Relationship Between Soil Microorganism/Microfauna And Organic Materials Decomposition

Stable isotope has frequently been used in ecology studies. In this study, we determinedthe decomposition of different substrates in different soil food webs with carbon stableisotopic technical. The soil food webs were changed from simple trophic levels to complextrophic levels. Right after determining the isotope fractionation of individual organism in abatch culture system with different simple carbon source, a simple food web containsbacterial, protozoa and nematodes were constructed to study the isotope fractionation indifferent trophic levels when predation happened. Subsequently, isotope fractionations weredetermined in a more complex food web with different organism diversities which wereobtained using dilution approach method. Meanwhile, carbon isotope compositions weredetermined when complex substrate added to soils that with different clay types and ironoxide contents decomposed. Priming effects were studied when a carbon stable isotopelabeled C4substrate was added to a C3soil. This study supplies a better way to understand theapplication of carbon stable isotope in soil food wed studies, as well as in farmland soiltransformation studies. It emphasized that organisms those in the primary trophic levels ofsoil food webs are extremely important for soil organic matter decomposition and nutritioncycle. The main results are shown as follows:1. Fractionation effects were measured when Escherichia coli was grown withC3-glucose and C4-glucose, under batch culture condition. The glucose from C3and C4plantwas fractionated differentially by E.coli. The δ13C-MBC of E.coli was significantly enrichedas compared with C3-substrate when using C3-glucose, while slightly depleted as comparedwith C4-sunstrate when using C4-glucose.The same trend was also happened to δ13C-CO2while without significantly difference. There were significant differences among differentmetabolic products of E.coli when using C4-glucose, especially those between fatty acid andacetate. The δ13C value of fatty acid was depleted as compared with that of C4-glucose, whileacetate was enriched. The results indicated that different way of using substrates may causethe isotope fractionation, since different carbon substrates those have different atomic ratiosmay pass through special bio-chemical ways in different speeds. Thus, it is important to takeindividual organism into account when study soil organic matter turnover and soil food webnutrition cycling. 2. A simple food web was constructed with bacteria and its feeder protozoa and/ornematode under liquid condition. The protozoa Cercomonas sp. and the nematodeCaenorhabditis elegans are common representatives of fast-growing bacterial feeders. Isotopefractionations were found in different trophic levels, particularly in the mixed tropic levelwhen protozoa and nematode both appeared. Meanwhile, the fractionation was much higherin the indirect trophic level where nematode was a predator. The results indicate that in asimple food web, preys may escape from predation through releasing some metabolicproducts. Thus, the predation will be slow down and the substrates turnover will also beaffected, the isotope fractionation will not be significant correspondingly. Nevertheless, whenthe trophic level is complex, the competition and predation between different predators willincrease the turnover of substrate, thus will increase the isotope fractionation. This will give atrace of carbon transformation in soil food web. Also, it helps to understand how the differenttrophic levels carry out the nutrient cycling in soils.3. Several different soil food webs were constructed through dilution approach method.Biolog-ECO results conform our hypothesis that within the high dilution level, a loworganism diversity occured. Isotope fractionations were determined when organism usingsimple carbon source in different food webs. The results show that the isotope fractionationincreased with decreasing dilution level. With the incubation, the δ13C-CO2values changedsignificantly in the low dilution level than that in the high dilution level. This supports ourinitial hypothesis that in the low dilution level, the food web is much more complex withdifferent diversity organisms. The substrate turnover was increased with increasingcompetition and predation among organisms. Furthermore, the fractionation indicates thegrowth stage of different organism, since with the incubation, more slow-growing organismsstarted to grow, hence, the fractionation increased with time. The present study demonstratesthat also carbon isotope shifts are coupled to trophic structure. It therefore shows that isotopicdiscrimination is a general phenomenon during substrate decomposition in soil and, further,that soil diversity can affect the variation of the isotopic composition of CO2evolution. Thisknowledge can add to a better understanding of carbon isotope fractionation during carbonturnover in soils.4. Simple mixing models have been criticized for underestimating the contribution fromC4plant material to soil organic matter formation in C3dominated system. This is allegedlybecause they do not take into account labelling inhomogeneity and changes in isotopecomposition during decomposition. In order to assess this experimentally, we constructedmicrocosms with essentially carbon free soil media. We followed the degradation of maizeleaves over the course of two successive laboratory incubations to determine carbon isotopic fractionation at successive stages of biodegradation. We measured changes in isotope13Csignatures of evolved CO2and in organic carbon during70days. We found that microcosmssubjected to compression of soil, or amendment with20%Montmorillonite clay, evolvedsignificantly less CO2, and therefore had a different13CO2signature over time. Compressedsoils contained fewer protozoa and more bacteria, which suggest unfavourable conditions forpredation. We found a strong correlation between the13C signature of CO2and the degree ofmaize-leaf decomposition, which indicates that in initial stages respired CO2was depleted in13C, while in the advanced stage there was a pronounced enrichment indicating turnover ofenriched microbial biomass. From the compressed soils we could harvest fungal myceliadepleted in13C relative to maize, indicating growth on relatively depleted carbon sources. Atthe end of the experiment we found depletion in addition to the one that could be ascribed topreservation of depleted maize carbon of up to0.5‰. Although we did not succeed inproviding a carbon free soil, the study indicates that an experimental mesocosm system foraccurate elucidation of13C signatures during soil decomposition of complex materials may beattainable.5. The effect of maize residues addition on farmland soil organic carbon and soilmicrobial biomass carbon were studied using13C stable isotope. The results showed that whenmaize residues were added, farmland soil carbon started to decompose, and carbondecomposed faster in the soil with higher carbon content. Nevertheless, the priming effect wassignificantly higher in the soil with lower carbon content when maize residues were added.Furthermore, the maize residues addition significantly increased both the total soil microbialbiomass and the microbial biomass of farmland soil.