Study On Optimal Nitrogen Rate,nitrogen Fate And Loss Characteristics In Cold Rice Fields

To address the problem that the relationship between nitrogen（N）quantity and N fate and loss in cold rice fields is unclear,the study collected 87 sets of data from N fertilizer gradient trials at different experimental sites in the Heilongjiang rice region,established a fertilizer effect function,and analyzed and determined the appropriate amount of N to be applied to cold rice fields in different regions.On this basis,a combination of field trials and 15N microplot trials were used to conduct N fertilizer gradient field trials in Jiansanjiang（2017）and Wuchang（2017 and 2018）to study the effects of different N levels on the group quality,yield and composition of cold rice populations;monitoring the occurrence of N losses and related indicators in the Wuchang experiment to study and analysis the occurrence of N losses and the mechanisms affecting them in cold rice fields;the 15N micro-zone experiment was conducted to identify the uptake,distribution and fate of fertilizer N in cold rice fields,and the fate of residual fertilizer N in the following year’s production,with a view to providing data support and a theoretical basis for N reduction and yield increase in cold rice fields.The main findings are as follows:（1）There are large regional differences in rice yield and N application in Heilongjiang Province,with significantly higher N application in the west,but lower yields than in other regions.The relationship between yield and N application was a quadratic function（R2>0.98）,with rice yield increasing and then decreasing as N increased.Yield without N application was significantly and positively correlated with maximum rice yield in a linear fashion,with higher ground yields yielding higher maximum yields obtained.Rice yields increased between 0.68-4.69 t/ha after N application,with a decreasing trend of N application from west to east when the highest yields were obtained,with the highest yielding N application in the eastern region being 135.7 kg N/ha and the corresponding value in the west being 157.8.4 kg N/ha.The range of economic optimum N application rate in Heilongjiang province was 129.0-150.7 kg N/ha,with the west being higher than the east.As N fertilizer use increased,N fertilizer agronomic efficiency and N fertilizer partial factor productivity decreased,with high fertilizer application and low N efficiency in the west.（2）The Microplot test showed that the N uptake by rice and residual in the soil was essentially equivalent to the N lost through other pathways,and the total（rice uptake and residual N in soil）recovery of 47.40%-52.55%and 46.77%-48.13%absorbed by rice and remaining in the soil in the first quarter of 2017 and 2018,respectively.The increase in N application increased the total recovery,but the increased uptake of fertilizer N by rice reduced fertilizer N residuals in the soil.N application increases the uptake of fertilizer N by rice,which will not facilitate the transfer of fertilizer N from straw to seed.N application increases the amount of fertilizer N left in each soil layer,with differences between treatments decreasing with increasing depth.The residual fertilizer N in the soil in 2017 was mainly concentrated in the 0-10cm soil layer and as the residual N increased more fertilizer N migrated downwards.In 2018 residual N was mainly distributed in the 0-20 cm soil layer;there was 38.45%-48.68%of the residual fertilizer N was still stored in the soil the following year,and mainly in the 0-10cm soil layer;only 4.48%-7.07%was absorbed by the second season rice;apparent losses accounted for 44.24%-56.62%of the residual N in the first growing season,with non-significant differences between treatments.Fertilizer N applied to the soil has a certain excitation effect on the soil,and both the increment of N taken up from priming effect and the contribution to N uptake from priming effect with the amount of N applied.（3）Fertilizer N losses include ammonia（NH₃）volatilization,leaching and runoff N losses.Ammonia volatilization mainly occurs during 7-10 days after fertilizer application,with the highest ammonia volatilization occurring during the tillering fertilizer stage and the lowest during the panicle fertilizer stage within the three application periods,which is determined by differences in rice populations and environmental temperatures.Total ammonia volatilization 0.30-4.91 kg N/hm²,and there was a quadratic function between total ammonia volatilization losses and N application（R²>0.96）.Difference in total ammonia volatilization between two years was mainly influenced by the NH₄⁺-N content of the field surface water,which directly influenced the daily flux of ammonia volatilization,and there was a linear correlation between the NH₄⁺-N content of field surface water and flux of ammonia volatilization（R²=0.6022）.Leaching occurred throughout the reproductive period,with leaching cumulative N losses was 2.94-6.77 kg N/hm² of in two years,respectively,with a quadratic function correlation（R²>0.89）with the amount of N.The inorganic N content of the leachate decreases as growing stage progresses,the NH₄⁺-N content decreases with increasing soil depth and the NO₃^--N content increases,N application can exacerbate leaching N losses by NO₃^--N content increasing.Runoff occurred after heavy rainfall,Runoff N losses ranged from 0.15-3.19 kg N/ha in both years,with inter-annual variation due to differences in rainfall.Rainfall in 2017 directly influenced the amount of runoff N loss,with a long interval between fertilizer application events and no significant differences between treatments;rainfall after fertilizer application in 2018 increased fertilizer N loss and N loss was linearly correlated with N rate,significant differences were observed between treatments.Based on the N balance and yield effects,the variation in N balance was relatively stable after increased fertilizer application,and the N balance in cold land rice fields was essentially flat.（4）Proper N application improves rice group quality,while excessive N application decreases population quality and reduces the N harvest index.N application increased the population size by increasing the number of rice divisions,while excessive N application increased the number of ineffective divisions and reduced the rate of spike formation.The N105 treatment of Jiansanjiang had the highest tiller spike rate of 93.13%,while the Wuchang N75 and N105 treatments had the highest tiller spike rate of 83.00%and 81.41%in both years,respectively.Accumulation of dry matter and N in rice with the highest accumulation rate at the jointing-heading stage and the highest accumulation at mature stage.The harvest indices of the two trial sites in Jiansanjiang and Wuchang were 57.40%-58.66%and 49.68%-54.01%,respectively,and the N harvest indices at maturity were70.94%-75.23%and 61.10%-68.73%,respectively.Compared to Wuchang,a higher proportion of dry matter and N accumulation distributed in grains was observed in Jiansanjiang.Both harvest index and N harvest index decreased with increased N application,while the N requirement of 100 kg grain increased significantly,causing excessive N application risks extravagant N uptake.（5）Rice yield could be promoted by proper application of N yet applying too much N causes reduction of N efficiency.N application can increase yield by increasing the number of spikes and grains.All yield components contributed positively to the yield of rice,while the indirect effect of the interaction between components on yield varied.Spike number and grain number are the most important factors affecting yield in cold rice fields,and the interaction between them both have a positive effect on yield,whilst fruiting rate has a more negative effect on yield through spike number and grain number than the direct effect of fruiting rate on yield,indicating a negative correlation with yield.Plant uptake and distribution of N could be coordinated during 105-165 kg N/ha N application,yet N recovery efficiency（21.42%-59.48%）and agronomic efficiency（18.80%-32.25%）would be reduced by excessive N supply,with inter-site difference present greater than inter-annual difference.In addition,physiological efficiency and partial factor productivity was 44.55-109.08kg/kg and55.57-128.83 kg/kg,respectively,which would consistently reduce with the increase of N application.Cold land rice fields have little NH3 volatilization,leaching and runoff losses,differential subtraction N fertilizer utilization was close to 50%,and N fertilizer agronomic efficiency is high and N application was less harmful to the environment,but excessive N application tends to reduce N efficiency by causing extravagant N uptake.The appropriate N application is lower in the south-central part of the cold rice fields than in the west,while yields are higher than the west.