Distribution Of Microbial Community In Soil Aggregates And Under Different Tillage Patterns

Soil is an essential and limited natural resource, and the fundamentals of agricultural and natural ecosystems, but many biological and biochemical processes occurring in the soil are still in "black box". Soil is made up of particles of different sizes and the study of distribution (or differentiation) of microbial community in soil structure is the first step to open the black box. Grasping the distribution of microorganisms in the soil structure is crucial to predict related biochemical processes such as the mineralization and de-nitrification of N, biological nitrogen fixation as well as the cycle of C, N etc, and the stability of soil structure as well as the degradation of organic pollutants in the soil. Based on the monitoring station of the fertility of purple soil of Southwest University, this paper preliminarily studies the distribution pattern of several microorganisms in the soil aggregates of different water-stability, as well as their functional mechanism with long-term(last for 18 years) research of the three different farming methods, that is, Combining Ridge and No-tillage(RNT), Conventional Tillage(CT), and Flooded Paddy Field(FPF), and explores the interaction between nitrogen cycle and microorganisms in soil aggregates. Through research, the findings can be concluded as the following ones:(1) In the soil structures of the three kinds of cultivation, the fractal dimension of soil samples of RNT is the minimum and lower than the CT with 0.049, and FPF with 0.322, meanwhile the bulk density of the RNT is higher than that of conventional tillage with 0.04 and the FPF with 0.107. In the composition of soil aggregates with different water stability, the percentage of soil aggregates in soil >4.76 mm is relatively higher and in RNT is further higher, reaching 40% of the totality. The percentage of soil aggregates of small size in the soil sample of the FPF has significantly increased, and the disparity among different aggregates reduces.(2)Soil organic matter has similar distribution pattern in aggregates of water stability, and measures of farming management have no significant impact on the distribution patterns of organic matter in aggregates. There are large amount of organic matter in aggregates in 2.0-0.25 mm particle-sized soil and the content is obviously higher than that of other sizes. There are smallest content of organic matter in <0.053 mm silt and clay component, which is 1/3-1/4 of the content in 2.0-0.25 mm particle aggregates. There are larger amount of organic matter of soil aggregates in RNT and FPF which the content is significantly higher than that of CT in the majority of particle sizes. The difference of the content of organic matter in the soil aggregates between different sizes decreases with same trend line under CT.(3)The content of the total nitrogen, available nitrogen, microbial biomass nitrogen, NO₃^--N and the urease activity in soil aggregates of water stability have a similar distribution pattern under three till-ages: Total nitrogen, available nitrogen and microbial nitrogen mainly distribute in large aggregates >0.25 mm particle size, urease activity is highest in the 2.0-0.25 mm aggregates, NH₄⁺-N and NO₃^--N in the >2.0 mm particle aggregates are higher than these in other particles. In the RNT, there are higher content of total nitrogen, available nitrogen and microbial biomass nitrogen in aggregates of every size than that in the FPF. There are lowest content in the soil samples of CT. The content of urease activity and inorganic nitrogen (NH₄⁺-N and NO₃^--N) in soil samples of RNT and FPF is totally higher and the content in soil samples of CT is low.(4) Microbial biomass carbon in the aggregates of soil samples of RNT and FPF has the same distribution pattern. The content of microbial biomass carbon in large-sized (>1.0 mm) aggregates is higher than that in small ones, and in aggregate of 2.0-1.0 mm soil samples of RNT the content of microbial biomass carbon comes to 1025mg/kg, which is obviously higher than that in other particle sizes. The microbial biomass carbon gradually decreases with the decreasing of particle sizes of aggregates in the soil samples of CT. The distribution of microbial biomass nitrogen in the aggregates of several particle sizes between 4.76-0.25 mm is high in the soil of CT and RNT, and there is also high content of microbial biomass nitrogen in the aggregates <0.25 mm particle size of the FPF. This study finds that the RNT obviously increases the content of microbial biomass carbon and nitrogen in aggregates and the distribution of microbial biomass in various levels of particle size is fundamentally consistent and the diversity obviously decreases.(5) The fungal and bacterial biomass in aggregates of water stability respectively has similar distribution pattern under three till-ages, and farming management has no obvious impact on the distribution pattern of fungi and bacteria biomass in different aggregates. In different soil aggregates, the quantity of fungal biomass is very large in large aggregates (>0.25 mm), and it is especially large in 4.76-2.0 mm particle-sized aggregates and the quantity of fungal biomass is the smallest in micro-aggregates (0.25-0.053 mm), and the former is 5 or 6 times of the latter. The bacterial biomass in aggregates of different particle sizes changes like the waves which 2.0-1.0 mm particle is most and the next is <0.053 mm particle.(6) There are large number of nitrobacteria in the aggregates of 2.0-0.25 mm and <0.053 mm particles and there is little in aggregates of 0.25-0.053 mm grain size. The number of nitrobacteria in the particle size >2.0 mm is not large but the activity of the nitrobacteria is significantly higher than the other particles. There is high nitrification intensity in aggregates >2.0 mm and <0.053 mm particle. This study proves that the number of nitrobacteria is larger and the nitrification intensity is higher in soil aggregates of RNT and FPF than that in CT. The amount of nitrobacteria in soil samples of FPF is 2.5 times of that in the CT and the nitrification intensity is 1.7 times higher.(7) The microbial biomass nitrogen, NH₄⁺-N and NO₃^--N of different aggregates have similar change trends during the process of mineralization. During the development process, the microbial biomass nitrogen firstly increases, then decreases and finally becomes stable, and the maximum is in aggregates of 2.0-0.25 mm particle size, but the range of change is relatively small. The NH₄⁺-N increases successively and the mineralization rate of NH₄⁺-N decreases gradually during the process of mineralization. The NO₃^--N first increases but while reaching the largest after 21 days, it begins to reduce during the process of mineralization, and the mineralization rate of NO₃^--N firstly increases and then decreases. The mineralization rate of NH₄⁺-N and NO₃^--N is highest in aggregates of 2.0-0.25 mm particle.Different fanning methods have certain effect on the change patterns of microbial biomass nitrogen, NH₄⁺-N and NO₃^--N in the mineralize process of soil aggregates. The microbial biomass nitrogen in soil aggregates of CT and FPF will reach the maximum after 7 days of cultivation, but in the RNT it will reach the maximum after 14 days. The microbial biomass nitrogen in soil aggregates of RNT is relatively higher in mineralization, which is 1.6 times and 1.4 times higher than that in FPF and CT. In the soil aggregates of RNT, the range of change of NH₄⁺-N and NO₃^--N and their mineralization rate are relatively higher, which are significantly higher compared with the soil samples of CT.(8) The microbial biomass nitrogen, total nitrogen and effective nitrogen in aggregates have obviously seasonal variations and good synchronization with the dynamic seasonal changes in soil aggregates in every tillage has been maintained, but different farming management can affect the dynamic seasonal changes of them in the soil aggregates. There are little microbial biomass in the soil aggregates in July and October under CT, and the quantity of microbial biomass decreases from April, followed by July, October and January under RNT; under FPF it gradually increases from the last October to the coming July.