Influences Of Long-term Conservation Tillage On Microbial Characteristics And Organic Matter Of Bulk Soil And Aggregates

The effects of conservative tillage on soil quality and soil organic carbon (SOC) dynamics and turnover are regulated by soil microbial activities and processes, but the mechanisms are not clear. This study was conducted to examine the influences of contrasting tillage and straw management systems on soil microbial biomass, residues, community structure, carbohydrates and SOC dynamics in bulk soil and various particle fractions, and their underlining mechanisms, based on a long-term tillage and straw amendment experiment in the North China Plain. The field experiment located at the Luancheng Agro-ecosystem Station of the Chinese Academy of Sciences. The cropping system was double cropping of winter wheat and summer corn. The experiment was initated in 2001 with three treatments: conventional tillage with no straw (CK), conventional tillage with straw incorporation (CT), and no tillage with straw covered on soil surface (NT). Soil samples were collected at the 0-5,5-10, and 10-20 cm layers in 2011 after winter wheat harvest, and soil bulk density of each layer was determined. The soil samples from 0-5 cm soil layer were further physically isolated into coarse particle organic matter (>0.25 mm), microaggregates (0.053-0.25 mm) and silt-and-clay (<0.053mm) fractions. We examined SOC and TN, phospholipid fatty acid (PLFA), amino sugar and neutral sugar concentration, and microbial community composition in bulk soil and various particle fractions. The thermal stability of soil organic matter (SOM) was also determined using the thermal analysis technique. The major findings are:A 10-year no tillage practice led to a significant stratification of SOC, soil microbial biomass and residues and carbohydrates in soil profile. When normalized to per dry soil weight, in the 0-5 cm soil layer, NT increased soil microbial biomass and residues by 48% and 53% as compared to CK and by 8% and 47% as compared to CT. Across the plough layer (0-20 cm), however, NT showed insignificant effects on SOC, soil microbial biomass and residues. When normalized to per SOC, NT decreased microbial biomass by 27% as compared to CT, but had a higher microbial residue accumulation relative to soil organic carbon by 11% as compared to CT and by 9% as compared to CK. The difference between turnover rates of microbial living biomass and nonliving residues indicated that NT decreased the mineralization of microbial residue (as a component of moderately active carbon pool), and therefore had the potential to improve long-term SOC sequestration.The results of carbohydrates showed that the ratio of hexose/pentose had a significant stratification in soil profile:as compared to surface layer, carbohydrates in the 5-20 cm layer existed mainly in microbes. Fungal amino sugars accounted for 52-61% of total amino sugar pool, manifesting that fungi might contribute to more microbial-derived organic matter than bacteria. In addition, conversion from CT to NT promoted fungal dominance in the 0-5 cm layer, but did not shift the overall soil microbial community structure in the plough layer. A positive correlated relationship between amino sugar ratio and SOC concentration was observed. Thus, NT could change SOC dynamics by altering microbial biomass, improving microbial residues sequestration and leading to a fungal-dominated microbial community.This study combined the concepts of physical fractionation of SOC and analysis methods to understand the characteristics of microorganisms in different particle size fractions. Our results showed that in the 0-5 cm soil layer, SOC, microbial biomass, residues and carbohydrates were higher in microaggregates than in silt-and-clay fraction; the soil microbial community in microaggregates was dominated by fungi whereas in silt-and-clay fraction was dominated by bacteria, manisfesting that microaggregates were major microenvironments for fungi. In addition, plant derived neutral sugar was mainly concentrated in microaggregates, whereas microbial derived neutral sugar was dominated in silt-and-clay fraction. Conversion from CT to NT improved SOC concentration in all fractions, and microbial biomass, residues and carbohydrates significantly in microaggregates and silt-and-clay fraction. NT enriched SOC, microbial biomass, residues and carbohydrates in microaggregates relative to bulk soil, microbial biomass, residues and carbohydrates.We applied thermal analysis to determine the influences of long-term conservation tillage on SOM stability. The results showed that NT did not alter the types of SOM, but decreased the thermal stability of SOM; and thermal stability of SOMs was higher in silt and clay fractions than that in coarce particle organic matter and in microaggregates. There was a positive linear relationship between SOM content from thermal analysis and that from elemental analysis.Overall, conservation tillage promoted SOC accumulation and improved soil quality, and thus is a viable management practice for reducing the greenhouse effect and maintaining soil sustainablity.