Spatial Variability Of Soil Nutrients In Topsoil Covered By Different Crops

As a continuous temporal and spatial variant,soil has a high degree of spatial heterogeneity.The spatial structure of the same nutrient attribute has a scale effect,that is,a single large-scale sampling will ignore the variation of the soil nutrient spatial pattern at a small scale.In recent years,with the widespread application of field precision irrigation and fertilization technology,small-scale farmland Soil nutrient research is very necessary.In order to explore the spatial variability of surface soil nutrients of fields covered by different crops,such as the appropriate monitoring density,the influence of crops in different periods on the spatial distribution pattern of soil nutrients,and the interaction between the upper and lower layers of soil nutrients in the orchard,the field nutrient distribution in the experimental base of Northwest Agriculture and Forestry University Caoxinzhuang Farm,the National Peach and Persimmon Germplasm Resource Nursery of the Arid Area Water-saving Agriculture Research Institute,and the experimental base of the Ningxia High-Efficiency Dry Farming Agricultural Science and Technology Park were studied respectively.The nutrient data of the surface soil（0-20 cm,0-40 cm）were collected,and the classical statistics and geostatistics combined with Kriging interpolation method were used,the effects of sampling interval of different plots（12 m×12 m nested 6 m×6 m）,different crop growth stages（winter wheat heading stage,ripening stage,summer corn/soybean seedling stage）and crop types（field crops,fruit trees）on spatial variation of soil nutrients in the field were analyzed.Determine the distribution characteristics of spatial variability patterns of surface soil nutrients in farmland.To quickly,economically and accurately determine soil nutrients,it provides a scientific basis for field nutrient monitoring and agricultural production refinement.The main conclusions are as follows:（1）The sampling spacing of 12m×12m mesh is betterThe available potassium（AK）in the 0-20 cm soil layer of farmland soil showed weak variation,the spatial variability of soil organic matter（SOM）,available phosphorus（AP）,total nitrogen（TN）,nitrate nitrogen（NO₃^?-N）,and ammonium nitrogen（NH₄^?-N）showed moderate variation.The suitable semi-variance model for topsoil nutrients of farmland was spherical model.The nugget coefficient of soil organic matter（SOM）and total nitrogen（TN）were less than 25%,and the spatial autocorrelation was strong;the nugget coefficient of available phosphorus（AP）,available potassium（AK）,nitrate nitrogen（NO₃^?-N）,and ammonium nitrogen（NH₄^?-N）were between 25%and 75%,and the spatial correlation was weak.The variability of farmland soil organic matter and total nitrogen is dominated by structural factors,while the available nutrients are dominated by random factors and are greatly affected by factors such as fertilization and crop consumption.The spatial distribution of soil nutrients in the farmland showed that the content of soil organic matter first increased and then decreased from north to south;available potassium and nitrate nitrogen both decreased from the center of the plot to the surroundings;and total nitrogen and ammonium nitrogen which content distribution is similar,with high and low values staggered.The effects of sampling intervals of 6 m×6 m and 12 m×12 m on the spatial variability of soil nutrients in farmland showed that the difference in the coefficient of variation of organic matter,available phosphorus,available potassium,total nitrogen,nitrate nitrogen and ammonium nitrogen is within the range of 0.00%to 43.33%;The difference of nugget coefficient is in the range of1.9%to 33.7%.The spatial variation characteristics of soil nutrients obtained by the two sampling intervals are basically the same.It is recommended to use 12 m×12 m grid sampling to save costs and improve benefits.（2）The spatial variability of soil nutrients tended to be stable from heading stage to ripening stage,but fluctuated at seedling stage due to tillage systemFrom the heading stage to the ripening stage of witer wheat,the mean value of soil nutrient mass fraction in the two plots decreased to different degrees,ranging from 2.30 mg/kg to 70.02 mg/kg,except for available potassium（AK）in the test field 2（adding oil residue）.From the ripening stage to the seedling stage,the average mass fraction of available phosphorus（AP）and nitrate nitrogen（NO₃^?-N）increased,while the average mass fraction of available potassium（AK）and ammonium nitrogen（NH₄^?-N）decreased.From the heading stage of winter wheat to the seedling of summer maize or soybean,the available potassium showed weak variation to moderate variation,and available phosphorus,nitrate nitrogen,and ammonium nitrogen still showed moderate variation,with fluctuations ranging from 1.13%to83.71%.At the 95%confidence level and the relative error range of 5%,the optimal sampling number of farmland soil nutrients decreased with the decrease of variation coefficient.The optimal model for available phosphorus,available potassium,nitrate nitrogen,and ammonium nitrogen for the two test fields was a spherical model,and the nugget value is greater than zero,which is affected by random factors.In general,with the change of growth stage of experimental crops,the nugget coefficient decreased,the spatial correlation increased,and tended to be dominated by structural factors.From the heading stage to the maturity stage of winter wheat,the distribution of soil nutrients is very similar.With the influence of tillage,maize/soybean sowing,fertilization and other planting systems and management methods,soil nutrients begin to move at the seedling stage,the distribution law is disrupted,and there are differences in spatial variability.（3）Soil nutrients in the 20-40 cm soil layer were more affected by random factors than those in the 0-20 cm soil layerThe content of available phosphorus（AP）and available potassium（AK）in garden soil is at level 1 and 2 of the soil nutrient classification standard,which is the rich level.The average content of available phosphorus,available potassium,nitrate nitrogen（NO₃^?-N）and ammonium nitrogen（NH₄^?-N）in 0-20 cm soil layer was greater than 20-40 cm soil layer.The available soil nutrients in the orchard showed medium variation.The variation is within the range of 3.86%to 38.68%in 0-20 cm soil layer,and the variation range of 20-40 cm soil layer is 5.56%to 52.72%.The coefficient of variation of the 20-40 cm soil layer in the wolfberry orchard was more active than that of the peach orchard and the persimmon orchard.The number of reasonable samples decreased with the decrease of coefficient of variation,and the number of samples in the 20-40 cm soil layer was more than that in the 0-20 cm soil layer.The most suitable semi-variance model for soil nutrients in the 0-40 cm soil layer of the garden was the exponential model.The soil nutrient nugget coefficient C₀/（C₀+C）value was less than75%,and the spatial correlation was moderate to strong.The spatial correlation difference in the 0-20 cm soil layer ranged from 0.100%to 49.955%,and that in the 20-40 cm soil layer ranged from 0.1%to 29.3%.The soil nutrients of the garden plot are mainly dominated by structural factors.The 0-20 cm soil layer is more affected by random factors.The distribution of soil nutrients in the upper and lower layers of available phosphorus,available potassium,and nitrate nitrogen in a persimmon orchard was relatively similar.