest chronosequence is likely to appear different results. Hence, accurate estimate on themechanism of SOC dynamics in forest chronosequences is necessary. The study investigatedthe variation patterns of organic matter of four selected Robinia pseudoacacia forests aged10,25,31,35years, and explore the rules. On basias of the rules, we made field and labortaryincubational experimentes to explore the effected factors. The main conclusions were showedas follows:(1)The study gives a anlysis on indexes such as SOC,Tatol nitrogen,particle size,CaCO3,CECand pH,etc,in the soil profile of Caragana Korshinskii Kom planted in1975,plowland since1974and Robinia pseudoacadia planted in1978. Soil organic carbonaccumulation in each site has a high correlation with the percent of <0.02mm diameter; somefactors in soil profiles exist unnormal changes in different depths, the <0.002mm clay matrialhas a substantial increase, represents the charactors of illuvial horizon; In the depth of100cm,it is C78>N75>H74on SOC content, however, the main difference was reflected in theupper30cm depth. The presence of ancient soil lead to16.684g/m3error on H74SOCassessment, as well as36.926g/m2Error on C78SOC assessment.(2)We investigated the variation patterns of organic carbon of four selected Robiniapseudoacacia forests aged10,25,31,35years, and a contrastive tillage site in similartopography condition. On average of20cm,40cm,60cm depth, SOC, active organic carbon(AOC), DOC gradually increase with the forest growth. Compared with tillage sites,AOC/SOC ratio, ROC (resistant organic carbon)/SOC ratio show a higher value, and LOC(slow organic carbon)/SOC ratio shows a lower value. DOC/DON is lower and UV absorptionat280nm shows a higher value in all forest sites. During forest restoration,25-year-oldRobinia pseudoacacia forest shows a special phase in four forest sites represented by depthdistribution. During forest restoration (i) not only AOC, but ROC take an accumulationprocess, Robinia pseudoacacia forest restoration contributes to SOC accumulation.(ii) DOC,DOC/DON, UV-absorbance appear changes but not obviously accordant to SOC, suggestingdifferent changes during SOC accumulation.(iii) SOC, AOC don't show a persistent increase in each depth of soil profile and which indicate a depth distribution. Our study found SOChad compounds-distribution and depth-distribution, but the relative machisms need to beexplore.(3)Analyzed the quantity of SOC, total nitrogen, particle size, CaCO3for the diversityanalysis and the correlation ratio in100cm soil profiles respectly in Caragana KorshinskiiKom forest planted in1975(N75),tillage since1974(H74) and Robinia pseudoacadia forestplanted in1978(N78). The results suggest that compared with tillage site, though SOC andtotal of nitrogen in two restored forest sites show obvious changes, the distinct diversity inC78appears mainly in the depth of0-20cm, and the same state appears at20-100cm for N75.At20-100cm in H74site, SOC, C/N show remarkable correlations with percent of0.002-0.02mm particle size. Below40cm in C78site, SOC are remarkable correlation with<0.002mm particle size, the correlation rate between C/N and <0.02mm changes from minusvalue for0-100cm depth to positive value for40-100cm. Our study indicate that after forestrestoration, changes of SOC and total of nitrogen mainly represent a obvious diversity at40cm and60cm soil layer for Caragana Korshinskii Kom forest, but diversity appearsat20cmfor Robinia pseudoacadia forest. SOC in deep soil layer of tillage and Robinia pseudoacadiaforest is closely adsorbed by fine particle size soil, with a steady quality.(4)We have observed the dynamics of SOC and it's relative factors of three depths(20cm,40cm,60cm) incubation holes, which were backfilled soils mixed with litter rags inadvance, in C78Robinia pseudoacacia forest, and took samples in March, July and November,2010. The results represent reasonal changes, at20cm and40cm, SOC, POC, TN in July arehighest, but decrease in November, and reverse states represent for DOC. Different with thoseat20cm, SOC at60cm decreases gradually in the process of incubation and TN and POChave increase trend, DOC in July appears a high value. During C accumulation, SOCincreases but decrease later, represents self-modulation function. DOC has different sourcesfor deeper soil layer with those at20cm, SOC suggests complex sources at60cm, theproduction of litters decomposition have limited effect.(5)Took soil and mixed litter samples in31year old Robinia Pseudoacacia forest anddetermined the accumulative CO₂-release of soil samples and DOC-removal soil samples(100%water content only) at three soil layers (20cm,40cm,60cm), three controlledlitter-mixed soil samples (mixed by complex litters, included Robinia Pseudoacacia littersand herbage litters), in laboratory under the different soil moisture levels of100%,20%,2%water content respectively. The results showed that the CO₂-release levels in different depthswere suggested by20cm>60cm>40cm in controlled100%and20%water content; In the condition of100%water content, when DOC were removed, the accumulative CO₂-releasequantity of20cm soil samples decreased, but obviously increase for samples at40cm andhad no marked change at60cm; The accumulative CO₂-release of complex litter-mixed soilsamples had highest value in the condition of100%water content, those of simply RobiniaPseudoacacia litter-mixed samples was obvious higher than that of herbage litter-mixedsamples in20%and2%water content, but slightly lower in100%water content. Theasynchronous effect of water content levels is suggested on the respiratory intensity of the soillayers with different SOC compositions and litter kinds, heavy precipitation removes DOCfrom soil subsurface layers, can weaken the soil respiratory intensity.(6)In two-year incubation experiment, litter-mixture soil (outside bags) and nolitter-mixture soil (inside bags) were insulated by nylon cloth in incubation holes, changes inthe LDOM portion of the soil were then monitored by comparing the WEOM levels outsidethe bags with inside the bags., In July, the ratio of locked WEOC (LDOC) to WEOC outsidebags (LDOC/WEOC) was low in the topsoil, and increased gradually with depth while thelocked WEON (LDON) to WEON ratio outside bags (LDON/WEON) suggested no obviousrule. At20cm In July, LDOC/WEOC were lower and LDON/WEON were higher than thosein March and November, solutions of the soil samples had highest UV-absorbance at280nm,The LDOC:LDON ratio at20cm was comparable in July with, but higher in March andNovember than WEOC:WEON ratio of samples outside bags, which implied thatLDOC:LDON has little difference compared to that of RDOM in July, but represents highervalue at other times of the year. The study proves that LDOM has seasonal variability anddepth distribution, which brings a difficult to distinguish the difference between WEOM andDOM (RDOM).