| The conservation tillage and liquid fertilization enhance the productivity of farmland.One of the soil fertilization mechanisms for earthworms is that the earthworm swallowed soil and excreted particle casts while burrowing in soil.In this process,the earthworm mucus plays critical roles in adhesion reducing,soil fertilizing and so on.In this work,we focused on three earthworm species(Eisenia fetida,Aporrectodea trapezoids and Amynthas pingi),which had different living habits.A low-voltage electro-stimulation device was developed to extract and collect earthworm mucus rapidly.The extraction efficiency of earthworm mucus reached about 15 wt.%in the first time,and 5 wt.%-6 wt.%in the second time after three days feeding,respectively.Meanwhile,the mucus constituents of three earthworm species were tested,including total carbohydrate content,total protein content and 16 kinds of amino acids content.The most important mechanism of reducing soil adhesion for earthworms was that earthworms secreted mucus continuously and formed a lubrication layer on the body surface during motion process.The rheological characteristics tests of earthworm mucus indicated that the movement of earthworm decreased the apparent viscosity of mucus,exhibiting shear-thinning rheological behavior.With decreasing adhesion between body surface and soils,the mucus exhibited drag-reducing effects on earthworm burrowing.When earthworm slowed down or stopped burrowing,the apparent viscosity of mucus in the adhesion layer and infiltration layer restored gradually due to thixotropic characteristics.The mucus adhered to soil particles and constituted the wall of earthworm burrows.The charge potentials tests of earthworm mucus showed that the mucus exhibited a negative potentials.While the earthworm moving in soil,the mucus with negative potentials was attracted by the body surface with positive potentials which made mucus layer adhere to the surface of earthworm firmly.The mucus layer attracted hydrated cation but repelled clay with negative potentials in soil,which thickened the mucus-liquid layer and reduced soil adhesion to body surface.The corresponding earthworm mucus contact angles were measured to test the wettability of the earthworm surface,which showed that the earthworm surface had good hydrophilicity.The secreted mucus wetted the body surface when earthworm expanded the muscles.The earthworm mucus infiltrated the body surface completely,which was beneficial to protect the earthworm surface.The mucus overflowed inter segmental grooves when earthworm contracted muscles.Then,the mucus tended to separate from body surface,which was conducive to renew the mucus layer.The retractable movement of earthworms constantly destroyed the mucus layer,which changed the contact area between the body surface and the soil.The flexible contact conditions resulted in a non-continuous contact interface among soil,mucus and body surface.The earthworm movement decreased the actual contact areas between earthworm surface and the soil,so as to reduce the soil adhesion and drag resistance.Inspired by amino acids composition of earthworm mucus,the bionic liquid fertilizer was prepared.The influence on corn emergence and growth of earthworm mucus,bionic liquid fertilizer and urea were studied with water as a control group.It can be confirmed by the cultivation tests that,earthworm mucus and bionic liquid fertilizer had fertilizer effects.The test results of three kinds of liquid fertilizer effects for seed vigor,seedling emergence rate and seedling quality of corn showed that the mucus treatments demonstrated the highest seedling emergence indexes.The influences for corn emergence rate were as follows:earthworm mucus>bionic liquid fertilizer>urea.The low concentration treatments were beneficial to the corn emergence while the high concentration treatments were beneficial to the corn growth and root development.The low concentration earthworm mucus was the most significant fertilizer treatment for the seedling quality.The test results of three kinds of liquid fertilizer effects for corn growth period indicated that the high concentration treatments provided more nitrogen nutrition than low concentration treatments.Among various fertilizer treatments,there is not a significant difference between the contents of available potassium and available phosphorus.Furthermore,all treatments had no significant effects on the pH value of soil samples.The stem height and stem diameter of corns were significantly promoted by the three kinds of liquid fertilizer.The significant influences of fertilizer treatments for stem height,stem diameter of corn,relative chlorophyll content(SPAD)and photo synthetic characteristic of corn leaves were as follows:bionic liquid fertilizer>urea>earthworm mucus.The earthworm mucus and bionic liquid fertilizer demonstrated obvious fertilizer effects,which increased soil nutrients,promote corn growth and dry matter accumulation.The profile curves of earthworm head and body surface were extracted and fitted using image processing technology.Geometric structure surface of sub-soil liquid fertilizer injection device was designed based on the contour curve equations,and the structural parameters were determined with reference to the burrows of the meadow voles.The sub-soil liquid injection fertilizer devices(3 injection holes and 6 injection holes)were designed,which matched with the bionic subsoiler.In order to maximize the pressure and flow rate of outlets,structural parameters of the devices were optimized as follows:anti blocking angle was 50°,distance of outlets was 20 mm,included angle between outlets was 70° by using parametric modeling&finite element analysis collaborative simulation and optimization technology.The significant influence of factors was the liquid injection,the number of holes,the materials and the surface structure for drag resistance,and the surface structure,the number of holes,the materials and the liquid injection for soil adhesion.The prototype performances were tested by using soil bin test system.The performance parameters of holes,liquid injection,material and geometrical structure surface were taken into account to present the effects on drag-reducing and anti-soil adhesion in tests.The significant influences of the performance parameters for drag resistance were as follows:liquid injection>hole number>material>surface structure.The significant influences of the performance parameters for soil adhesion were as follows:surface structure>hole number>material>liquid injection.The optimal level of each parameter was the liquid injection,6 holes,UHMWPE material and geometrical structural surface.The optimized prototype was selected from 8 types of sub-soil liquid fertilizer injection devices,which was ultra-high molecular weight(UHMWPE)in material,with geometry structure surface and 6 liquid injection holes.In the same experiment conditions,the mean value of the drag resistance of the selected bionic prototype was 283.48 N,the mean weight of soil adhesion was 10.93 g,which was lower than that of other prototypes.According to the experimental optimum design methods,the working parameters of tillage depth,flow rate and velocity were chosen to evaluate the effects on drag-reducing and anti-soil adhesion by using soil bin test system.The working parameters of tillage depth,flow rate and velocity influenced drag resistance and soil adhesion significantly.Tillage depth and velocity presented interaction effects,which influenced drag resistance and soil adhesion highly significantly.The significant influences of the experimental factors for drag resistance and soil adhesion were in turn:the tillage depth,velocity and mass flow rate.The working parameters were optimized as follows:the tillage depth was 11 cm,velocity was 1 m/s,mass flow rate was 350 g/s.Under the above condition,the mean value of the drag resistance of the selected bionic prototype was 260.01 N,the mean weight of soil adhesion was 8.73 g.This study provided technical references for the mechanization of sub-soil fertilizing engineering. |
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