Effect Of Bio-tillage On The Least Limiting Water Range Of Clayey Red Soil

The subtropical red soil region in China,summer and autumn is prone to occur seasonal drought because of high temperature with little rain,which is characterized by lack of water in the surface layer.However,the red soil is sticky and heavy with poor aeration and it is difficult for crop roots to absorb and utilize water of deep layer.Deep root rotation crops can improve soil physical properties,promote water absorption under the root system of crops in the later season,and alleviate seasonal drought by bio-tillage,but the improvement mechanism and effect are still unclear in clayey red soil.The soil least limiting water range(LLWR)is a range of water content which is suitable for crop growth,determined by penetration resistance,air filled porosity,field capacity and permanent wilting point,reflects the comprehensive physical quality of the soil.This paper studies the effects of several autumn and winter cover crops on the LLWR of clayey red soil through field experiments,evaluates the ways and effects of bio-drilling to improve the physical properties of clayey red soil,it provides a basis for selecting suitable cover(rotation)crops and mitigating seasonal drought in the region.The field experiment was carried out at the of Huazhong Agricultural University Red Soil Comprehensive test station from October 2020 to October 2021.The soil is Quaternary red clay.A total of 7 bio-tillage treatments in autumn and winter were set up:common rape(RapeC),1-year alfalfa(Luc1y),perennial alfalfa(Luc5Y),1-year vetiver(Vet1y),deep-rooted rape(RapeD),perennial vetiver(Vet5Y),and blank(plant-free)treatments(Control).When sowing(or transplanting)in October,shallow tillage of 15 cm,application of the same amount of NPK basal fertilizer,manual weeding.Harvesting of the upper part of the field and sampling to determine root and soil properties.The main results are as follows:(1)The bio-tillage ability of cover crops in soil varies significantly.The root leng th density RLD(cm/cm~3)of crops at depths of 0～20 cm were RapeC(2.37)>Vet5Y(2.18)>Luc5Y(1.93)>Luc1y(1.90)>Vet1y(1.41)>RapeD(0.99).The roots of the two kinds of rape are mainly distributed in the soil shallow layer(0～20 cm).In the soil lay er below 20 cm,with the increase of soil depth,the roots of RapeC decreased rapidly,while the roots of RapeD decreased.the roots of vetiver and alfalfa not only decrease d slowly,but also increased in local soil layers.The RLD(1.76)and root penetrate de pth(RPD)of Vet5Y were larger than those of other crops,and the biological tillage ab ility was stronger.(2)Bio-tillage improves soil physical properties,and the improvement effect varies with root characteristics.There was no significant difference in the effect of bio-tillage on soil available water content(the difference between field capacity and permanent wilting point),but the effect of Luc5Y and RapeD in increasing air-filled porosity was higher than other crops.All bio-tillage reduced soil bulk density slightly and reduced soil penetration resistance greatly,among which Vet1y reduced soil bulk density the most(7.84%),RapeC mainly reduced shallow soil penetration resistance(18.23%),while several other deep-rooted crops also significantly reduced the penetration resistance of soil layers below 20 cm(28.58%～50.83%).But even so,the penetration resistance of soil below 20 cm is still very high(≥2500 kPa),which is the biggest soil physical factor to reduce LLWR.(3)Bio-tillage improve LLWR mainly by reducing penetration resistance.in the shallow soil layer(0～20 cm),the soil field capacity and permanent wilting point determine the upper and lower limit of LLWR,while bio-tillage has no significant effect on the least limiting water range of this layer.With the increase of soil depth,the factor that determines the upper limit of LLWR changes from field capacity to 10%air-filled porosity,while the lower limit changes from permanent wilting point to 2000kPa penetration resistance,penetration resistance and air-filled porosity become the main limiting factors.In the soil layer below 20 cm,the decrease effect of bio-tillage on penetration resistance is Vet5Y>Luc5Y>RapeD>Vet1y>Luc1y>Control>RapeC,and the first three have the best effect on the improvement of air-filled porosity,so LLWR of the three is the largest(0.31～0.38 cm~3/cm~3)after bio-tillage,followed by Vet1y(0.39～0.44 cm~3/cm~3),while the LLWR of the control is almost zero,which is not suitable for crop growth.(4)There was no significant correlation between the maize yield and soil LLWR in the next season of bio-tillage,because the precipitation in the growing season of maize was too much this year,and the soil water content in the main production period of maize was higher than the upper limit of LLWR for a long time,which masked the difference of LLWR between bio-tillage treatments.The results showed that bio-tillage increased soil LLWR mainly by reducing the penetration resistance of red soil,and the improvement effect was related to the RLD and RPD of crops,but not to whether the main root system or the fibrous root system was not related.RapeD、alfalfa and Vet5Y can be used as autumn-winter rotation cro ps through bio-tillage.The main reason of LLWR stenosis is the higher penetration re sistance of clayey red soil.The classical penetration resistance value of water content corresponding to<2000 kPa as the lower limit of LLWR is too low,which needs furth er improvement.