Impacts Of Soil Fertility Management On Nitrogen Use Efficiency And Greenhouse Gas Emissions In A Double Maize Cropping System In Southern China

Burgeoning population worldwide calls for an increase in food production to meet the demand for food.Fertilizers play important role in improving crop production and they have been a major contributor to impressive crop yields worldwide.Routine applications of fertilizers to enhance the fertility of the soil to boost crop production are an essential component of soil fertility management in arable crop production systems.However,improper soil fertility management（SFM）in c ropping systems is one of the major causes of high greenhouse gas（GHG）emissions and poor nitrogen use efficiency（N UE）.The objectives of the study were,therefore;（i）to evaluate the effect of SFM through fertilization on NUE and crop productivity in a double maize cropping system in Ultisols in Southern China;（ii）to investigate the impact of the SFM on field scale GHG emissions and ammonia（NH₃）volatilization from long-term fertilizer amendments in a double maize cropping system and（iii）to evaluate the factors that play important roles in the emissions of the GHGs and NH₃.To achieve the objectives a two-year field study was conducted in a long-term experiment which has been in existence since 1986 in Jinxian in Jiangxi Province,China.The field trials of the present study were conducted from 2015 to 2016with six treatments arranged in a randomized complete block design with three replications:no fertilizer（CK）,manure（OM）,chemical nitrogen fertilizer（N）,chemical NPK,combined NPK fertilizer and manure（NPKM）and double NPK（DNPK）.The DNPK was considered in the second year.Each year consisted of two seasons.The OM,NPKM,DNPK and NPK significantly impacted on nitrogen uptake,crop productivity and NUE.Significantly higher nitrogen uptak e was observed in the NPKM treatment at flowering and harvest in all the four seasons.The OM significantly increased the NUE as compared to the other treatments.N treatment significantly had the poorest nitrogen uptake and NUE.Maize crop productivity was improved under OM,NPKM,DNPK and NPK treatments.Thus,aboveground biomass,yield components and grain yield were all enhanced significantly by those treatments.Manure combined with NPK（NPKM）significantly had the highest aboveground biomass and grain yield.Nitrogen treatment significantly recorded the lowest aboveground biomass,grain yield and yield components.The results indicated that NPKM benefited the double maize cropping for higher dry matter production and grain yield.Moreover,the OM and NPKM significantly increased soil p H,SOM and TN,and decreased soil bulk density.However,the quality of the soil did not improve under N treatment.Nitrous oxide（N₂O）and methane（CH₄）emissions measured using the static closed chamber method varied co nsiderably between the treatments.N itrous oxide flux varied between the treatments with peaks appearing within three to five weeks after fertilization in the entire four seasons under study.The highest peaks occurred in the OM and NPKM treatments and the lowest occurred in the CK.The cumulative N₂O emission showed significant differences between the treatments（P<0.05）.The seasonal cumulative N₂O in2015 ranged between 0.33 kg N₂O ha^-1 and 0.81 kg N₂O ha^-1 in the first season and between 0.47 kg N₂O ha^-1 and 1.19 kg N₂O ha^-1 in the second season.The NPKM had the highest cumulative N₂O in the two seasons but the values did not differ significantly from OM.The annual cumulative N₂O emission was highest in the NPKM in 2015(1.95 kg N₂O ha^-1)but this was not significantly different from OM(1.92 kg N₂O ha^-1).The lowest annual cumulative N₂O was observed in the control(0.8 kg N₂O ha^-1)in 2015.A similar trend was followed in 2016 with NPKM having the highest annual emission(2.42 kg N ₂O ha^-1)which again didn’t differ significantly from OM(2.15 kg N₂O ha^-1).However,between the years there was no significant difference in cumulative N ₂O emission.The interactions between treatment,season and year did not differ significantly in cumulative N₂O emission.In this study,NH₄⁺and NO₃^-concentrations increased but varied considerably between the treatments as well as between the seasons.Fertilization increased nitrification rate but between organic and mineral fertilizer there w ere no significant differences.Manure significantly enhanced denitrification with the manure treated soils（OM and NPKM）having very high denitrification potential as compared to the other treatments.During the observation for CH₄ flux,each treatment was either seen as a sink orsource.Thus CH₄ emissions alternated with consumption.There were seasonal variations in the cumulative CH₄ with the treatments mainly acting as sink except in 2015 where OM,N and NPKM acted as a minor source.The annual cumulative CH₄ also varied considerably between the treatments and years.In 2015 OM,NPKM and N had CH₄emissions while NPK and CK experienced CH₄ consumption.However,in 2016,all the treatments experienced CH₄ consumption.The contribution of N₂O to GWP was much greater than CH₄.The global warming potential（GWP）which was calculated as CO₂ equivalent（CO₂-eq）based on a 100-year time horizon by multiplying the seasonal CH₄ and N₂O emissions by their respective radiative forcing potential showed significant differences between the treatments.In the first season of 2015,the NPKM had the highest area-scaled GWP(288.56 kg CO₂-eq ha^-1)and OM recorded the highest in the second season(358.08kg CO₂-eq ha^-1)but in each season the differences between the two were not significant.In 2016 OM rather recorded the highest area-scaled GWP(329.63 kg CO₂-eq ha^-1)in the first season but in the second season it was NPKM treatment that had the highest(393.45kg CO₂-eq ha^-1).Between 2015 and 2016 the area-scaled GWP of NPKM,NPK and OM increased by 8.2%,4.8%and 2.8%,respectively but that of CK and N decreased by 26.4%and 6.5%,respectively.The results suggest that manure application stimulated higher N ₂O emission as compared to its inorganic counterpart.The GWP per unit grain yield also showed variations between the treatments with N treatment having significantly high yield-scaled GWP for all the seasons.The yield-scaled GWP was not determined in N treatment in the last season because no yield was recorded.However,OM and NPKM had the lowest yield-scaled GWP which did not differ significantly from NPK and DNPK.Thus,treatments with higher yields recorded lower yield-scaled GWP.Maximum yields occurred in the manure treated soils so by employing the yield-scaled GWP makes OM and NPKM the best choice because their high yields could offset the negative impact of N ₂O emissions.Ammonia volatilization measured with the Draeger-tube method after field application of the different fertilizers was highly variable under the different treatments.Greater portion of the NH₃ loss was realized within four days of fertilizer application and as a result,the peaks of the NH₃ volatilization fluxes were seen within these days.Intermittent rainfall disturbed the measurements and affected the results of t he NH₃volatilization.The volatilization was higher in OM and NPKM treatments.Per the method used to determine NH₃ volatilization the control treatment had zero volatilization.Higher soil temperature,moisture and wind enhanced NH₃ volatilization.The entire findings from the studies suggest that optimal maize yield could be achieved with high N UE and simultaneous reductions in yield-scaled GWP by manure application which also has the capacity to improve the soil quality.