| Alternate wetting and drying irrigation can significantly save water and increase/stabilize yield in paddy fields.However,its wet and dry processes result in frequent soil oxidation and reduction,enhance soil nitrification-denitrification,increase nitrous oxide(N2O)emissions,cause environmental problems such as the Greenhouse Effect,and hinder the green sustainable development of the paddy ecosystem.Therefore,effective management strategies are urgently required for ameliorating the adverse effect of alternate wetting and drying irrigation on N2 O emissions in paddy fields.In this study,zeolite,an effective soil amendment,was applied to reduce N2 O emission in paddy fields under alternate wetting and drying irrigation through its characteristics of water holding,nitrogen retention,and pollution control.Based on the paddy field in a seasonal frozen region,Northeast China,this study investigated the distribution and emission of N2 O in paddy fields during the rice-growing season and freeze-thaw process during the fallow period by 24-months field in-situ monitoring,using in-situ soil profile gas collection system and static chamber-gas chromatography technique.The potential mechanism of zeolite controlling the production,distribution and emission of N2 O under alternate wetting and drying irrigation was clarified by observing the apparent morphological traits of roots,dynamic changes of soil environmental factors(p H and redox potential),and spatiotemporal changes of nitrogen in the soil profile.The main results are as follows:(1)Compared with continuously flooded irrigation,alternate wetting and drying irrigation did not significantly affect rice yield and aboveground biomass.The application of zeolite remarkably increased rice yield by 12.1% and aboveground biomass by 5.1%.Both alternate wetting and drying irrigation and zeolite application significantly increased root biomass,root length,root surface area,and root volume.(2)During the rice-growing season,alternate wetting and drying irrigation significantly reduced soil p H values during the water control period(tiller and panicle fertilizer periods);zeolite application significantly increased soil p H during water control period under alternate wetting and drying irrigation.This indicates that application of zeolite can remarkably improve paddy soil acidification induced by alternate wetting and drying irrigation.During the fallow period,there was no significant difference in soil p H values between treatments under the freeze-thaw process.(3)During the rice-growing season,alternate wetting and drying irrigation significantly decreased the NH4+ concentrations in the soil profile during the tiller fertilizer period(0-30cm)and the panicle fertilizer period(10-50 cm),and significantly increased the NO3-concentration in the soil profile during the water control period(10-50 cm).This indicates that the frequent wet and dry cycles accelerate the nitrification reaction and promote the conversion of NH4+ to NO3-.Zeolite application significantly increased the NH4+concentration in the soil profile(0-50 cm)and the NO3-concentration in the surface soil profile(0-30 cm)during the water control period,and decreased the NO3-concentration in the soil profile(30-50 cm).This indicates that zeolite can improve soil nutrient supply while reducing soil NH4+ oxidation and leaching of NO3-induced by alternate wetting and drying irrigation.(4)During the freeze-thaw process in the fallow season,alternate wetting and drying irrigation significantly reduced the freezing period NH4+ concentrations(10-30 cm),increased the thawing period NH4+ concentrations(10-40 cm)in the soil profile,and increased the NO3-concentrations in the soil profile during the freezing period(10-30 cm)and the thawing period(0-30 cm).This indicates that alternate wetting and drying irrigation accelerates the nitrification during the freezing period,promoting the conversion of NH4+ to NO3-.During the thawing period,the increased organic nitrogen mineralized substrates under alternate wetting and drying irrigation and enhanced the inorganic nitrogen content in the soil profile.Zeolite application significantly increased the NH4+ concentrations in the soil profile(0-50 cm)during freezing and thawing periods and increased the NO3-concentrations in the soil profile(0-30 cm)during freezing and thawing periods.This indicates that zeolite application improves the inorganic nitrogen concentrations in the soil profile during the fallow season.(5)During the rice-growing season,the alternate wetting and drying irrigation promoted N2 O emissions during the water control period and significantly increased the cumulative N2 O emissions by 22.5%.Zeolite application mitigated N2 O emissions during the water control period and significantly reduced the cumulative N2 O emissions by 8.8%under alternate wetting and drying irrigation.This indicates that zeolite application effectively mitigates N2 O emissions induced by alternate wetting and drying irrigation.Regarding the N2 O concentration and distribution in the soil profile during the growing season,alternate wetting and drying irrigation significantly increased the N2 O concentrations in the soil profile(0-50 cm)during the water control period(15.7-73.6%),and the increase was significantly greater in the surface(0-30 cm)soil profile(25.1-73.6%)than in the deep(30-50 cm)soil profile(15.7-38.7%).Zeolite application significantly decreased the N2 O concentration in the soil profile(0-10 cm)during the water control period(6.6-6.7%).Therefore,the hot moment for alternate wetting and drying irrigation and zeolite to regulate N2 O production during the rice growing season is the water control period,and the hotspot is located at 0-30 cm and 0-10 cm soil profiles,respectively.(6)During the fallow period,the thawing period is the hot moment of N2 O emissions.The thawing period accounts for 61.8-85.7% of the cumulative fallow season emissions.Alternate wetting and drying irrigation and zeolite application did not significantly affect N2 O emission during the freezing period,but significantly increased N2 O emissions during the thawing period by 30.6% and 15.8%,respectively,and the cumulative fallow season emissions by 24.3% and 10.9%,respectively.This indicates that zeolite and alternate wetting and drying irrigation had a synergistic promoting effect on N2 O emissions during the fallow period,which was different from the mitigation effect of zeolite application on N2 O emissions during the growing season.Regarding the N2 O concentration and distribution in the soil profile during the freezing and thawing process in the fallow season,alternate wetting and drying irrigation and zeolite application did not significantly affect the N2 O concentration in the soil profile during the freezing period,but significantly increased the N2 O concentration in the soil profiles of 0-30 cm(3.7-19.0%)and 0-20 cm(7.1-10.3%)during the thawing period,respectively.Therefore,the hot moment for alternate wetting and drying irrigation and zeolite to regulate N2 O production during the fallow season is the thawing period,and the hotspot is located at 0-30 cm and 0-20 cm soil profiles,respectively.(7)Alternate wetting and drying irrigation significantly increased the annual cumulative N2 O emissions and N2 O emission intensity.Zeolite application significantly reduced annual N2 O emissions by 4.6% under alternate wetting and drying irrigation.Zeolite application significantly reduced N2 O emission intensity by 9.1% and 15.4% under continuously flooded irrigation and alternate wetting and drying irrigation,respectively.This indicates that zeolite effectively increases rice yield and mitigates N2 O emissions induced by alternate wetting and drying irrigation.(8)The fallow season is an important part of the annual N2 O emissions,accounting for21.2% to 30.5%.Zeolite application significantly increased the ratio of fallow to annual N2 O emissions(10.5-17.8%).If the contribution of the fallow period is overlooked,the N2 O emission from paddy fields will be underestimated,and the underestimation will be more significant in zeolite-amended treatment.Meanwhile,the mitigation effect of zeolite on N2 O emission will be overestimated without considering the fallow season.Altogether,zeolite(15 t ha-1)effectively mitigates N2 O emissions in paddy fields under alternate wetting and drying irrigation.This study highlights that the differential response of N2 O emission between the growing season and fallow season should be fully considered when evaluating the mitigation effect of zeolite on N2 O emissions,identifies the hot moment and hotspot of soil N2 O production regulated by zeolite and alternate wetting and drying irrigation,and reveals the potential mechanisms of zeolite regulating the N2 O production,distribution and emissions under alternate wetting and drying irrigation.Our study will be helpful to understand the potential mechanism of how zeolite regulates the nitrogen cycle in paddy fields under alternate wetting and drying irrigation. |
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