The N-Supply Characteristics Of Paddy Soil In Cold Area And The Relationships Between N Supply Rates And Rice Yield

Reasonable nitrogen(N)supplement is the main means to increase rice yield in cold area.During the whole growth period of rice,N is mainly obtained from soil and fertilizer.Clear for the characteristics of soil N supplement and rice N uptake,and coordinate the balance of N supplement and demand of rice in cold area is important theoretical basis for rational N application.However,not much correlation was observed between rice N uptake in Treatment N0(No N fertilizer applied)and N mineralization rate measured with the traditional in situ incubation method,which decreases the accuracy in in measuring soil N supplying capacity,and affects the rational application of fertilizer.To solve this problem,modification was made of traditional method in an attempt to improve its accuracy in measuring soil N supplying capacity.Furthermore,a field experiment was conducted,and the accumulated N amount,rice yield and other indicators under different N-supply levels were determined to study the effect of N supplement on rice N uptake,yield and N use efficiency.The main results were as follows:Each incubation time of phase incubation method should not be too long.About mixing times of soil that is used as in situ incubation should be appropriate,too few mixing times may lead to incomplete mixing.On the contrary,too much mixing times may increase NO3--N content.Consequently,mixing times should be 2 times.When incubation went on continuously over 40 days,it might pose a risk of inhibiting N mineralization.So,each incubation time should not last too long or over 40 days.When measuring soil mineralized N in paddy field,measurements using the phase incubation one be cited as an indicator to evaluate soil N supplying capacity.There were noticeable differences in soil inorganic nitrogen content of different sites or different years.And inorganic nitrogen content during transplanting and mature stage was irregular.There were differences in soil mineralized nitrogen content of different sites or different years as well.The content of mineralized N measured with the phase incubation method was found to be closely and positively related to rice N uptake measured in Treatment N0,with R2 being 0.621(P<0.01).Correlative analysis between rice N uptake at mature and soil mineralized nitrogen or soil nitrogen supplywere performed respectively,their corresponding coefficients of determination(R2)were 0.827(P<0.01)and 0.857(P<0.01).Rational N supplement could promote rice N accumulation.The N contents of leaves and stems showed a downward trend from tillering stage to mature stage.The largest decline was from tillering stage to jointing stage.Leaf N content decreased 40.71%~48.89%,and stem N content declined 44.22% ~ 54.39%.Increasing panicle fertilizer nitrogen supply appropriately could inhibit the decrease of N contents in leaves and stems after jointing stage.N content of panicle was declined if N supplement was excess.The N accumulation in rice before jointing stage was increased with the increase of N supply.The peak period of N accumulation was from jointing stage to heading stage,which period N accumulation was 31.87% ~ 53.65% of total N accumulation.Increasing the amount of N supply at late stage of rice could lead to the decrease of N accumulation in panicle.Consequently,rational N application could help improving rice N accumulation.Rational N application could increase rice yield and N use efficiency remarkably.The panicles per unit area were increased with the increasing of N supply rate.However,when the N-supply exceeded 196 kg hm-2,the increasing of the panicles per unit area was not significant.The grain number per panicle showed significant increase with the increase of N application.On the contrary,filled grain rate and 1000-grain weight decreased with the increase of N supplement.There was a quadratic curve relation between N-supply rate and rice yield.The optimal N fertilizer supply rate is 102.76 kg hm-2.There was a quadratic curve relation between N-supply rate and N recovery efficiency or N agronomic efficiency.The amounts of N fertilizer supplied was 87.19 kg hm-2,when reach the highest efficient of N utilization.