Influence Of Long-term Fertilization On Microbial Communities Within Soil Aggregates Under Different Cropping Systems

Soil plays a key role in major global biogeochemical cycles and hosting the great amount of organisms on land. Hence, soil aggregate, as basic units of soil structure, is an important place where soil biogeochemical cycles happen. Microorganisms are the most active component of soil ecosystem. Their activities affect formation and stability of soil aggregate, and cycles of soil carbon and nitrogen. Therefore, it is essential to study distribution of soil microorganism within soil structurefor understanding these biogeochemical processes. Based on long term experiments in the North China Plain, we studied effects of fertilization on soil nutrients, microbial community structure within bulk soil, and concentrations of soil organic carbon and nitrogen and microbial community in soil aggregates under different cropping systems.In this study, three experiments of cropping systems were selected:(1) vegetable greenhouse systems with organic management and conventional management; (2) wheat-maize rotation systems with organic fertilizer, chemical fertilizer and no fertilizer application, and a natural fallow; (3) wheat/alfalfa-maize rotation systems with organic farming (alfalfa-maize rotation), conventional farming (winter wheat-summer maize rotation), no nitrogen fertilizer application (winter wheat-summer maize rotation) and a natural fallow. The soil aggregates with different size, including macroaggregate (> 0.25 mm), microaggregate (0.053-0.25 mm) and silt and clay (< 0.053 mm), were isolated using wet sieving method. Meanwhile, microbial communities in these aggregates were studied with quantitative real-time PCR, Terminal restriction fragment length polymorphism (T-RFLP) and clone-sequencing methods. The results showed that:In the vetetable greenhouse systems, organic management improved soil nutrient, SOC and TN within soil aggregates. Compared with conventional management, the concentrations of SOC and TN within soil aggregates were 13%-153% and 30%-146%, respectively. Uner both management systems, the concentrations of SOC and TN in aggregates declined when fraction size decreased. The contents of SOC and TN in microaggregates explained 42%-65% of SOC and 43%-66% of TN, respectively, in bulk soil. Compared with conventional management, organic management siginifcantly increased microbial abundance in soil aggregates while increased biodiversities of microbial community without significance. Redundancy analysis demonstrated that the distribution of microorganism denpended on soil aggregate structures. The results of T-RFLP and clone library indicated that Proteobacteria, Actinobacteria and Bacteroidetes were found in all three aggregate fractions, while Acidobacteria was found only in microaggregate and silt and clay fractions. Soil fungi concentrated in the macroaggregate fractions. The RFLP of 257 bp (Ascomycota),272 bp and 294 bp (Zygomycota) were found in all three fractions.In long-term wheat-maize rotation system, organic fertilization increased soil nutrient and chemical fertilizer application only increased soil available phosphorus content. Compared with no fertilizer treatment, concentrations of SOC and TN increased within all aggregate fractions under organic fertilization while increased within microaggregate under chemical fertilization. However, the concentrations of SOC and TN decreased in macroaggregate and silt and clay fractions. The bacterivorous nematode showed a positive relationship with bacterial biomass while the fungivorous nematode had a negative relationship with fungi. The soil mean weight diameter and concentrations SOC and TN showed the positive relationships with promote microbial biomass. The microbial abundances in all aggregate fractions show a similar trend to SOC and TN, i.e. decreasing with fraction size decreases. The bacterial phylum of Proteobacteria mainly inhabit in macroaggregate and microaggregate while Bacteroidetes in microaggregate and silt and clay. The fungal community composition within macroaggregate was similar to that within bulk soil, and meanwhile the composition within microaggregate was similar to that within silt and clay fraction.In the third experiment, cropping systems had different effects on concentrations of SOC and TN, microbial community composition within soil aggregates. Compared with no nitrogen fertilizer treatment, organic farming increased the concentrations of SOC and TN within all size aggregates while conventional farming increased SOC concentration in silt and clay fraction and TN concentration in microaggregate and silt and clay fractions. The concentrations of SOC and TN in soil aggregates decreased following fraction size decreased. The contents of SOC and TN within microaggregate explained 52%-66% of SOC and 52%-69% of TN, respectively within bulk soil. Compared with no nitrogen fertilizer treatment, organic and conventional farming increased abundance and diversity of bacteria, decreased those of fungi. The fungal abundance was positively affected by the concentrations of SOC, TN and Olsen P, and the Fu2 guild while was negative affected by the Fu3 guild. The bacterial phylum of Proteobacteria distribution mainly concentrated in macroaggregate, while Acidobacteria mainly in silt and clay fraction. The distribution of fungal communities was mainly influenced by cropping system, for example the organic and conventional farming had significantly different effects on fungal community composition.In conclude, the fertilization improved the concentrations of SOC and TN, microbial community composition within soil aggregates under different cropping systems. However, the fertilization had different effects on bacterial and fungal community composition among different cropping systems. Bacterial community distribution within soil aggregate was mainly influenced by aggregate structure, SOC and TN concentrations within aggregate while fungal community was mainly affected by cropping system and fertilization.