Study Of Nitrogen Leaching Process From Urban Green Turf Under Reclaimed Water Irrigation

Urban green turf have important ecological functions of improving water environment and regulating microclimate.With the growth of urban population and the expansion of construction scale,the area of urban turf is increasing year by year,and the contradiction between supply and demand between large amount of irrigation water and shortage of fresh water resources is becoming more and more acute.Reclaimed water provides a reliable and economical source of irrigation water for turf,and its reasonable use can effectively improve soil fertility and promote good plant growth.However,excessive nitrogen input from reclaimed water into the soil-groundwater system will lead to the deterioration of the ecological environment of water bodies.The development of a safe and efficient reclaimed water irrigation scheme is important for realizing the virtuous cycle of water resources and preventing water environment pollution.In order to promote the construction of the water-covering function area and ecological environment support area of Zhangjiakou capital,and to address the current ecological problems such as water scarcity,soil erosion and severe non-point source pollution in the region,the article selects a typical local turfgrass（Poa annua L.）as the research object,the effects of three irrigation modes,namely single reclaimed water irrigation（RI）,mixed reclaimed water-clear water irrigation（MI）and reclaimed water-clear water alternating irrigation（AI）,on soil nitrogen leaching and accumulation characteristics were investigated by outdoor column leaching experiment in 2022.The HYDRUS-2D model was used to simulate soil water infiltration,root water uptake,nitrogen loss and accumulation under different reclaimed water irrigation scenarios(R_1/4I_100%～R₄I_60%).The scientific and reasonable reclaimed water irrigation scheme for turf was investigated to achieve efficient use and safe management of reclaimed water.The main findings are as follows:（1）The average concentrations of TN（2.94 mg/L）,NH₄⁺-N（1.26 mg/L）and NO₃^--N（1.27 mg/L）in the drench solution at 120 cm soil layer of the turf under AI treatment were the highest,followed by RI and MI treatments.The overall fluctuation of nitrogen leakage concentration from April to June was greater than that from September to October;the over-standard rate of NH₄⁺-N concentration in the drench solution reached more than 80%,and the reclaimed water irrigated turf might cause shallow groundwater pollution.（2）The main form of mineral nitrogen in the soil profile under the RI treatment was NO₃^--N,which was significantly different from the MI and AI treatments（P<0.05）;in the vertical direction,the distribution of soil total nitrogen and alkali-hydrolyzable nitrogen in the three treatments showed an"inverted"trend,while the distribution characteristics of soil NH₄⁺-N and NO₃^--N were different;soil nitrogen was positively correlated with Cl^-,and Cl^-was an important factor influencing regional nitrogen changes.（3）The HYDRUS-2D model can better simulate the soil water and nitrogen transport process under RI treatment.The Root Mean Squared Error（RMSE）and the consistency index（d）of soil water and deep seepage simulations in the rate and validation stages ranged from 0.002 to 3.26 and 0.60 to 0.96,respectively;the RMSE and d of soil nitrogen leaching and accumulation ranged from 0.10 to 0.92 and 0.74 to0.90,respectively.（4）The responses of water infiltration and nitrogen leaching from turf to rainfall and irrigation were basically the same.The water content of all soil layers ranged from 0.209 to 0.308 cm³/cm³,from high to low 90 cm,50 cm,70 cm,and 30 cm,respectively;the maximum amount of deep water seepage（40.62 mm）occurred on June 28,accounting for 27.99%of the total seepage（712.39 mm）during the peak growth period.The total amount of NH₄⁺-N and NO₃^--N leaching occurred mainly in spring and summer（April to August）and autumn（September to October）,with 2.79kg/hm²and 1.97 kg/hm²,respectively.（5）The varied effect of different reclaimed water irrigation scenarios on soil water and nitrogen transport patterns in turf.The cumulative infiltration（421.54 mm）was significantly the least under the R_1/4I_60%to R₄I_60%scenarios,while the root water uptake（548.18 mm）was basically unchanged and the water utilization rate was the highest;the cumulative nitrogen loss under the R_1/2I_60%and R₁I_60%treatments increased by 1.39%and 4.16%,respectively,compared with the R_1/4I_60%treatment,and the nitrogen leaching rates were smaller,ranging from 2.38%to 5.72%and 1.22%to 2.86%,respectively;among them,the nitrogen accumulation in each soil layer under the R_1/2I_60%scenario decreased by 3.36%to 33.55%compared with the R₁I_60%scenario,therefore the R_1/2I_60%reclaimed water optimized irrigation scheme is recommended for the study area.