N₂O Emission From The Agricultural Soil Under The Supply Of Nitrogen And Water In Wheat-maize Rotation System

Agricultural soil is one of important sources of N2 O emission.It is very meaningful for reducing N2 O emission to study the influence of managements of water and nitrogen and mechanism of N2 O from agricultural soil in North China Plain.Fertilization before rainfall or followed by flooding irrigation is a general management practice for a wheat-maize rotation in the North China Plain.The rate of nitrogen influencing N2 O emission has been studied extensively,but the results are different.At present,more and more wheat and maize residues have been returned to the field.Yet,little is known on the impacts of rate of irrigation,pattern of fertilizing and crop residues returning to field on N2 O emission.We conducted a field experiment and a cultivation experiment to study mechanism of N2 O emission,further finding a way to reducing N2 O emission without sacrificing grain yield.The influence of coupled water with nitrogen on N2 O emission was studied through the field experiment.The cultivation experiment was conducted to study crop residue combined with nitrogen affecting N2 O emission and denitrifier abundances of Ochri-Aquic Cambosols and Hapli-Aquic Vertosols soils.The experiment included two nitrogen rates(250 and 200 kg N ha-2)and patterns(broadcasting and ditching)of fertilizer and two amounts(60 mm and 90 mm)of irrigation,totaled eight treatments: 250 kg N ha-2 with broadcasting or ditching plus 60 or 90 mm of irrigation(CS250,CK250,TS250 and TK250)and 200 kg N ha-2 with broadcasting or ditching plus 60 or 90 mm of irrigation(CS200,CK200,TS200 and TK200).Cultivation experiment contained seven treatments of each type soil: a control with no N fertilizer application(CK,N0),200 kg N ha-1(N200),250 kg N ha-1(N250),maize residue plus N200(MN200),maize residue plus N250(MN250),wheat residue plus N200(WN200)and wheat residue plus N250(WN250).The results of field experiment showed the bulk of the N2 O emissions occurred when the application of fertilizer before rain or followed by irrigation.During the period of experiment,9 peaks of N2 O emission of occurred,7 peaks of which happened were caused by the rain or irrigagion after fertililzation.There were obvious differences of the N2 O annual and seasonal acumulative emission among all treatments.The N2 O accumulative emission of broadcasting fertilization was higher than that of the deep-ditch fertilization under the other factors were same.The N2 O accumulative emission increased with the rate of fertilizer increasing during the maize growing season,but this phenomenon was not found during the wheat growing.The amounts of two irrigation treatments did not apparently influence the N2 O accumulative emission and grain yield of crop in this experiment,the rates of fertilizer application did not obviously affect the grain yield of crop,but apparently influenced the N2 O accumulative emission.The N2 O acculative emission of the CK200 treatment(1.9 kg.ha-1)treatment was lowest,decreasing 44% compared to the highest CS250(3.39 kg.ha-1)in all treatments.Meanwhile,grain yield decreased slightly under CK200(14659 kg ha-1)compared to CS250(14379kg ha-1).Therefore,CK200 is recommended as promising management regimes to attain the dual objectives of sustaining grain yield and reducing N2 O emission from the field under wheat-maize rotation system in the North China Plain.The results of cultivation experiment showed that,in the HAV soil,MN250 and WN250 increased the cumulative N2 O emissions by 60% and 30% compared with N250 treatment,respectively,but MN200 and WN200 decreased the cumulative N2 O emissions by 20% and 50% compared with N200.In the OAC soil,compared with N200 or N250,WN200 and WN250 increased the cumulative N2 O emission by 40%-50%,but MN200 and MN250 decreased the cumulative N2 O emission by 10%-20%.Compared with CK,addition of crop residue or N fertilizer resulted in significant increases in N2 O emissions in both soils.The cumulative N2 O emissions from the treatments of 250 kg N ha-1 were 1.1-3.3 times higher than those of treatments with 200 kg N ha-1 in both soils with adding equal amounts of the same type of crop residue.Abundance of the 16 S r RNA gene did not significantly change in all treatments in two soils,but the nos Z and nir S genes were more abundant in soils amended with crop residues compared with CK or N-only treatments.N2 O emission,however,were not related to the abundance of denitrifier containing nir S or nos Z.This research provided some information regarding supply of nitrogen and water influencing N2 O emission from the field under wheat-maize rotation systerm and the effect of crop residues with N fertilizer on N2 O emissions and denitrifier abundances in two soils.A pattern of nitrogen and water supply was found to attain the dual objectives of sustaining grain yield and reducing N2 O emission.If this pattern was applied to all the field under wheat-maize rotation system in the North China Plain,the N2 O emission will be reduced 12000 tons,meanwhile decreasing urea use of about 400000 tons.Our results also imply the property of crop residue and rate of N fertilizer are important influencing factors of N2 O emission when crop residues combined with N fertilizer are applied to different agricultural soils.