| It is an important way to realize sustainable rice production to reduce the ineffective loss of water and nitrogen and improve the water-nitrogen utilization efficiency of rice field.The transformation from dry land to paddy field is a common form of farmland transformation in China.The difference of transformation time leads to the formation of a series of paddy fields with different cultivation history.The difference cultivation history significantly affected the physical properties of soil,and then changed its hydraulic parameters,resulting in the difference in paddy field water-nitrogen migration and transformation process.However,few studies have considered the effect of different cultivation history on water-nitrogen loss in rice fields,especially lack of studies on nitrogen balance in rice fields based on measured data correction and optimization of irrigation design.Based on this,the paddy fields and continuous tillage in 1998,2009 and 2015 were studied.The 100-year old rice field(1900)was selected as the research object.Through field in-situ monitoring,laboratory experiments and numerical simulation(HYDRUS-1D model),the dynamic changes of soil water content and nitrogen concentration in paddy field profiles under different cultivation history were revealed,and the components of water-nitrogen balance and their dynamic changes were quantified.The key paths and time nodes of water-nitrogen loss were identified,and an optimal irrigation scheme for rice field was designed considering the different cultivation history.The main conclusions are as follows:(1)The soil water content and nitrogen concentration were higher in paddy fields with the increase of cultivation history.With increasing depth,the volume water content and nitrogen concentration decreased gradually,and the fluctuation range of water content and nitrogen concentration in the surface soil(0-20 cm)was much greater than that in the bottom soil(20-100 cm).The mean volumetric water content increased with the increase of cultivation history.The volumetric soil water content in 1900 was 1.08,1.10 and 1.16 times of that in 1998,2009 and 2015,respectively.Soil nitrogen concentration also increased with the prolongation of cultivation history.The average soluble total nitrogen content in paddy soil in 1900 was 1.15,1.10 and 1.06 times of that in 1998,2009 and 2015,respectively.(2)The simulation results of the HYDRUS-1D model can be used to further analyze the characteristics of water-nitrogen balance.The RMSE of water content(0.00～0.058 cm-3 cm-3)and nitrogen(0.00～5.14 mg L-1)was lower,and R2 of water content(0.66～0.97)was higher.The NSE of water(-16.16-0.81)and nitrogen(0.09-0.86)were mostly close to 1.(3)The results of water balance showed that the cumulative runoff rate increased and the cumulative leakage decreased with the increase of cultivation history.From 2019 to 2021,an average of 89.50 cm of irrigation and rainfall input was carried out in the four paddy fields.Because groundwater recharge had a certain amount of capillary rising water,runoff,leakage and cumulative evapotranspiration were high,but evaporation was low.The average capillary rising water,runoff,leakage,transpiration and evaporation were 9.84,28.06,36.49,20.31 and 10.00 cm,respectively.With the increase of cultivation history,the surface runoff increased,but the deep leakage decreased.In 2015,2009,1998 and 1900,the average annual leakage was 86.96,96.30,68.75 and 39.89 cm,respectively,and the average annual runoff was 30.89,47.91,66.01 and 79.60 cm,respectively.With the increase of cultivation history,the changes of evapotranspiration and evaporation of paddy fields were small.Average evapotranspiration and evapotranspiration varied from 39.80～41.14 cm and 20.04～21.61 cm,respectively.The results of nitrogen balance showed that the longer the increase of cultivation history,the greater the nitrogen volatilization,denitrification and runoff loss.During the growth period,200 kg ha-1 nitrogen fertilizer was applied to each of the four paddy fields,and the nitrogen entering the paddy field was hydrolyzed,mineralized and nitrified.A large amount of nitrogen was absorbed by rice,and nitrogen was also lost through runoff,leaching,volatilization and denitrification.The average annual nitrogen hydrolysis,mineralization,nitrification,rice root uptake,runoff,leaching,volatilization and denitrification were 125.18,49.21,11.72,134.76,79.73,0.69,51.42 and 128.68 kg ha-1,respectively.Nitrogen runoff and denitrification increased with the increase of cultivation history.In 2015,2009,1998 and 1900,the average annual nitrogen runoff was 11.75,13.90,16.51 and 21.62 kg ha-1,respectively,and the denitrification was 237.99,255.71,255.88 and 279.85 kg ha-1,respectively.The changes of nitrogen hydrolysis,mineralization,leaching,root absorption and nitrification were small with the increase of cultivation history.The range of nitrogen hydrolysis,mineralization,leaching,volatilization,root uptake and nitrification in the four paddy fields were 120.06～131.36,43.65～51.30,0.01～3.30,46.69～63.19,112.00～142.53 and 106.74～128.42 kg ha-1,respectively.(4)The simulation results showed that paddy fields with different cultivation history had higher irrigation water-saving potential,and paddy fields with different groundwater levels had different water-saving potential.When the groundwater level and the irrigation time was not changed,the single irrigation water was reduced by 3.2,1 and 1 cm for the rice fields in 1900.1 998,2009 and 2015,respectively and the average water loss was reduced by 15.19 cm and the average nitrogen loss was reduced by 38.53 kg ha-1.The average water loss and nitrogen loss of the four paddy fields decreased by 20.03 cm and 52.10 kg ha-1 when the irrigation time was changed and 1-3 cm irrigation was applied to the paddy fields with short hydrotillage history in the rainy season.All the above methods can ensure that rice can absorb proper amount of water and nitrogen of more than 100 kg ha-1.When the local water level was-50 cm and-100 cm,the irrigation input was reduced by 3 cm,and the average water loss was reduced by 28.75 cm and 27.77 cm,and nitrogen loss was reduced by 16.58 and 15.45 kg ha-1,respectively,without changing the groundwater level condition and irrigation time.When the local groundwater level was150 cm,the single irrigation water decreased by 3.2.1 and 1 cm in 1900,1998,2009 and 2015,respectively,the average water loss decreased by 15.06 cm and nitrogen loss decreased by 13.59 kg ha-1.Changing the irrigation time,0,2,1 and 3 cm irrigation were applied respectively in-50 cm underground water,and 13.50,15.50,19.25 and 19.75 cm irrigation were applied respectively in-100 cm underground water.and in-150 cm underground water,When 31.50,31.00,24.00 and 17.50 cm irrigation were applied,the average water loss decreased by 38.01.18.99 and 8.26 cm,and the average nitrogen loss decreased by 24.63,20.43 and 20.83 kg ha-1,respectively.All the above methods can ensure that rice can absorb proper amount of water and nitrogen of more than 100 kg ha-1. |
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