| High salinity and alkalinity,drastically upward movement of salt,vertical variability of salt content and so on,which are general problems in saline-alkali lands.Usually it led to lower and unstable productivity and less economic profits,and therefore soil salinization has become one of the main constraints of agricultural production.The spatial distribution characteristics of soil salinity mainly focused on the two-dimensional(2D)research scale and fewer researches discussed the interaction between the upper and lower layers of soil currently.Hence,it will be helpful for accurate salinization governance and layout of agricultural production to master the spatial distribution of soil salt content in the saline-alkali regions,especially for the three-dimensional(3D)research scale.This study has chosen the Yinchuan Plain of Ningxia which suffered a lot from the serious salinized soil and the salt-affected area of the Yanqi basin in Xinjiang province as our study regions.Then we utilized the field sampling and the electromagnetic induction method to monitor the soil salt content in the four nested scales(field,farm,Tongfu Village and Pingluo County)of the Yinchuan Plain and salt-affected soil amelioration area(about 70 ha)of Xinjiang.Then the geostatistics,3D inverse distance weighting method(IDW)and GIS method were employed comprehensively to investigate the 2D and 3D spatial distribution characteristics of soil salt content in the four nested scales of Yinchuan Plain and amelioration area of Xinjiang,revealing the major influencing factors of the spatial distribution of soil salinity under different scales in different regions.Moreover,precautions of utilizing 3D-IDW method to make spatial interpolations of soil salt content were discussed.Finally,Hydrus-1D method was used to simulate the leaching of soil salt at 0?80 cm layers with different irrigation treatments in the salt-affected amelioration area of Xinjiang.Soil salinity at the four scales in the salt-affected area of Yinchuan Plain exhibited moderate variation in the horizontal direction,and the statistical characteristics(Standard variation,maximum and minimum)of salt content at the same layer decreased with the scaling down.Besides,salt accumulation in the topsoil occurred at all scales,the total salt content under each scale diminished vertically.Soil salinity at different layers of the four scales had significant spatial autocorrelation and the spatial ranges for field,farm,village and county scale were 80 m,800 m,1km and 23?29 km respectively,showing an increasing trend with the scaling up.The spatial ranges of soil salinity at all scales increased with the soil layers deepening except that the spatial range of the content of the topsoil is larger than that of the lower layers under county scale.There were also obvious scale effects for the spatial distribution of soil salinity in the Yinchuan Plain.The soil salinity under county scale was higher in the northwest and lower in the southeast.Areas with high salinity were mainly located to the south of the Gaozhuang and to the north of Sand Lake,among which Xidatan district was the most severely affected by salinization and it belongs to unused lands.The soil salinity under village scale reduced from southeast to northwest and it became lower with the increasing distance to the Yellow River;the soil salinity under farm scale decreased from the southwest to the northeast;and the soil salinity at all soil layers under field scale decreased from west to east and the salt content was relatively low in the northern part.The soil salinization under county and village scales were mostly mild and moderate salinization and there was also severe salinized soil in some local area;and it was mainly mild salinization soil with some parts belonging to moderate salinized soil under farm and field scale.The main factors for the soil salinity variability under county scale were terrain,landforms,groundwater tables,land use patterns,and the concentrated irrigation canals and drains;the spatial variability of soil salt content was affected mainly by topography,groundwater tables,the irrigation canals and drains towards,the distance from sampling points to canals or drains and lateral recharge of the drain moisture under village scale;the most influence factors of soil salinity for farm scale was micro-relief and the distance from the drainage ditch;the influence factors for the field scale were the presence of the irrigation drains,soil texture,groundwater tables.Soil salinity at the topsoil varied most drastically in the typical region of the Pingluo County and its coefficient of variation was much larger than that at the lower layers.The spatial correlation distance of soil salinity in the horizontal and vertical direction was 6000 m and 1.2 m respectively.The 3D interpolation results showed that:the soil salt content was higher in the West and North,lower in the East and South of the study area.And the salt content in the majority of the study area fell in the range of 1-2 g kg-1,belonging to the mild salinized soil,and the 3D interpolation accuracy was much higher than that of 2D Kriging results.In addition,areas with higher salt content at topsoil also had relatively higher salt content at the lower layers.The 3D spatial distribution of soil salinity was mainly affected by topography,geomorphology,layout of irrigation canals,sampling intervals and soil textures.It was obvious that the soil profiles could be divided into two layers with the variability of soil salt content in the salt-affected soil amelioration area of Xinjiang,which included the 0?140 cm soil layer with the average salt content of 1.84?2.11 g kg-1 and the 140?200 cm soil layer with the mean of soil salt contents of about 1.75 g kg-1.The statistical characteristics(mean,standard deviation and coefficient of variation,etc.)of soil salt contents for all soil layers showed a decreasing trend with the increasing soil depths and salt accumulation occurred at 0?40cm layers.The spatial correlation distance of soil salt content increased with soil layers deepening and it had significant spatial correlation at all soil layers.Soil total salt content for all layers in the study region was substantially less than 6 g kg-1 and the soil content in most area was less than 2.5 g kg-1.And it had lower soil salt content near the north and south boundary,belonging to non-salinized soil;however,soil salt content over 4 g kg-1 was mainly distributed in the local areas of central and south areas.About 80%of the study area was non-salinized and light-salinized soil,and the 20%was moderate and heavy salinized soil.The main influence factors of soil salt content were soil parent material,climate,topography,hydrogeology,soil texture and irrigation.The spatial interpolation of soil salinity was finished with the 3D-IDW method and we explored the effects of vertical expanding multiples and searching points on the interpolation results.The results showed that the root mean square error(RMSE)of the measured and estimated values decreased with magnifying ratios of the vertical distances,but it increased with the increasing number of searching points,and the RMSE varied from 0.1 to 0.4.The optimal 3D-interpolation result was obtained when the vertical size expanded 300 folds and searching points was 6.The 3D spatial distribution map of soil salinity was obtained with the IDW method following the above determined parameters,and the results showed that the spatial distribution of soil salinity agreed well with the measured values.The spatial distribution pattern of soil salinity was consistent with the 2D interpolation results,but the accuracy was higher for the former.The main factors influencing the spatial distribution of soil salinity were irrigation or not,local terrain,the position of the claypans,the depth and mineralization of groundwater.Therefore,if the ratio of sampling intervals was large enough in different directions,it was crucial to choose a suitable expanding multiple for the direction with the minimum sample spacing and the numbers of searching points to guarantee the prediction accuracy for 3D-IDW interpolation.Simulation with the HYDRUS-1D model showed that it could remove almost 60?80%of the salt in the 0?80 cm soil mass if the irrigation water supply was 60 mm per day for 5 consecutive days.With small amount irrigation water,it could lower the soil salt content significantly supplying more water;however,the effect of over-irrigation on soil salt leaching was not obvious.The average irrigation amount was 164 mm.The mean of salt content in the 0?80 cm soil body before irrigation showed a significant spatial correlation and it became weaker after irrigation.Before irrigation the soil salt content on the north-south axis in the 0-80 cm soil mass was higher than other areas,and it was 1.5?2.5 g kg-1 in most of the study area.After irrigation the soil salt content became less than 1 g kg-1.91%of the interest area demanded less than 250 mm water supply for salt-leaching,and the other areas which demanded more water were located in the high salinity area with clay texture.It needed 180 mm water for flood irrigation in the whole study area and it would save about 40%of the conventional irrigation amount(300 mm).Soil salt storage quantity in the 0-80 cm soil body before irrigation was 10-45 kg m-2,and it was less than 15 kg m-2 after irrigation;and the total salt storage quantity decreased from 17647 t to 8560 t,salt-leaching rate in the 0-80 cm soil body was 51%,achieving the aim of desalination. |
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