| In the process of deepening the high-quality development of agriculture,it is necessary to make effective use of water and fertilizer resources,improve the environmental quality and physical and chemical properties of soil,and achieve the purpose of soil environmental restoration and management,which is a necessary measure to ensure the healthy and sustainable development of modern agriculture.γ-Polyglutamic acid(γ-PGA)is a kind of environmentally friendly polymer material,which has good ability to enhance soil water holding capacity and water and fertilizer retention.At present,the research mainly focuses on the effect of γ-PGA on soil water and fertilizer characteristics,and less on the characteristics and mechanism of soil water movement and its parameters.In view of the above analysis,this paper uses a combination of laboratory experiments,theoretical analysis and numerical simulation to explore the effects ofγ-PGA application amount(0,0.2%,0.4%,0.6%,0.8%)and application method(mixed and surface application)on the variation characteristics of soil moisture and hydraulic parameters of three texture soils(silty loam,sandy loam,sandy soil)and its mechanism,aiming to promote the use of γ-PGA in farmland systems.The main research results are as follows:(1)Effects of mixed application of γ-PGA on soil water vertical infiltration process:The application of γ-PGA to the soil infiltration characteristic parameters(cumulative infiltration,wetting front migration distance)is to inhibit infiltration.With the increase of application amount,the water content of each soil layer increased.In addition,the characteristics of soil water infiltration in the surface application of γ-PGA were the same as those in the mixed application treatment.However,compared with the mixed application,the soil cumulative infiltration and wetting front increased slowly with time.For the infiltration process with γ-PGA,the consistency between the simulated value of Philip model and the measured value is higher than that of Kostiakov model.(2)The effects of soil type and γ-PGA mixed application amount on soil water evaporation:The evaporation rate of the control group of the three soils was the highest.The average evaporation rate of the silty loam treated with γ-PGA was lower than that of the control group(CK)by 6.25%,4.61%,3.62%,13.16%,9.87%;the average evaporation rate of sandy loam decreased by 2.70%,8.56%,4.95%and 7.66%respectively compared with CK.The average evaporation rate of sand decreased by 5.78%,19.74%,12.98%and 22.48%respectively compared with CK.It shows that the application of γ-PGA in different soils can reduce the soil evaporation rate.The soil evaporation process after adding γ-PGA,through regression analysis,the fitting coefficient R2 of the power function is above 0.98.(3)The variation characteristics of hydraulic parameters of different soil textures under the condition of applying γ-PGA(mixed application):three soil textures can reduce the soil saturated hydraulic conductivity Ks at the beginning of the test,and the greater the decrease with the increase of the amount of application;under the same water content and the same soil texture condition,the less the application amount,the greater the soil water diffusion rate,and the exponential function relationship with the application amount of γ-PGA.The soil water characteristic curve moved to the right with the increase of γ-PGA application,that is,the soil water holding capacity was enhanced,which was manifested as:0,0.4%,and 0.8%.Under the condition of high suction,the difference of water content was obvious.In the silty loam,the volume water content of 0.4%increased by 0.27%and the volume water content of 0.8%increased by 28.94%compared with the control group(CK)at the stage of suction pF of 4.12.In sandy loam,compared with CK,the volume water content of 0.4%increased by 4.99%and the volume water content of 0.8%increased by 18.08%at the stage of suction pF of 4.13.Compared with CK,the volume water content of 0.4%increased by 92.42%,and the volume water content of 0.8%increased by 484.34%.The RETC software was used to fit the soil moisture characteristic curve,and the VG model was selected.The fitting coefficient R2 was above 0.99.(4)The dynamic characteristics of soil aggregates and pore structure in different soil textures under the condition of applying γ-PGA(mixed application):The distribution of water-stable aggregates with different particle sizes in silty loam,sandy loam and sandy soil after applying γPGA in each group of treatments showed that compared with the control group,the content of>0.25 mm aggregates in the soil could be significantly increased.Compared with the control group,the number of soil macroaggregates R0.25 in the silty loam 0.2%,0.4%,0.6%and 0.8%increased by 2.64,0.55,0.51 and 0.82 times,respectively;the R0.25 of each treatment in sandy loam soil increased by 0.70,0.46,0.96 and 1.93 times compared with the control group.The R0.25 of each sand treatment increased by 0.75,0.86,0.39 and 0.393 times compared with the control group.The geometric mean diameter GMD and mean weight diameter MWD evaluation indexes of soil aggregates also increased significantly.The fractal dimension D of soil particles was lower than that of the control group,and the D value decreased in the low application amount treatment(0.2%,0.4%).The D value increased in the high application amount treatment(0.6%,0.8%).The comprehensive index showed that the water-stable aggregates were more stable than the control group.At the same time,when the amount of γ-PGA was large,the aggregate evaluation index was significantly worse than the lower application amount,indicating that γ-PGA promoted the formation of large soil aggregates.For silty loam and sandy loam soil,the application of higher amount of γ-PGA effectively increased soil porosity and enhanced water retention capacity.For sand,applying a lower amount of γ-PGA will lead to a decrease in soil pore size and enhance soil water retention.(5)Analysis of the influence mechanism of γ-PGA on soil water movement and hydraulic characteristic parameters:The analysis results show that γ-PGA affects the hydraulic characteristic parameters through the regulation of soil structure,and then realizes the regulation of soil water movement and water holding capacity.At the same time,there are indirect and direct regulation methods for the regulation of soil water holding capacity:the regulation of soil structure is due to the continuous effect of γ-PGA application.The smaller the soil bulk density is,the larger the porosity is.The quantitative relationship between the amount of γ-PGA applied and the total porosity is polynomial(R2>0.92),and the influence on the equivalent pore size is fluctuating.The effect on aggregate structure increased the proportion and stability of large-size aggregates in soil.However,there is a threshold value for its promotion effect.This is because although γ-PGA adheres to soil particles and increases the viscosity of aqueous solution,the cohesion of γ-PGA is reduced because the experimental step is wet sieve,which has dilution.Due to the change of soil pore characteristics and aggregate structure characteristics,the soil hydraulic parameters,water movement and water holding capacity have changed.With the increase of γPGA application,the suction value of saturated soil when it begins to enter the air during dehydration increases,and the soil Ks and unsaturated water diffusivity decrease.Soil water infiltration rate decreased linearly(R2=0.93).The infiltration characterization values showed a decreasing trend,and the soil evaporation rate was inhibited.The direct effect of γ-PGA on enhancing soil water holding capacity is due to the strong water absorption capacity of γ-PGA itself. |
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