Differences And Mechanism Of Soil Nitrogen Supply In Oilseed Rape Planting Season Under Rice-Rapeseed And Cotton Rapeseed

Nitrogen(N)is an important factor that limited the yield of oilseed rape in the rice-oilseed rape rotation and cotton-oilseed rape rotation system in the Yangtze River Basin.The yield of oilseed rape in the paddy-upland rotation system is lower than it in the upland-upland rotation system.Therefore,to understand the differences of yield soil N supply under oilseed rape season in different rotation system,the characteristics of soil N mineralization and N uptake by oilseed rape in different growing periods are estimated.And more focuses on the analysis of soil N transformation and N losses with 15N-lablelded technologies are paid during different growing periods.Further considering the difference of soil organic matter fraction and soil water content in the different rotation system,the effects of soil organic matter fractions removed on soil N mineralization are studied in the different soil water contidions,which is helpful to deeply understand the interaction of soil organic matter fraction and soil water content on soil N turnover in the different rotation system.Overall the evaluation of the differences of soil N supply and the key factors is important to develop the optimum fertilizer application technologies and achieve the high yield and high nutrient use efficiency for oilseed rape in the different rotation system in the Yangtze River Basin.The main results are listed as follows:1 Rotation system significant affect yield of oilseed rape.The yield of oilseed rape in RR rotation was significantly lower than that of CR rotation about 15.4% in average.N fertilizer input is the main factor affecting the yield of oilseed rape which was increased with the increase of N fertilizer input.The N fertilizer input in the previous crop of oilseed rape can increase the yiled of oilseed rape also.The apparent utilization efficiency of oilseed rape under CR rotation was 13.5% higher than that of RR rotation.The contribution rate of N fertilizer in oilseed rape season under RR rotation was 7.7% higher than that of CR rotation.The results showed that the yield of winter oilseed rape was lower under RR rotation than that of CR rotation,and the requirement of N fertilizer under RR rotation was higher than that of CR rotation.The effect of N application on oilseed rape under CR rotation was higher than that of RR rotation.2 The N residue in previous crop season had a significant effect on the yield and N uptake of oilseed rape.15 N isotope marked analysis showed that the residual N content of 0-40 cm depth soil was significantly different in the two crop rotation modes,that is,29.3% and43.4% respectively after harvest of rice and cotton respectively,in the application of N fertilizer(150 kg N/ha)treatment.The N loss in rice season was accounted for 36.9% of the gross N input.The N supply capacity was enhanced by the relative higher N residue from CR rotation.The aboveground biomass and N accumulation of oilseed rape in CR rotation were significantly higher than those of RR rotation.About 7.8% N was from the residue of cotton season,which was higher than the recovery efficiency of residual N in the rice season(4.2%).3 The correlation relationship between soil N supply and crop N absorption is the main reason for the lower dry matter quality and N uptake of rapeseed from RR rotation.The results of in situ mineralization experiments showed that the net mineralization of soil N was significantly different at each developmental stage of the oilseed rape growing season.The net mineralization of soil N in the early stage of the oilseed rape growth(transplanting-flowering stage),which was significantly higher in the CR rotation than that of RR rotation.While the net mineralization of soil N under RR rotation was higher than that of CR rotation after the flowering stage.N fertilizer application significantly increased soil N net mineralization accumulation.15 N dilution experiments showed that the gross mineralization rate and gross nitrification rate under RR rotation were significantly lower than that of CR rotation during the overwintering period;but the gross mineralization rate and gross nitrification rate of RR rotation were significantly increased and significantly higher than CR rotation after the flowering stage.The gross immobilization rate showed the opposite trends.In the overwintering period,the gross immobilization rates was higher in the RR rotation than in the CR rotation,and the gross soil immobilization rate showed a decreasing trend in both rotation systems,but the RR rotation was higher than that of CR rotation on the whole.The N uptake of oilseed rape was inconsistent with that of soil N supply.The N uptake of oilseed rape under CR rotation was higher than that of RR rotation no matter of N fertilizer application or not.The N uptake of oilseed rape under CR rotation was 35.9 kg/ha higher than that of RR rotation in the early stage(transplanting-flowering stage),while the N uptake of oilseed rape under RR rotation was 21.1 kg/ha higher than that of CR rotation.Due to the accumulation of N is insufficient in the early stage;the dry matter and N accumulation of rapeseed harvested from CR rotation were significantly lower than that of RR rotation.4 RR and CR rotation affect the soil N supply by changing the quantity and quality of soil organic matter component.The results showed that soil organic C and total N content from RR rotation were 19.9% and 16.3% higher than those in CR rotation,respectively.Soil readily oxidizable organic C,particulate organic C(POM-C)and N(POM-N)in the soil of RR rotation were 35.6%,31.4% and 35.9%,higher than those of the CR rotation,respectively.Soil potential mineralization N(PMN)was positively correlated with soil organic N content.After removal of active organic matter from RR and CR rotations,the soil PMN decreased significantly by 38.0% and 45.6%,respectively.The contribution of MBN to PMN was significantly positively correlated with PMN decline.The analysis of the soil organic matter by infrared spectroscopy showed that the active organic matter component accumulated the aromatic compounds which are difficult to decompose in the paddy-dry rotation conditions,and they reduced the biological activity and affected the soil N supply capacity.5 The types and quantities of organic matter affect the soil organic matter components.The contents of soil organic carbon and gross N were increased after rice root and cotton leaves added.There was no significant difference in soil organic C and gross N content between the two organic materials under the same input,but the contents of soil organic matter increased significantly.The contents of POM-C and POM-N with addition of rice root were significantly higher than those with addition of cotton leaves.The soil C accumulation and mineralization rates in group of addition of rice root were 36.0%-50.6%,respectively,which was significantly lower than that of cotton leaves group(58.5%-70.1,respectively),leading to more stubble staying in the soil.The soil PMN in group of addition of cotton leaves were increase 9.5-42.3 kg/hm2,but was descrease 2.7-14.8 kg/hm2 with addition of rice root.6 Soil moisture and organic matter components have a significant interaction with N conversion.Water is an important factor affecting soil N mineralization.The soil mineralize-susceptible organic N content in the appropriate water treatment was significantly higher than the other two treatments.Although the soil moisture content in stimulated dried soil at the last week of culture was the same with that of appropriate water treated group,the content of organic N in the whole culture process was significantly lower than that of the appropriate water treatment group.The difference of soil organic matter composition mainly affected the soil N mineralization rate.The N mineralization rate of lower POM treated soil was significantly higher than the higher POM treated soil.There is a significant interaction between soil moisture and soil active organic matter components.The gross mineralization rate and the gross nitrification rate were the highest at the appropriate water treatment,while there was no significant difference in the gross mineralization rate between the simulated dried and simulated flooded soils.The gross nitrification rate in stimulated dried group was significantly higher than that of the simulated flooded group.Soil organic matter content and soil moisture content have a significant interaction.The gross immobilization rate of simulated dried soil was significantly higher than that of the other two treatments.Higher POM treated soil showed high gross mineralization rate and gross immobilization rate,while there is no significant difference on the gross nitrification rate between high POM treated soil and low POM-C treated soil treatment.In summary,the differences in soil physical,chemical and biological properties caused by different rotation systems can affect the transformation and supply of soil N in oilseed rapse season.It is beneficial to soil organic matter(SOM)fractions accumulation,but not conducive to the biological availability of SOM and its components under RR rotation.The net mineralization of soil N under the RR rotation in the early stage of the oilseed rape growth(transplanting-flowering stage),which was significantly lower than that of CR rotation,is not conductive to the eraly growth of oilseed rape.Therefore,it should be increase N fertilizer input,promote N nutrient accumulation of oilseed rape.In view of the soil texture features,is not easy to mineralization,it should advance ploughing drying soil,promoting soil N mineralization under RR rotation.After flowring period,due to the enhancement of soil N mineralization,we can ruduce the N fertilizer input under RR rotation.In the late growth stage of oilseed rape with the increase of soil temperature and SOM decomposition,enhance the ability of mineralization of soil organic N,and the yield of rapeseed is the key transfer at the late growth stage in rape nitrogen,therefore appropriate to reduce the rape growth late nitrogen fertilizer can.Reasonable distribution of N fertilizer according to soil N supply characterstics,can realize high yield of oilseed rape,therefor the effect of rotation system on soil N supply should be take into full considueration in the N management on oilseed rape.