| Conventional cultivation with chemical nitrogen fertilizers has resulted in lower nitrogen fertilizer utilization and soil degradation,and "nitrogen reduction" has become a hot topic of research in rice production.At the same time,organic food consumption is growing rapidly around the world,but organic farming in China has started late,and there is a lack of systematic research on the long-term effects of organic cultivation on arable land quality and rice production.In this study,the cultivation methods（organic cultivation[OF]and conventional cultivation[CF]）and N levels(pure N 180 kg·hm-2[N12]and pure N 270 kg·hm-2[N18])were combined into four treatments for field trials from 2020 to 2021.Nanjing 46（a high yield rice with superior taste）and Huaixiangjing 15（a high yield rice with moderate taste）were selected.By studying the correlation between soil environmental factors and bacterial community characteristics,rice population growth indicators,nitrogen,phosphorus and potassium uptake and rice yield and quality,the systemic laws of cultivation methods and nitrogen levels on soil nutrients-plant nutrients-rice yield and quality formation were revealed to provide a theoretical basis for exploring reasonable cropping systems and nitrogen application rates.The main results are as follows:1.Effects of cultivation methods and nitrogen levels on soil fertility and bacterial community characteristics:In terms of soil nutrients and enzyme activity,organic cultivation increased TN,AN,AP,SOM content,alkaline phosphatase and sucrase activities in 0-10 cm soil,while decreased SOM content and urease activity in 10-20 cm soil.Under organic cultivation,the activities of sucrase,urease,and alkaline phosphatase at the N12 level increased by 5.22%,16.98%,and 26.38%compared to the N18 level in the mature 0-10 cm soil in 2020,and increased by 11.18%,29.61%,and 6.42%in 2021,respectively.This indicates that organic cultivation with appropriate nitrogen reduction treatment can improve soil enzyme activity.It was demonstrated by a diversity indices that the N18 level remarkably undermined soil bacterial richness and diversity.Venn diagrams and NMDS analysis manifested great difference in bacterial communities and an apparent clustering tendency under different treatment conditions.Species composition analysis indicated that the total relative abundance of Proteobacteria,Acidobacteria and Chloroflexi retained stable in paddy soil.LEfSe results revealed that a low nitrogen organic treatment could elevate the relative abundance of Acidobacteria in 0-10 cm soil and Nitrosomonadaceae in 10-20 cm soil,thereby tremendously optimizing the community structure.Moreover,Spearman correlation analysis was also performed,which proved the significant correlation of diversity with enzyme activity and AN concentration.Additionally,redundancy analysis disclosed that the Acidobacteria abundance in 0-10 cm soil and Proteobacteria abundance in 10-20 cm soil exerted conspicuous influence on environmental factors and the microbial community structure.2.Effects of cultivation methods and nitrogen levels on plant growth traits and nutrient absorption:After heading,Nanjing 46 had a higher dry matter allocation ratio of leaves under N18 compared with N12,but reduced the dry matter allocation of panicles at the later stage,indicating that moderate N reduction could optimize and balance the dry matter allocation ratio of both leaves and panicles.The variation of various morphological indicators of the root system varied by variety and period.In conventional cultivation,The nitrogen content of panicles of Nanjing 46 and Huaixiangjing 15 under N18 in 2021 increased by 13.49%and 31.98%respectively compared to N12,with significant differences;In organic cultivation,it increased by 4.94%and 1.99%respectively compared to N12,with no significant difference;It can be seen that increasing the application of nitrogen fertilizer in organic cultivation does not significantly increase the nitrogen content of panicles.The phosphorus content of panicles of Huaixiangjing 15 under conventional cultivation during the maturity period in 2021 was 51.03%and 36.84%higher than that under organic cultivation at the N12 and N18 levels,respectively;However,the phosphorus content of panicles of Nanjing 46 remained at 1.72-1.93 g·kg-1 and 2.61-2.79 g·kg-1 during the heading and maturity periods in 2021,and there was no difference between treatments;It shows that the phosphorus content of Huaixiangjing 15 is more influenced by the cultivation method than Nanjing 46.In terms of potassium content,its pattern is consistent with that of phosphorus content.3.Effects of cultivation methods and nitrogen levels on rice yield and quality formation:Both rice species would have lower theoretical and actual yields under organic cultivation than conventional cultivation.In 2021,the actual yield of Nanjing 46 under organic cultivation was 3.10%and 3.52%lower than that of conventional cultivation under N12 and N18,respectively;and in 2021,Huaixiangjing 15 was 10.25%and 6.89%lower than that of conventional cultivation,respectively;indicating that Nanjing 46 had a stable yield advantage.In terms of processing quality,organic cultivation significantly improves the milled rice rate and head rice rate.In terms of appearance quality,organic cultivation reduces chalkiness rice rate and chalkiness degree.In terms of cooking and taste quality,OF and N12 treatments reduced the protein content of Huaixiangjing 15,but had no effect on Nanjing 46.In the RVA spectrum characteristic value,organic rice showed higher breakdown value and smaller setback value,especially the OFN12 treatment significantly optimized the grain structure.In the cross section of the grain,the number of cavities in the grain interior showed organic cultivation<conventional cultivation.In the characteristic value of DSC spectrum,both organic and low nitrogen treatments could reduce the termination temperature,gelatinization range,enthalpy and peak index of starch.In terms of starch crystal structure,in 2021,compared to 2020,the starch peak intensities between CFN18,OFN12,and OFN18 treatments gradually approached.Based on the above results,organic cultivation with a nitrogen application rate of 180 kg·hm-2 can effectively balance dry matter distribution and improve nitrogen fertilizer utilization efficiency,improve various environmental factors in paddy soil,optimize soil bacterial community structure,enhance soil fertility level,and have the potential for increasing yield and quality of rice.This study can provide a theoretical basis for sustainable cultivation in the Jianghuai region. |
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