Distribution Characteristics Of Annual Fertilizer And Straw Nitrogen In The Plant-soil System Under Oilseed Rape-rice Rotation

Oilseed rape-rice crop rotation was an important planting pattern to ensure grain and oil security.However,the unique dry-wet alternation conditions of flood and drought rotation formed a dynamically changing soil environment,resulting in various ways of nitrogen（N）loss and low N utilization.In order to reduce the input and loss of fertilizer N and improve the utilization of agricultural waste,straw returning to the field had attracted more and more attention.However,there was still lack of quantitative research on the use of different exogenous N（fertilizer N,deciduous stubble N and straw N）in the current season and residual effect in the later season under the paddy-upland rotation.This study was conducted in the located experiment,using the ¹⁵N isotope labeling technology to quantitatively analyzed the crop uptake and soil residues of fertilizer N,leaf and stubble N,and straw N under oilseed rape-rice rotation.We analyzed the effect of different exogenous N additions on soil organic N pools,aiming to clarify the"supply"and"storage"characteristics of residue N in the organic N pool.The research results would provide a reasonable basis for the reduction of chemical fertilizers,the improvement of soil fertility,and the green and healthy development of agriculture.The main results were as follows:（1）Based on the located field experiment of ¹⁵N fertilizer in the crop-soil system under the oilseed rape-rice rotation,we found that the utilization rate of ¹⁵N fertilizer was 26.0%,and the residual rate of ¹⁵N fertilizer in soil was 31.5%.The residues amount of ¹⁵N gradually decreased with soil depth.The contents of different forms of organic N in the 0-20 cm were acid insoluble N（AIN）>hydrolyzed unknown N（HUN）>hydrolyzed ammonium N（HAN）>amino acid N（AAN）>amino sugar N（ASN）.The contents of three active organic N（HAN,AAN and ASN）in 0-20 cm soil layer accounted for 25.5%of the total organic N.However,in 20-40 cm soil layer,the proportion of three active organic N decreased,the proportion of HUN increased slightly,and the content of AIN was relatively stable.Fertilizer ¹⁵N was mainly converted into AIN-¹⁵N and HUN-¹⁵N,accounting for 57.2%and 26.7%of the total residual,respectively.The proportion of ¹⁵N in the three active organic N remained relatively stable.（2）Fallen leaves could provide 9.4%of its own N for the growth of oilseed rape in the current season,and 32.2%of N from fallen leaves and stubble returned to the field could be absorbed and utilized by rice in the next season.Due to the dilution of soil N,the reuse rate of fertilizer ¹⁵N by the subsequent crop to the residual soil in the previous season decreased（19.8%）,which was only 6.2%of the total ¹⁵N input.After rice harvest,the proportion of soil HUN content in total organic N increased significantly,with an increase of 48.0%.Compared with that after rape harvest,the proportion of the three active N（HAN,AAN and ASN）in the total organic N decreased by 7.2 percentage points after rice harvest,with the decreasing rate of 28.2%.There were still 16.8%fertilizer ¹⁵N residues after rice harvest Compared with the proportion of ¹⁵N residual fertilizer entering each organic N pool,we found that the residual fertilizer ¹⁵N after rice harvest further increased its proportion in HUN and AIN.（3）Based on the field experiment of ¹⁵N fertilizer and ¹⁵N labeled straw in rice season,we found that under the condition of long-term straw return,the N utilization rate,soil residual rate and the unknown N loss rate of ¹⁵N fertilizer was 40.0%,18.3%,41.7%,respectively.Meanwhile,the N utilization rate,soil residual rate and the unknown N loss rate of ¹⁵N straw was 34.5%,18.3%,47.2%,respectively.The ¹⁵N residues of fertilizer and straw in rice season were both mainly concentrated in 0-40 cm.However,the distribution of different exogenous ¹⁵N in soil organic N com layers was various.Taking the 0-20 cm soil layer as an example,compared with fertilizer ¹⁵N,straw ¹⁵N was distributed higher in the AIN-¹⁵N（accounting for 71.7%）but lower in the HUN-¹⁵N（accounting for 14.1%）.The proportion of different exogenous N in the three active N pools(HAN-¹⁵N,AAN-¹⁵N,ASN-¹⁵N)in the total ¹⁵N residue was similar（14.2%-14.7%）.In summary,under the annual oilseed rape-rice rotation,the N sources in the rice growing season include soil N pool,N fertilizer application in the current season,straw N in the previous season,residual N in the previous season and residual N in the previous season.Among them,the soil N pool was an important source of N absorption by rice,followed by the application of fertilizers in the current season,and the residual N of previous seasons fertilizers,straw and stubble N also had a small amount of N supplementation.The straw and stubble returning to the field had a small contribution to the current season of crop N utilization.However,from the long-term effect,making full use of the input of straw and residue in the previous season to expand the soil N pool is an effective way to improve soil fertility and replace N fertilizer.