| Straw treatment and nitrogen（N）application decision-making are two important cultivation management measures in the rice-rape rotation that is one of the most important cultivation patterns in southern China.As a way of nutrient return,straw incorporation（SI）can improve soil structure,increase soil available nutrient content,and provide a solution to reduce N fertilizer use and ensure yield.However,there is still a lack of systematic explanation on how to determine the appropriate amount of N application in rice under SI to achieve N reduction and stable yield.For this reason,based on the positioning experiment of the annual SI combined with N fertilizer application and using straw removal（SR）plus 0 kg ha-1（N1）,75 kg ha-1（N2）,150 kg ha-1（N3）,225 kg ha-1（N3）as control,we studied the response characteristics of rice growth（dry matter accumulation,canopy agronomic characteristics,photosynthesis physiology,root characteristics,yield and yield composition）,N use（including N absorption and distribution）and soil properties（physical,chemical and biological properties）to SI combined with N fertilizer application.At the same time,we also analyzed the difference in the soil-rice system correlation network between SI combined with N fertilizer application and the control,and compared their sustainability index.Through these studies,it was clearly understand the agronomic physiological basis on increasing and stabilizing yield,N reduction effect and soil improvement ability under SI combined with N fertilizer,which provided theoretical support for the decision of N fertilizer in the rice sustainable development under SI.The main research results are as follows:（1）SI could increase and maintain rice yield,and improve rice quality,but its effect was affected by N application rate and climate conditions.During 2020-2021,the yield of low N condition（N0,N1）under SI was higher than that of under SR,and the difference was significant（16.97%higher）under N0 level in 2021.Under the condition of high N（N2,N3）,the yield of SI in 2020 was lower than that of SR,however,it was higher than the yield of SR in 2021.In 2021,the variation coefficients of yield and yield composition under SI combined with N fertilizer application all were smaller than that of the control.Compared with SR combined with N fertilizer application,the advantage of SI to increase yield was to increase the effective panicles number so as to obtain higher total spikelets m-2.The correlation analysis showed that the yield under SR was significantly related to the effective panicles number,while the yield under SI was related to the seed-setting rate,which indicated that the management measures for rice yield increase under different straw treatments were different.Climatic conditions had very significant effects on seed-setting rate and 1000-grain weight,and were also the main factors affecting rice quality（especially brown rice rate,chalkiness,alkali loss value and protein content）.Although there was no significant difference rice quality between SI and SR,on the whole,SI improved various indexes of rice quality and changed the specific response of N application rate to rice quality.（2）There were difference in the response strategies of rice aboveground and underground part to N application rate between SI and SR.Compared with the SR,plant height,leaf area index,and dry matter accumulation under SI had shown advantages at jointing stage in rice（except N3）.The dry matter after heading was the main source of rice yield,with the contribution rate reaching 86.03-112.75%.For leaf organs,as a whole,with the increase of N application,canopy leaf growth,leaf area and N accumulation,SPAD value and net photosynthetic rate increased,while transmittance and specific leaf weight decreased.In addition,the“quick investment”strategy was adopted for leaf organs under SI compared with SR,and the leaf dry weight and its proportion of SI were higher than those of SR（the difference of leaf dry weight at heading stage reached a significant level,P≤0.05）.However,the leaves response under the SI to the investment of N belonged to the“leaf increase”type,therefore,there were significantly positive correlation between grain yield and leaf organ related indicators.While it belonged to“leaf passing”type under SI,and there were no significant advantage in photosynthetic characteristics for leaf under SI.Which increases the risk of yield reduction caused by adverse climatic factors under high N treatment.Besides,the rice root system mainly adopts the response strategy of“laziness high efficiency”to the N application rate,that was,under low N conditions,the rice root system actively grows with relatively high root dry weight,root length,root tip number and root absorption area;while under high N conditions,it had relatively high root activity and average root diameter.The complexity of the relationship between root and shoot indexes of rice was affected by whether straw is returned to the field or not.For SR,the absorption area and root length of rice root system were the main correlation points.However,the root dry weight and the of root tips number except the absorption area and root length were also significantly correlated with the above-ground part under the SI.（3）N uptake,accumulation,distribution and use in rice were affected by straw treatment and N application rate.Under the SI,the N absorption of rice leaf and root would be actively promoted before heading.At jointing and heading stage,SI had a higher proportion of leaf N.Especially at jointing stage,N accumulation in roots of N1 and N3conditions under SI was 27.54%and 26.08%higher than that of N1 and N3 conditions under SR,with a significant difference（P≤0.05）.The accumulation ratio of P-N and K-N under low N condition was higher than that under high N condition,and the accumulation ratio of K-N under SI was higher than that of condition of SR.Due to the high yield and N accumulation at the N0 level,the N agronomic efficiency and contribution rate of SI were low.And the N uptake efficiency and partial factor productivity varied with the year and N application rate.Only N3 level had N surplus under SR,however N1 level could maintain relatively low and stable N surplus under SI.The model estimated that the N reduction efficiency of SI could reach more than 40%（based on fertilizer effect function model）or19.34-26.75%（based on leaf value model）,and the recommended N application rate of Yixiangyou2115 was 88 kg ha-1under the annual SI in the rice-rape rotation.（4）The responses of soil physical,chemical and biological property to N application rate were affected by straw treatment.Although there was no significant difference in soil physical properties between SI and SR,the analysis of X-ray micro CT scanning technology under N2 condition showed that SI could improve the soil pore structure.Especially in the7-14 cm cultivated-layer,the numbers of connected pores and independent pores under SI were significantly higher than that under SR,and the pore distribution was more uniform under SI.SI could improve soil nutrients,enhance the correlation between nutrient indicators,especially significantly（P≤0.05）increase the content of available potassium in the 0-14 cm cultivated-layer.And the content of available potassium and its increase range under high N conditions were higher than those under low N conditions.The composition and structure of soil bacterial and fungal communities varied with rice growth period,straw treatment and N application rate.LEfs E analysis showed that Actinobacteria were significantly enriched at jointing stage;Acidobacteria,Gemmatimonadetes,Latescibacteria,Nitrospirae,Planctomycetes,Rokubacteria,Spirochaetes and Ascomycota were significantly enriched at heading stage;Bacteroidetes,Firmicutes,Proteobacteria,Rozellomycota,Basidiomycota and Mortierellomycota were significantly enriched at maturity stage.Bacteroidetes and BSV26were significantly enriched in the three growth stages under SR,while SI significantly enriched Proteobacteria and Variovorax at heading and maturity stage.However,at mature stage,the enrichment degree of fungi community under SI was weaker than that under SR.Microbial alpha diversity had no significant response to straw treatment and N application rate,but Non-metric multidimensional scale analysis showed that N application rate had a certain impact on microbial beta diversity.And SI could promote and enhance this effect.（5）Whether straw returned to the field or not effected the response of the soil-rice system to N application rate and its internal correlation.Based on coefficient variation as the evaluation standard for the sensitivity of the response of the“soil-rice”system to N application rate,microbial related indicators were more sensitive,followed by soil available nutrients and the rice canopy leaf traits and photosynthetic characteristics.On the whole,the sensitivity of aboveground growth traits,mineral element absorption and distribution,and soil physical and chemical properties under SR was higher than that under,while the sensitivity of root characteristics and bacterial communities under SI was relatively higher.In the correlation network constructed based on variables screened by the sensitivity and random forest importance scores,in addition to N application rate and yield,there were 54nodes under SR,forming a complex correlation network,while there were only 32 nodes under SI,and the correlation between nodes was simple.Leaf area indexes at jointing or heading stage were the main node of the two straw treatments.In addition,leaf dry weight at heading stage,specific leaf weight of L1,specific root length at joining stage,root-shoot ratio at heading stage,N photosynthetic use efficiency,available potassium soil,specific root length at jointing stage,Paraphaeosphaeria and Coprinellus at jointing stage and Firmicutes were the most relevant network nodes under SR.Different from SR,Ek,r ETRm,Y（II）and seed-setting rate participated in the construction of the correlation network under SI.In addition,the sustainability indexes of N3,N1 and N0 levels under SI were 20.34%,8.00%and 6.96%higher than the critical value 1.30,respectively.And the sustainability indexes of N1 under SI were significantly higher than that under SR.For each N treatment level under SR,however,their sustainability indexes all did not exceed 1.30.In conclusion,the combination of“storing grain in technology”and“storing grain in land”could be achieved by returning straw and inputting reasonable N fertilizer rate in rice-rape rotation.SI could increase the total spikelets m-2and promote the growth of leaves.Therefore,it is necessary to pay attention to the sensitivity of rice varieties to N application rate and configure appropriate planting and management methods in production.Meantime,SI has the effect of active potassium,which can significantly increase the content of available potassium soil.Hence it should be paid to the balanced application of N,P and K fertilizers.In this study,it was suitable for Yixiangyou2115 to input 88 kg N ha-1in rice season when rice and rape straws were returned to the paddy.And the grain yield was 8.5-8.9t ha-1under such condition. |
