Effect Of Seasonal Freezing-thawing And Subsoiling Preparation On The Recovery Of Compacted Black Soil

Seasonal freeze-thawcycles and agricultural subsoiling operations are two dominant factors affecting soil structure in typical black soil region of Northeast China.Based on the simulation of mechanical compaction and followed by subsoiling operations in the field,the influences of mechanical compaction and subsoiling operations on soil structure,especially on the micro-aggregatecharacteristics were revealed by measuring and analyzing the soil structural indicators and the soil micro-aggregates distribution,fractal dimension(D)and dispersioncoefficient in twoconsecutive years at different soil depths.At the same time,the effects of mechanical compaction and subsoiling preparations on the seasonal freeze-thawcharacteristics and the soil temperature and humidity ofcropland soil in black soil area were discussed.The resultscan provide a theoretical basis for the restoration of compacted black soil.The main findings are as follows:(1)Mechanical compactioncan influence the soil structure and micro-aggregatecharacteristics in the soil depth of 0?80cm.The mechanical compaction mainly increased soil bulk density and soil saturation in the surface area(0-20cm),while the soil porosity and generalized soil structure index(GSSI)were reduced gradually with increasing the passes of traffic compaction,especially when the mechanical compaction passes increasing to 12 times,the soil porosity is as high as 0.95,and the mean weight soil specific area(MWSSA),D value and dispersioncoefficient were significantly higher thancontrolcondition,which indicates that repeated compactioncouldcause a significant reduction on soil micro-aggregate stability.In the subsurface area(20?40cm),soil porosity was significantly reduced by mechanical compaction,but it was not related to the number of compaction passes,and the soil micro-aggregate stability was weakened by the accumulate compaction passes.For the deeper soil(40?80cm),the total soil porosity decreased by 22.27%after 12 compaction treatments,soil saturation and GSSI were also reduced gradually with compaction passes,and even few passes of compactioncould significantly reduce the stability of the soil micro-aggregates,which indicated that the mechanical compaction had a significantcumulative effect on the deeper soil layer.(2)For the compacted farmland black soil of different compaction passes,the diurnal and the annual variation of the soil temperature both decreased with the increase of soil depth,thecorrelationcoefficients of monthly average temperature and the mean soil temperature of 5cm,15cm,30cm,60cm and 100cm depth were 0.994?0.988?0.956?0.852 and 0.650 respectively,which reflects the important influence of air temperature on soil temperature,and this trend is weakened with the increase of soil depth.In the black soil area of NortheasternChina,mechanical compaction increased the soil freezing rate and maximum frost depth.The freezing rates of soil without compaction, compacted for 3 passes,6 passes and 12 passes were 0.95,1,12,1.11 and 1.15cm/d,respectively,and the maximum frost depth increased by 10cm when the soil was compacted.This situation lasted for more than 2 years,but the difference decreased in the second year.The surface soil began to melt in the end of the March,but because of the soil temperature in deeper depthchanges in the lag,frozen depth was still increasing at the beginning of the thawing period.The thawing rate of the compaction soil was faster than the un compacted soil,so the date of thawing completely was earlier,too.Mechanical compaction increaseed the daily fluctuation of soil temperature at 5cm and 15cm depth,the date of the soil in 60cm depth begins to freeze was 3 days earlier,and the date of the soil in 30cm and 60cm depths when melt completely was 10 days earlier in thawing period,also the soil watercontent was lower than un compacted soil in 0-60cm depth.(3)The seasonal freeze and thawcycles mainly reduced the porosity of un compacted soil,decreased the GSSI in the deeper soil when the mechanical compaction treatment was only 3 passes,which indicated that the seasonal freeze and thawcycles may have acertain destructive effect when the soil was not compacted or compacted only in few times.But the seasonal freeze and thawcycles increased both the soilcapillary and non-capillary porosity in thecontinuous tow years,and the distribution of soil pore was better thancontrolcondition after two seasonal freezing-thawing processes.At the same time,the D value of soil micro-aggregates and dispersioncoefficient were significantly higher thancontrolcondition when the mechanical compaction treatment was only 3 passes,although the soil dispersioncoefficient was also higher thancontrolcondition when the mechanical compaction passes increasing to 12 times,the D value of soil micro-aggregates was no significant difference compared with thecontrol.In general,the seasonal freeze and thawcycles generally exacerbated the damage of micro-aggregates and reduced the stability when the soil was compacted only a few times,but displayed acertain recoverycharacteristic when the mechanical compaction passes were extremely increased.(4)Subsoiling operations was beneficial to increase the maximum snow depth,decrease the freezing rate and the thawing rate in freezing-thawing period.The maximum frost depth was reduced after subsoiling operations,the time was earlier when the soil thawing completely.For uncopacted soil,the maximum frost depth was reduced after subsoiling to both 15cm and 35cm,but the difference between the subsoiling depths was small.And for the soil when mechanical compaction treatment was only 3 passes,although the impact of subsoiling to 15cm depth was small,but has restored to un compaction situatio.But for 6 and 12 passes,the maximum frost depth was reduced significantly by the subsoiling depth,especially when subsoiling to 35cm,the maximum frost depthchanged by 38.5cm and 29cm,respectively.This situation lasted for more than 2 years,but the difference decreased in the second year.The daily fluctuation of compacted(12 passes)soil temperature at 5cm depth was reduced after subsoiling operations,and the fluctuation of the soil temperature was between the un compacted soil and the compacted soil(12 passes)with no subsoiling,indicated that subsoing preparation measurescan alleviate thechanges in temperature fluctuationscaused by mechanical compaction.At the beginning of the freezing,the soil temperature after subsoiling was decreased slowly,incontrast with the impact of mechanical compaction,the date of the soil in 30cm depth begins to freeze was 3?4 days later.Similarly,at the beginning of the thawing,the soil temperature with subsoiling rose slowly,and the temperaturecurve during the thawing period was basicallyconsistent with the un compacted soil,itcan be said that the soil temperaturechangecaused by mechanical compaction was completely restored by subsoiling.(5)Subsoiling operationscan have a direct effect on soil bulk density and porosity,thus affecting the composition and stability of soil micro-aggregates,and the influences of subsoiling operations will last for more than 2 years.Subsoiling to 15cm and 35cm depths both reduced the soil bulk density,while increased soil porosity of the un compacted soil in the first year after subsoiling,the soil bulk density also declined after 2 years in the surface area(0?20cm).The soilcapillary porosity only reduced in the surface area(0?20cm)when the mechanical compaction treatment was only 3 passes,but increased in both subsurface area(20?40cm)and the deeper area(40?80cm).Subsoiling to 15cm and 35cm both decreased the soil bulk density in 2 years when the mechanical compaction passes increasing to 12 times,and increased the soil porosity in all soil layers(0?80cm),and the soil porosity was increased with the depth of subsoiling.At the same time,for both un compacted soil and compacted soil only by 3 passes,subsoiling operations reduced the stability of the soil micro-aggregates in the first year after subsoiling,but had a positive impact in the second year.Also,subsoiling to 15cm mainly influence the upper layer(0?40cm),while subsoiling to 35cm improved the stability of the soil micro-aggregates from deeper soil layer.And it is important that the stability of micro-aggregates increases with the depth of subsoiling in the studied soil layers(0?80cm).