Spatial Distribution And Migration Mechanism Of Soil Water And Salt In The Newly Cultivated Land After The Project Of "Gully Land Consolidation" In Loess Hilly Region

The Loess Hilly region of China suffers from very serious soil erosion,which has previously led to significant ecological environmental damage in this region.Upon implementing a project of returning farmland to forests in 1999,the ecological environment in this area improved dramatically,the intensity of soil erosion dropped significantly,the increase of the years of the silt deposition and the sharp decrease of the silt deposition effect in the traditional dam.The project of"Gully Land Consolidation"was implemented in 2008,which further improved the ditch area environment and developed a large area of the newly cultivated land to increase the amount of high-quality cultivated land.However,due to elevation of the groundwater table caused by the"Gully Land Consolidation"project,some of the newly cultivated land has been subject to soil salinization,which seriously affects the effective use of the high-quality resource benefits that come from the newly cultivated land.It is undoubtedly necessary to study the soil salinization in the newly cultivated land.Spatial distribution and migration characteristics both soil water and salt were studied,which could clarify the soil water–salt coupling mechanism,reveal the soil salinization process mechanism,promote the further development of soil salinization knowledge,for soil salinization prevention and scientific control,ensuring the efficient use of newly cultivated land,providing an important scientific basis for effectively achieving sustainable regional development in the Loess Hilly region.This study examines newly cultivated land in the Nangou watershed,Yan’an City,Shaanxi Province.This study used field sampling and indoor testing,classical statistics and multifractal method,and revealed the spatial differentiation characteristics both soil water and salt,the dynamic migration law,the regulation effect of tillage measures on soil water and salt,and the coupling mechanism between soil water and salt in the newly cultivated land.The main conclusions are as follows.（1）The spatial distribution characteristics of soil water and salt in the newly cultivated land were determined.1)Along the head to the end of the ditch,the soil water,and salt levels gradually decreased.The average soil water and salt levels in upstream land were 17.6%and0.81 g·kg^-1,respectively,23.0%and 14.1%higher than the downstream land.The depth of the groundwater influence zone also decreased along with the head to the end,showing a significant positive correlation between the average soil water content and the average salt content（P<0.05）.The depths of the groundwater influence zone were–1.33 m and–3.67 m in upstream and downstream land,respectively.2)The spatial variability of the soil water and salt was higher in upstream land than in downstream land.The runoff and sediment generated from the slope invaded the upstream land,and uneven deposition occurs,which was an important reason for the high spatial variability of water and salt in the upstream land.In addition,the depth of the groundwater influence zone was also an important factor affecting the spatial variability of soil salt,which showed a significant correlation with the multifractal parameters of soil salt（P<0.05）.These results showed that the shallow depth of the groundwater influence zone facilitated the water exchange between groundwater and soil water in the upstream land,which improved the soil water content and promoted the accumulation of salt in the soil.Thus,upstream land is the key area for soil salinization prevention in the future.（2）The dynamic migration characteristics of soil water and salt in the newly cultivated land were clarified.1)After a rainstorm,the dynamic soil water migration was divided into the fast rise stage and the slow fall stage in the whole soil layer.Potential evapotranspiration affected all dynamic soil water migration（P<0.05）,rainfall,and depth of groundwater influence zone affected the fast rise stage and slow fall stage of soil water,respectively（P<0.05）.2)After a rainstorm,the dynamic salt migration in the 0–40 cm soil layer was divided into three stages—leaching stage,rapid accumulation stage,and slow accumulation stage,which were affected by rainfall,potential evapotranspiration,and the groundwater influence zone depth,respectively（P<0.05）.The dynamic salt migration in the 40–80 cm soil layer was divided into the rapid accumulation stage,the leaching stage,and the slow accumulation stage.In the leaching and rapid accumulation stages,dynamic migration of soil salt was affected by rainfall（P<0.05）.In the slow accumulation stage,dynamic migration of soil salt was affected by the depth of groundwater influence zone and potential evapotranspiration（P<0.05）.The different migration stages in soil water and salt were controlled by different influencing factors.Based on different migration stage characteristics between soil water and salt allows adopting appropriate measures to reduce the influence of control factors in corresponding stages on soil salt to lower salt accumulation in soil and provides a new idea for preventing soil salinization.（3）The regulation effect of tillage measures on the soil water and salt in the new cultivated I was clarified.1)Soil salt was accumulating in the newly cultivated land during the monitoring period.In the 0–40 cm and 40–80 cm soil layers,the average salt accumulation was 0.08 g kg^–1 and 0.06 g kg^–1,respectively.For the entire soil layer,average soil salt accumulations under the flat cropping,ridge planting,plastic mulching,and straw mulching measures were 0.19,0.16,0.08,and 0.12 g kg^–1,respectively.Without corresponding control measures（flat cropping measures）,the newly cultivated land will transform into slight saline-alkali land after ten years.2)The soil salt accumulation after plastic mulching treatment was the lowest,33.3%,50.0%,and 57.8%lower than straw mulching,ridge planting,and flat cropping measures,respectively.At the same time,the soil water level under plastic mulching measure was the highest,4.9%,8.9%,and 9.7%higher than straw mulching,ridge planting,and flat cropping measures,respectively.These results showed that plastic mulching had a good effect on water conservation and salt suppression in the newly cultivated land,and recommended for agricultural production application.（4）The soil water-salt coupling mechanism in the newly cultivated land was revealed.The spatial distribution of soil water and salt has high correlation in different spatial sampling sites,and the essential reason is the coupling effect of soil water and salt.Soil water is the carrier of salt migration.As salt moves with soil water,the spatial distribution of soil water largely determines the spatial distribution of salt.When the relative soil water content was 0.52–0.75,soil water and salt had a coupling effect,which promoted a high correlation between soil water and salt spatial distribution.When the relative soil water content exceeded 0.75,the correlation between soil water and salt spatial distribution was low due to the uneven soil salt dissolved by rainfall.When the relative soil water content did not exceed 0.52,low soil water content caused salt crystallization.Solid crystalline salt reduced the pore size and volume of the soil,affected soil water and salt spatial distribution,and thus reduced the correlation between soil water and salt spatial distribution.These results expanded the understanding of the impact of water salt coupling on the spatial distribution of soil water and salt.These results also gave the range of the relative soil water content when there was a high correlation between soil water and salt spatial distribution;this can improve the simulation accuracy of the soil water salt coupling model and predict the spatial distribution characteristics of soil water and salt.