| Soil organic carbon content is high and fertile in the mollisol soil region of northeast China,which plays an important role in ensuring regional food security,increasing soil carbon sinks,and coping with global changes.However,due to long-term overutilization,the mollisol soil of northeast China has been severely degraded and organic carbon has been significantly reduced.Erosion and tillage are the main driving factors of land degradation in this region.At present,researchers have systematically analyzed the driving role of soil erosion on mollisol soil degradation and organic carbon reduction,while there are fewer studies on organic carbon reduction caused by the change of biological processes under tillage disturbance,which limits the understanding of soil degradation mechanisms in this region.In this study,the typical agricultural soil in the mollisol soil region of Northeast China was taken as the research object.Through the indoor simulated tillage disturbance experiment,different tillage intensity(light,moderate and heavy tillage),tillage frequency(tillage every four-week,three-week,two-week,and one-week),soil water content(40% and 70% field capacity)and fertilization application(no fertilization and fertilization)treatments were designed to study their effects on soil and aggregate organic carbon,related processes and soil microbial properties,and the differences of tillage disturbance under different soil water and nitrogen application conditions were analyzed,and the relationship between soil organic carbon change and microbial properties under tillage disturbance was established.To study the relationship between soil organic carbon(SOC)changes and extracellular enzyme activity,microbial residual carbon and acterial community composition and functional microbial properties in detail,this study did not consider the factor of planting crops,so as to reduce the impact of crop growth differences on related processes.The main results of this study were as follows.(1)Soil organic carbon tended to decrease and then increase with increasing tillage frequency and intensity,and was significantly lower in the moderate tillage frequency or light and moderate tillage treatments(-0.8%,-0.5% and-0.8%)compared to the control,while the differences were not significant in the high tillage frequency and intensity tillage.Tillage frequency and intensity affects the soil aggregates content and stability,with higher aggregates content(+44% and +15%)and mean weight diameter(+26% and +9%)of the 2-7 mm grain size under moderate frequency and intensity tillage compared to the control group Soil aggregate organic carbon content was only affected by low frequency tillage and light tillage,with higher organic carbon content of of all aggreagtes under low frequency tillage compared to the control,and higher organic carbon content in 0.25-1 mm aggregates under light tillage(+3%).Increasing soil moisture and fertilization application attenuated the effect of tillage on soil organic carbon,but the response of soil aggregates and their organic carbon content to tillage was not affected by changes in moisture,nitrogen application and their interaction.(2)With the increased of tillage frequency,soil mineral nitrogen content decreased significantly,nitrogen mineralization rate did not change significantly,and carbon mineralization showed a decreasing trend.With the increased of tillage intensity,soil mineral N content and N mineralisation rate did not change significantly and carbon mineralisation tended to increase,but the differences were not significant.The response of soil N mineralisation to tillage frequency was not driven by moisture and fertiliser application,the effect of tillage intensity was not influenced by fertiliser application and interacted only with moisture conditions to influence soil nitrate-N content.The effect of tillage frequency and intensity on carbon mineralisation was only related to moisture conditions and its effect is greater at low moisture than at high moisture.(3)All tillage frequencies increased soil enzyme activity,while the effect of different tillage intensities on extracellular enzyme activity varied.Compared with the control group,the activities of β-1,4-glucosidase(BG)and β-D-cellobiose hydrolase(CBH)were higher(+8.50% and 10.62%),and the activities of phosphatase(AP)were lower(-8.53%)under intensive tillage,the activity of β-1,4-N acetylglucosaminidase(NAG)was lower under moderate tillage(-17.54%),and the activity of L-leucine aminopeptidase(LAP)was lower under mild tillage(-25.80%).All tillage frequency treatments reduced carbon limitation and increased phosphorus limitation,while different tillage intensity treatments reduced both carbon limitation and phosphorus limitation.Both increased moisture and fertiliser application contributed to the effect of tillage disturbance on enzyme activity,with moisture playing a greater role.The effect of tillage frequency on microbial nutrient limitation was significantly higher under high moisture conditions than under low moisture conditions,and significantly higher under fertiliser application than under no fertiliser application.Tillage intensity enhanced carbon limitation and reduced phosphorus limitation under low moisture conditions,and reduced soil carbon limitation and increased phosphorus limitation under high moisture conditions.(4)Soil amino sugar content,soil fungal and total microbial residue carbon and their proportion of soil organic carbon decreased significantly with increasing frequency of tillage.Light tillage treatments significantly reduced the amino sugar content(except for muramic acid)and light and moderate tillage treatments significantly reduced fungal and total microbial residual carbon and its proportion of soil organic carbon.The effect of tillage on microbial residual carbon as a proportion of organic carbon was related to both moisture and fertiliser application,with the effect being greater under low moisture conditions than under high moisture conditions,and under no-fertiliser conditions than under fertiliser conditions,and the effect of moisture on the intensity of tillage being more regulated than that of fertiliser application.(5)Soil moisture conditions,fertiliser application and tillage frequency/intensity all significantly altered soil bacterial alpha diversity,community and functional composition,and high moisture and fertiliser application amplified the effects of tillage frequency and tillage intensity The construction of soil bacterial communities at different moisture,fertiliser and tillage frequencies/intensities was dominated by homogeneous selection and drift.Specific ecological clusters of the soil co-occurrence network were more sensitive to tillage frequency/intensity effects than overall network properties,and this response was more pronounced under high moisture content and fertiliser application conditions.Combining the above studies on soil organic carbon and its related processes with microbial and enzymatic properties,and combining the analysis of variance decomposition and structural equation modelling,it was found that: soil organic carbon in the black soil region of northeast China is mainly influenced by aggregate distribution,microbial residues and microbial community composition,and this influence is related to tillage frequency,moisture and fertiliser application,and is less influenced by tillage intensity.The effect of tillage disturbance on soil organic carbon is mainly achieved by changing the distribution of aggregates and microbial residues,and is influenced by soil moisture and fertiliser application.Under different moisture conditions,the frequency and intensity of tillage disturbance affects soil organic carbon by changing the enzymatic activity of the carbon cycle(BG and CBH).Under unfertilised conditions,both frequency and intensity of disturbance affected organic carbon independently of microbial properties;under fertilised conditions,frequency of disturbance affected soil organic carbon indirectly by altering microbial residues,and intensity of disturbance affected soil organic carbon directly.These results lay the foundation for further study on the microbiological mechanisms of tillage affecting soil organic carbon,which have important theoretical value and guiding significance for improvement of mollisol soil fertility,nutrient resource utilization and mollisol soil land conservation. |
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