| Litter is the important component of connecting soil carbon pool and plan’s, and itis the material and energy sources of the decomposer. So it has a significant impact on carboncycle in terrestrial ecosystem. This study adopted field decomposition method and incubationmethod and measured the contents of soil organic carbon, DOC, MBC, LFOC and PLFAs inthe process of litter decomposition, with the purpose of researching the effect of litterdecomposition on soil organic carbon and soil microbial community structure in the processof vegetation restoration in loess hilly-gully region, and revealing the mechanism oftransformation and fixation of soil organic carbon affected by soil microorganism. The mainconclusions are as follows:(1) The effect of litter decomposition on soil nutrients was greatly influenced byclimate condition in the field, and the effect was mainly presented in0-5cm soil layer.Affected by litter decomposition, soil organic carbon increased rapidly from August toNovember in2012, and then stayed stably. In June and July, soil organic carbon decreasedrapidly. The content of soil organic carbon affected by litter decomposition was higher thansoil without litter. Soil total nitrogen mainly comes from organic matter formed by plantresidues decomposition and synthesis, so the change of total nitrogen is similar with organiccarbon. Soil NO3--N was significantly fluctuated with time. From August to November in2012, soil NO3--N decreased and NH4+-N increased, and soil NO3--N increased rapidly fromNovember to March next year. Litter decomposition had an effect of increasing soil NO3--N,but little impact on NH4+-N. Different plant types had different effect on soil nutrient, andCaragana microphylla had the most significant influence on soil organic carbon, totalnitrogen and NO3--N. Caragana microphylla is the main tree species in local afforestation,having significant effect on improving soil nutrient. Root decomposition had a similar effectwith leave decomposition on soil nutrient. The effect of root decomposition on soil organiccarbon was even higher than leave decomposition, which showed the considerable role ofroot in improving soil nutrient.(2) The results of mineralization characteristics of litters when they were added tosoil showed that: organic carbon mineralization of soil with litters involved two stages.Mineralization rate was very high and declined quickly in prior period (0-11d), and then became steady in later period. The final mineralization rates were5.37~13.98percent ofinitial rates; Affected by mineralization rate, the cumulative CO2-C increased fast and thenturned slow. The cumulative CO2-C in prior period even accounted for30.43~50.51percentof that in the whole incubation period; We simulated the curves of the cumulative CO2-Creleased by litters according to the first order kinetic equation and got the data ofmineralization rate constant (k) and decomposition rate of litters. The decomposition raterevealed that Agropyron cristatum> Stipa bungeana> Artemisia gmelinii> Artemisia giraldii> Thymus mongolicus, which showed that Agropyron cristatum and Stipa bungean is easierto decompose than others. The results indicated that the mineralization rate constant (k) anddecomposition rate showed a significant negative correlation with lignin content of litters.That meant that the lignin content of litter was related to the mineralization rate anddecomposition rate, and the more the lignin content was, the harder the litter decomposed.(3) In incubation experiment, soil TOC, DOC, MBC and LFOC added litters werehigher than that without litter. The content of soil organic carbon without litter was2.60~2.90g/kg, and the content of soil added litters was8.80~13.90g/kg. Soil TOC decreased with theloss of organic carbon caused by mineralization. Soil DOC increased at the7thday and thendecreased because of using by microorganism. The correlation coefficient between soil MBCand LFOC added litters was0.817~0.995, which showed a significant correlation. Soil LFOCwas the important source of carbon and energy to soil microorganisms. Without the influenceof climatic conditions, the chemical composition of litters determined the litter decompositionprocess. The variation trend of soil organic carbon was similar between the soil addedAgropyron cristatum and Stipa bungeana, also similar between Artemisia gmelinii andArtemisia giraldii. This showed that the similar chemical composition caused this result. Atthe14thday, soil DOC added Agropyron cristatum and Stipa bungeana maximized and soilLFOC was completely decomposed. The soil added Artemisia gmelinii and Artemisia giraldiishowed the same result at the28thday, which revealed that Agropyron cristatum and Stipabungeana were more easier to decompose than Agropyron cristatum and Stipa bungeana.(4)19PLFAs were determined in the result, and16:0,18:1ω9c,18:2ω9,12and18:1ω9t were more than others. The percentage in total PLFA were respectively22.26~45.58%,1.66~49.48%,3.62~15.41%and1.59~10.40%. The content of soil total PLFAadded litters (5.78~61.79n mol/g) was higher than that without litter (1.71~6.91n mol/g).The total PLFA decreased with litter decomposition. Compared with soil without litter, thepercentage of fungi and gram-positive bacteria increased and the bacteria decreased in soilmicrobial community with litters. The fungi/bacteria and G+/G-also increased because oflitter decomposition, which showed that the fungi and gram-positive bacteria played an important role in litter decomposition, and litter decomposition caused the change of soilmicrobial community structure. The chemical composition is the most important impactfactor to microbial community structure in incubation experiment. Soil total PLFA, fungi, andfungi/bacteria showed that Stipa bungeana> Artemisia gmelinii> Thymus mongolicus, andthe bacteria and gram-positive bacteria showed that Stipa bungeana <Artemisia gmelinii <Thymus mongolicus. This revealed that Thymus mongolicus was harder to decompose andmore favorable to the increase of fungi, and Stipa bungeana was more favorable to theincrease of bacteria because it was easier to decompose.(5) Soil TOC showed significant positive correlation with total PLFA and fungiPLFA. This revealed that the content of soil TOC was an important impact factor to thequantity of soil microorganism. Soil LFOC and MBC had similar correlativity with soilmicrobial group, which showed that they were all significant correlative with fungi PLFA andfungi/bacteria. This revealed that the change of soil MBC is mainly caused by the change offungi’s quantity, and fungi played an important role in soil organic carbon transformation.Soil TOC without litter was only significant correlation with gram-positive bacteria. Thisshowed that fungi had little effect on organic carbon transformation in the soil without litterand the gram-positive bacteria showed important effect. |
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