| Environmental pollution monitoring has always been the hot spot in science research. Soil is an effective information carrier for environmental pollution, and can reflect degree of environment pollution by monitoring soil pollution. In recent years, environment magnetism has been broadly researched in the domestic and overseas. Especially, the rising technology of magnetic susceptibility monitoring has been widely used in environmental work for many advantages of simpleness, promptness, low cost and no destructiveness. At present, research of soil magnetic susceptibility is mostly applied in urban and industrial area, but less in agricultural soil which is directly related to agricultural product safety and human health. Domestic agricultural soil pollution situation is not optimistic, and needs to be further studied. However, agricultural soil is different from conventional soil, for there is often a large area of crop straw covered on surface. Traditionally, the operation of measuring soil magnetic susceptibility is measuring bare soil magnetic susceptibility by constantly removing straw, which is inefficient for monitoring farmland soil magnetic susceptibility under a large area of crop straw covered.This thesis has researched influence of farmland straw mulching on soil magnetic susceptibility. Previous studies have shown little effect of soil moisture on magnetic susceptibility. The research mainly analyzes influence of straw coverage rate and coverage thickness on soil magnetic susceptibility, and the effect of straw coverage on spatial structure of soil magnetic susceptibility. Rice field, sugarcane field, and corn field are selected to do above test. The results show that:1. Under 100% straw coverage rate, spatial distribution characteristic of straw mulching magnetic susceptibility is similar to bare soil magnetic susceptibility in each field, and both of them have extremely significant positive correlation, so straw mulching magnetic susceptibility can reflect relevant bare soil magnetic susceptibility.2. There are two measurement methods under 50% straw coverage rate, one is setting all measurement points on someplace where covered by straw, the other is randomly setting measurement points on whole test area. The effect of first method is equal to 100% straw coverage rate, its straw mulching magnetic susceptibility can reflect spatial distribution characteristic of bare soil magnetic susceptibility. However, magnetic susceptibility under straw mulching measured by second method includes straw mulching magnetic susceptibility and bare magnetic susceptibility, its spatial distribution characteristic is unlike relevant bare magnetic susceptibility, and correlation of both is not significant, so magnetic susceptibility under straw mulching cannot reflect relevant bare soil magnetic susceptibility.3. Straw coverage thickness has significant index negative correlation with the superficial magnetic susceptibility measured values of rice filed, #1 and #2 sugarcane filed, but has significant linear negative correlation with corn field's susceptibility values. When thickness is 0~5cm, magnetic susceptibility measured values will uniformly decrease with increase of thickness, so it can reflect bare soil magnetic susceptibility in some extent. When thickness is over 6cm, magnetic susceptibility measured values decrease slowly. When thickness is over 10 cm, magnetic susceptibility measured values are close to zero.4. Semi-variogram models of bare soil magnetic susceptibility of rice field, sugarcane field and corn field are linear model, Gaussian model and spherical model respectively. Semi-variogram models of straw mulching magnetic susceptibility of all crop fields are linear model, but theirs R-squared is lower than bare soil magnetic susceptibility.5. Nugget of straw mulching magnetic susceptibility of all fields is bigger than bare soil magnetic susceptibility, but partial sill is smaller, which means straw mulching increases random spatial variation but decreases structural spatial variability and total spatial variation. Nugget coefficient and range of straw mulching magnetic susceptibility of most fields are bigger than bare soil magnetic susceptibility, which means straw mulching decreases spatial autocorrelation but increase distance range of spatial autocorrelation. |
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