Changes In Microbial Biochemistry And Molecular Ecology Under Different Ecological Conditions And The Significance In Soil Quality Indicators

Paddy soil, as a unique anthropogenic type of soils in China, plays an important role in C sequestration and food security of China as well as a crucial role in the global CH4 budget. Fertilizer application, one of most normal agricultural management practices for more production, directly or indirectly causes changes in chemical, physical and biological properties of soils and in soil mircroorganisms' community structures and functions. In this present study, the influence of different fertilizer treatments on the microbial community structures in a paddy soil was conducted using polymerase chain reaction and denaturing gradient gel electrophoresis (PCR-DDGE), as well as on the soil microbial activities. The major results are showed as follow:1) Statistical analysis indicated a significant increase both in microbial gene abundance and diversity of the paddy soil by fertilization comparing to that under no fertilizer treatment. However, much higher soil microbial activity and gene diversity was reached by combined fertilization of inorganic and organic fertilizers than that under inorganic fertilization only. Coupling with the data of annual rice yield of the different plots, it was found that very close correlation of soil microbial gene diversity and abundance index to the level and stability of rice yields. Thus, the soil microbial gene abundance and diversity in paddy soil, as an important soil quality indicator, coped well with the primary production and ecosystem stability of rice fields. The present study indicated that high soil quality in paddy soils receiving combined fertilization of both inorganic and organic fertilizers sustained a high but stable rice production while supported high microbial activity and diversity. It suggested that chemical fertilizers should be applied in combination with organic fertilizers for sustainable agriculture. And soil quality changes under shifting agricultural performance in croplands could be traced by ecological footprints by using molecular techniques of PCR-DGGE.2) Significant changes of microbial community structure were found both with soil particle size fractions and under the different fertilizer treatments. More species and higher diversity of bacteria survived in the size fraction of＜2μm, in which the bacterial community structure was more significantly affected by fertilizer application practices than in the other fractions of 2-20μm, 20-200/tm, and 200-2000μm. However, the size fractions of＜2μm harbored less species and smaller diversity of methanogenic archaea than the other size fractions. While different fertilizer application affected slightly on the communities of methanogenic archaea in all size fractions. Although methanogens found in the particle size fractions were dominated by Methanomicrobialaceae and Methanosarcinaceae in this paddy soil, different fertilizer treatments influenced bacterial community structures in different particle size fractions to various degrees and bacterial species and their diversity was affected to a greater extent by the particle size than by fertilizer application practices. The present study evidenced that influence of both fertilizer practices and the size of particles was stronger on the methanogenic archaeal activity than on the methanogenic archaeal community, which may affect the methane production and emission from the paddy while different fertilization is performed. Smallest size fractions were suitable habitats for microbes with not only higher bacterial species richness but also diversities due to the vicinity between microbes, access to carbon resource outside the microaggregates, and smaller pore size as protective agent, rather than to providing carbon resources stored inside.Karst ecosystem is defined as the ecosystem that is restrained by the kast environment, which is characterized by its extreme vulnerability and subject to severe water and soil erosion under abuse of the shallow sloping lands of intensive agriculture. Rocky desertification, a land surface process of rapid decline in vegetation cover and in soil thickness and quality on the sloping karst lands, has occurred extensively since 1960s' as a result of growing population and intense cultivation in the area. For the last 5 years, national and provincial programs described in the outline of the 10th five plan for national economic and social development (2001-2005) have been initiated in some selected locations for controlling and remedying the rocky desertification. The purpose of the present study is to:1) demonstrate the changes of soil nutrients, soil microbial and biochemical activity (biomass, community diversity and enzyme) as affected with desertification and restoration procedures; 2) illustrate the linkage of these different parameters to soil quality and ecosystem health and functioning of the karst ecosystems; 3) search for which remedying method is more suitable for rebuilding degraded soil quality. The results are as follow:1) Results evidenced that cultivation and vegetation degradation caused a marked decline in most of examined parameters, while the degree of soil quality deterioration being associated with the time since native vegetation degradation. However, in karst areas, rather than metabolic quotient (basic respire/microbial biomass carbon), these parameters more responsive to disturbed ecosystem such as soil cultivation and vegetation degradation were: soil organic carbon (C_org), extractable extracellular carbon (C_ext), potential respiration (PR), basal respiration (BR), microbial biomass carbon, enzyme activities (urease and alkiline phosphatase), and bacterial community structures measured by molecular method. Moreover, microbiological parameters were more suitable to use as indicators to evaluating changes of soil quality, especially in predicting potential of soil degradation. All information in this study indicated that if karst system was not protected seasonally, soil nutrient pool and microbial function and diversity would decline dramatically with karst system degradation associated with rocky desertification.2) Soil nutrient pool (organic carbon (C_org), total nitrogen, and alkali-hydrolyzadle nitrogen) restored more rapidly than microbiological parameters (potential respiration (PR), basal respiration (BR), potential respiration (PR) microbial biomass, enzyme activities (urease and alkiline phosphatase)), indicating the most important using microbiologial parameters to evaluating the effect of the soil quality restoration. Moreover, among the several methods of recovering from farm land to forest or grass land, soil quality indicated by biochemical and biological parameters mentioned above increased better and more rapidly by naturally rebuilding methods than artificial methods, and the naturally recovering from farmland to grass was the best way to recover soil quality.3) Comparing to those parameters three years before, soil total carbon, nitrogen, phosphate and microbial biomass carbon have increased evidently under different types of vegetation restoration. The degree of the betterment of soil quality depended the type of vegetation, the grassland was the best to enhance soil quality for high restoration speed, however, cypress forest improved the soil quality slowly. The soil quality improved dramatically quickly at the initial stage from bare land to weak grassland, which is foundation of the restoration in the future.