Mechanism Of Nitrogen Use And Loss Affected By Nitrogen Managements Under Typical Paddy-upland Rotation System In The Middle Reaches Of The Yangtze River Basin

The middle reaches of the Yangtze River Basin are rich rain and heat resources.The agricultural planting mode is mainly the rotation of two crops per year,which is an important grain and oil production base in China.Application of nitrogen（N）fertilizer is an important way to ensure crop yield.Different N management affect the characteristics of N cycle in soil-plant system.However,the change of N surplus in different rotation systems and the effect of seasonal dry-wet alternation on the loss of active N made the N cycle in paddy upland rotation system complicated.To clarify the effects of different N management on N use and loss in paddy-upland rotation system is of great significance for regulating N application in typical areas and reducing the loss of active N.Based on the long-term experimental station,the characteristics of N use and loss in oilseed rape-rice（OR）and wheat-rice（WR）rotation systems were studied under different nitrogen management（no N application:PK,balanced fertilization:NPK,balanced fertilization with straw return:NPK+St and high fertilization:HNPK）.The ¹⁵N isotope labeled field experiment was conducted to determine the reabsorption of fertilizer N remaining in soil organic N pool in winter and subsequent rice by oil rice and wheat rice rotation systems.Through microculture experiment,the effects and related mechanisms of water and drought rotation on N₂O emission and NH₃ volatilization in winter were explored.The main results are as follows:（1）Rational application of N fertilizer improved the utilization of N fertilizer in rice.In OR and WR rotations,NPK fertilization significantly increased N use efficiency（NUE）and partial factor productivity of N fertilizer（PFPN）as compared with the HNPK treatment.Compared with the NPK treatment,the NPK+St treatment enhanced rice yield by 11.71%and 14.92%under the OR and WR rotations,respectively,increasing the NUE by 6.15 and10.31 percentage points.Under the same N management,the OR rotation increased rice yield and aboveground N uptake as compared with the WR rotation.However,there were no significant differences in NUE and PFPN between the two rotation systems.（2）The effects of different N management on NH₃ volatilization and N₂O emission were different between the paddy and upland season.In terms of cumulative emissions,high fertilizer inputs increased NH₃ and N₂O emissions at each fertilization stage of the rice season.However,compared with the NPK treatment,the NPK+St treatment in rice season did not significantly affect NH₃ volatilization and N₂O emissions at each fertilization stage of the two rotation systems.In terms of emission factors（EF）,the EF values of NH₃ and N₂O in rice season under different N management were 8.39-9.94 and 0.10-0.12,respectively,and there was no significant difference between rotation and treatment.In the winter season,the treatment with straw return mainly increased NH₃ volatilization during the topdressing stage（with an increase of 154.82%-187.22%）and N₂O emission during the base and bolting/jointing period（with an increase of 22.80%-104.64%）.The difference between cropping rotations was mainly reflected in the total N₂O emission of winter crops during the whole growth period,but not in each fertilization period,i.e.,the increase of N₂O emission induced by soil N would be the main factor leading to the increase of soil N₂O emission in OR rotation system（3）The difference of soil substrate N concentration in different rotation systems depends on the annual N surplus of rotation and the proportion of surplus N into the soil N pool.The aboveground N uptake of oilseed rape（8.28-115.71 kg N/hm²）was lower than that of wheat（32.01-238.55 kg N/hm²）.Therefore,the annual N surplus of OR rotation was significantly higher than that of WR rotation,with an increase of 4.54%-69.38%.After three years of rotation planting,soil total N content and soil NO₃^-content of OR rotation were significantly higher than that of WR rotation.After winter harvest,soil NO₃^-content of OR rotation was 19.43%-43.96%higher than that of WR rotation.Soil total N of OR rotation was 6.80%-19.72%higher than that of WR rotation.（4）Different N management and the NUE of winter crops affected the residual fertilizer N in soil and the absorption of residual fertilizer N by rice in the next season.The¹⁵N tracer experiment showed that fertilizer N residues in OR rotation increased by 15.17%-34.02%compared with those in WR rotation after winter crop harvest.Similar to the characteristics of N residue,the reabsorption of residual N in winter under OR rotation was also 30.08%-64.26%higher than that under WR rotation.The amount of residual N reabsorbed by rice in the NPK+St treatment was higher than that in the NPK and HNPK treatments.Further analysis of residual fertilizer N in soil organic N pools showed that NPK+St treatment enriched more residual fertilizer N in amino acid N（AAN）and amino sugar N（ASN）pools,and those enriched in acid hydrolyzed ammonium N（HAN）pools in HNPK treatment.Compared with WR rotation,OR rotation concentrated more residual fertilizer N in HAN and AAN pools than WR rotation.（5）The typical seasonal alternation of drying and wetting is an important factor affecting the loss of fertilizer N.Based on the simulation of different soil wet and dry conditions,we found that the gradual decrease of soil moisture（i.e.,WFPS decreased from70%to 50%）could reduce the emission of N₂O and NH₃ caused by urea addition.After rewetting,soil N₂O emission recovered to a certain extent.However,it was still lower than that of constant moisture soil with the same moisture content.However,soil NH₃volatilization decreased continuously during soil rewetting.The decrease of soil NH₄⁺-N concentration during the drying process led to the shortage of substrate supply for the ammonia oxidation process and the enhancement of soil adsorption capacity for NH₄⁺.And the change of amo A-b and ure C community structure and the decrease of the abundance resulted in the decrease of ammonia oxidation and urease producing microbial activities.In conclusion,the characteristics of annual N use and loss were different under different management.Different paddy upland rotations affected N absorption by rice and N₂O and NH₃ emissions in winter.Compared with the WR rotation,OR rotation increased the residue of fertilizer N in soil acid hydrolyzable ammonium N and amino acid N pool in winter,and then increased the absorption of N by later season rice,reducing the impact of different N management measures.Compared with the WR rotation,OR rotation would change NH₃ volatilization and N₂O emission in winter by increasing soil N content and/or affecting soil wetting and drying degree.