| Quantifying the availability of effluent nitrogen(N)to crop is essential for developing environmentally sound fertilization strategy when applying treated sewage effluent.Overestimating the availability of effluent N leads to the insufficient of soil N,thus reducing the crop growth and yield.Conversely,treated sewage effluent irrigation may reduce the N use efficiency,but also increase the risks of nitrate leaching and agricultural non-point source pollution.Drip irrigation is one of the most appropriate irrigation methods applying treated sewage effluent because it can increase the irrigation and fertilization efficiencies and diminish the ecological hazards and health risks under treated sewage effluent irrigation.Accordingly,identifying the effluent N availability can provide scientific evidences for the treated sewage effluent irrigation.To mitigate or eliminate the influences of rainfall and spatial variability of soil nutrients on the responses of water quality and N application rate to maize(Zea mays L.),pot experiments of maize under drip irrigation were performed using 15N isotope tracer method plus fertilizer equivalence(FE)approach to quantify the effluent N availability.In the pot experiments,four N application rates ranging from 0 to 2.64 g/pot(0-210 kg/hm~2 equivalently)were established in 2014 and 2015 applying either secondary sewage effluent(SW)or groundwater(GW).Additionally,four irrigation sources,including groundwater(G),secondary sewage effluent and groundwater blended at 4 to 2(S67%)and 5 to 1 ratios(S83%),and secondary sewage effluent(S100%)were tested in 2015.During maize growing seasons,the distribution of soil N contents at different depths,N balances of sewage effluent and fertilizers and effluent N availability were determined.Meanwhile,plant height,leaf area index(LAI),dry biomass,N uptake,root weight density,root length density,yield,yield determinants and qualities of maize were evaluated.Field experiments were conducted to confirm the findings of pot experiments.In the field experiments,four N application rates ranging from 0 to 180 kg/hm~2 were investigated in 2014 and 2015 applying with SW and GW.During the maize growing seasons,the distribution of soil N contents at different depths,deep percolation and nitrate leaching at 70 cm depth were determined.Additionally,plant height,LAI,dry biomass and N uptake of above ground,yield and yield determinants of maize,and N use efficiency were determined.Furthermore,a crop growth model of DNDC was established to evaluate the influences of water quality,N application rate and irrigation amount on maize growth more systematically.The main conclusions of this study are as follows:(1)Pot experiments revealed that 15N decreased with the increasing radial distances in silt loam soil,while effluent N increased first and then decreased,with the maximum value at 12 cm from the center of the pot.Both 15N and effluent N were mainly distributed within the top 35 cm soil layer,being in accord with the root distribution of maize.Pot and field experiments showed that SW irrigation enhanced the N contents,particularly increased the N accumulation at the lower soil layer compared to GW irrigation.(2)Higher N application rates decreased the N uptake derived from effluent.The percentage utilization of effluent N declined from 45%to 33%as N application rates increased from 0 to 2.64 g/pot on average.Compared with GW irrigation,SW irrigation facilitated the uptake of 14N labeled urea,thus increasing the total N uptake.However,SW irrigation reduced the percentage of plant N derived from fertilizers compared to GW irrigation.Unlike the N use efficiencies decreasing with increasing N application rates under GW irrigation,the percentages utilization of total N(including effluent and fertilizers)under SW irrigation averagely increased from 45%to 55%when the N application rates increased from 0 to 1.76 g/pot,and subsequently decreased to 46%at the N application rate of 2.64 g/pot for pot experiments.Similarly,the apparent recovery efficiency of applied N(51%)peaked at 120 kg/hm~2 for the field experiments.Furthermore,increasing sewage proportions evelated the assimilation of effluent N by maize.(3)Pot experiments indicated that FE approach could be applied to quantify the effluent N availability,with the relative errors between calculated and measured values of yields in 2014 and 2015 were only-0.2%and-2.6%under SW irrigation.The FE values decreased quadratically with increasing N application rates.The FE values ranged from 0.58 to 0.79 g/pot(46-63 kg/hm~2 equivalently).The N availability of effluent was only 50%-69%as available as an equivalent application rate of fertilizer urea N,being attributed to the N compositions of effluent and the coupling relationship between fertilizers and effluent.SW irrigation could provide approximate 58%and 13%urea N reductions for pot and field experiments to attain a satisfactory yield(95%of the maxmum yield)in comparison to GW irrigation.In addition,increased the application rate of effluent N could increase the FE values,but reduced the relative FE values.(4)Field experiments suggested a low risk of nitrate leaching in silt loam soil when applying properly managed drip irrigation.However,heavy rainfall dramatically increased the nitrate leaching.Additionally,the ranking of factors increasing the leaching of nitrate was fertilizer N,deep percolation,irrigation N and initial soil N.Therefore,SW irrigation coupled with reduced fertilizers was helpful to reduce the seasonal nitrate leaching.(5)Pot and field experiments indicated that SW irrigation and higher N application rates both increased growth and yield of maize.However,higher N application rates reduced the sewage-derived effects.N application rate had greater effects on maize than water quality and pot experiments strengthened the sewage-derived effects than field experiments.The apparent N loss values were enhanced mostly with increasing N application rates,followed by increasing initial soil N and nitrogen containing in irrigation water(irrigation N).(6)DNDC model was used to simulate the maize growth and yield when irrigated with SW by hypothesizing three relative FE values of 1.0,0.65 and 0.3.Results showed that the best simulation of maize was recorded at the relative FE value of 0.65 and DNDC model could be used to simulate maize growth of using SW irrigation,thus simulating the influences of different water and N management measures on maize.SW irrigation and higher N application rates also improved the maize performances,while higher irrigation amount leads to a slightly yield reduction.To achieve satisfactory yields and high N use efficiencies,N application rates of 140～160 kg/hm~2 for SW irrigation and 160～180 kg/hm~2 for GW irrigation coupled with the 75%of full irrigation were recommended under drip irrigation in the North China Plain.In conclusion,SW irrigation facilitated the uptake of 15N labeled urea,thus increasing growth and yield of maize.However,increased N application rate reduced the N availability of effluent,and there was quadratic curve relationship between effluent N availability and N application rate,with the relative FE values ranging from 50%to 69%.Additionally,nitrate leaching and the apparent N loss were both increased with increasing N application rates and irrigation N.Therefore,SW irrigation coupled with reduced N application rate could effectively reduce N loss and enhance N use efficiency.In the North China Plain,75%of full irrigation and N application rates of 140～160 kg/hm~2 were recommended for maize to achieve a satisfactory yield and high N use efficiency,with approximate 20 kg/hm~2 urea N reduction when applying drip irrigation with SW. |
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