Effects Of "Organic Fertilizer Substitution" On Soil Enzyme Activities And Nematode Communities In Northeast Rice Fields

As an important commercial grain base in China,rice fields in Northeast generally rely on the excessive application of chemical fertilizers,especially nitrogen fertilizers,in their growth process,leading to non-point source pollution,ammonia volatilization,greenhouse gas emissions,soil fertility decline and other ecological and environmental problems.Appropriate substitution of organic fertilizers for nitrogen fertilizers（referred to as"organic fertilizer substitutions"）can reduce environmental pollution and promote the sustainable development of farmland systems while maintaining high crop yields.Soil enzyme activity and soil nematodes are important indicators of soil fertility.Understanding the soil enzyme activity and nematode community response to organic fertilizer replacement is of great significance for guiding the rational application of this technique.Therefore,this study took Northeast paddy fields as the research object,relying on the"organic fertilizer substitution"field experiment,through field sampling and laboratory experiment analysis to study the influence of organic fertilizer substitution technology model on the soil enzyme activity and nematode community in Northeast paddy fields,and then discussed the driving factors of soil enzyme activity and nematode community changes under the organic fertilizer replacement model.The main findings are as follows:（1）Compared with conventional fertilization（CF）,soil organic carbon（SOC）,available phosphorus（AP）,porosity(R_0.25)and carbon to nitrogen ratio（C:N）were significantly increased by13.3%,9.4%,9.2%and 8.9%under the organic fertilizer substitution mode,but there was no significant change in rice yield.（2）Organic fertilizer replacement significantly increased the total enzyme activity,and the enzyme activity related to the carbon metabolism e.g.α-glucosidase（AG）,β-glucosidase（BG）,β-cellobiohydrolase（CBH）,β-xylosidase（XYL）,and the enzyme activity related to the nitrogen metabolism such as N-acetylglucosidase（NAG）related to carbon metabolism（NAG）and L-leucine（LAP）and the enzyme activity related to phosphorus metabolism i.e.phosphatase（PHO）activity.Soil enzyme activity representing carbon to nitrogen ratio,namely BG/（NAG+LAP）and NAG/LAP,also had an increasing trend,while the evenness index of enzyme activity changed insignificantly.（3）Correlation analysis showed that there were significant positive relationships between soil SOC content and aggregate stability(R_0.25)and most enzyme activities,indicating that substrate resources and physical environment were important factors driving the changes in enzyme activity.The result of variance partitioning further showed that the contribution of R_0.25（6.28%）was slightly higher than that of SOC（5.52%）.In addition,the enzyme activity representing carbon to nitrogen ratio（BG/（NAG+LAP））has a negative correlation with the substrate carbon to nitrogen ratio（SOC/TN）,indicating that the enzyme group index can be used to reflect the quality of substrate resources.（4）Under the organic fertilizer replacement treatment,the total number of soil nematodes increased by 40.67%and 31.33%in June and September,respectively.The relative abundance of bacterial-feeding nematodes（BF）,fungal-feeding nematodes（FF）,and plant parasitic nematodes（PP）were all significantly reduced,while the relative abundance of predatory/omnivorous nematodes（OP）was significantly promoted.There were close correlations between the soil TN,R_0.25,SOC and BF,FF,PP and OP.TN explained the largest variation（28.4%）for all trophic groups.In addition,the substitution of organic fertilizers significantly increased the abundance,Shannon Index and evenness index of soil nematodes,indicating that organic fertilizer substitution can maintain the growth of more species of soil nematodes.（5）The organic fertilizer substitution treatment increased the maturity index,enrichment index,structure index and nematode channel ratio（NCR）of free-living nematodes,and reduces the channel index（CI）.These findings indicate that organic fertilizer substitution alleviate soil environmental stress and improves the contribution of bacterial decomposition channels in the soil debris food web.There were significant correlations between the soil SOC content and aggregate stability and the diversity index and ecological index of soil nematodes.The variance partitioning results showed that R_0.25explained more of the variation in soil nematode diversity than SOC（37.27%vs.16.18%）.This study shows that the substitution of organic fertilizers improves soil enzyme activity and soil nematode community structure and diversity.The increase in substrate resources and the improvement of soil pore status are the main driving factors for soil enzyme activity and nematode community changes.These findings provide a scientific theoretical basis for reducing the application of nitrogen fertilizer and improving soil fertility in Northeast rice fields.