| Soil fertility, as an important component of soil quality, has an important influence on cropland productivity and it was affected by soil erosion. Therefore, mastering the spatial variation of soil nutrients, knowing the soil fertility quality and its respon to soil erosion were of great significance to guarantee food security and promote the sustainable development of Agriculture. The spatial variation of soil nutrient of cropland in Hengnan County was studied by geographic information system(GIS) technology based on the data of 3692 sampling points obtained by soil testing and formulated fertilization. Besides, the reasonable sampling number was thoroughly analyzed on the basis of the above study. Finally, soil fertility of cropland in Hengnan County was evaluated by using the fuzzy mathematics method, and then the effect of erosion on soil fertility was analyzed. Main results were as follows:(1) The variability of available nitrogen(AN), available phosphorus(AP), available potassium(AK), slowly available potassium(SAK), p H and organic matter(OM) of cropland soil in Hengnan County were studied. The range of coefficient of variation(CV) was from 13.07% to 66.70%, the coefficient of variation(CV) increased as the sequence of p H<AK<AN<OM<SAK<AP, while AP had a maximum CV. All these indicators belonged to medium variation intensity and the best fitted models were exponential model with the C0/(C0+C) less than 25%, which showed high spatial correlation. The p H of cropland was mainly neutral and acidic in Hengnan County. The contents of OM and AN were abundant, OM was mainly distributed in the second and third grade accounting for 89.85% of total cropland area, AN was in the first three grade for 97.34%. The contents of AP and AK were relatively lack and mostly distributed in fourth grade for 77.09%, 95.62% respectively. The content of SAK was seriously lack and mostly distributed in fifth grade accounting for 87.48%. Therefore, it should be paid attention to the application of phosphate fertilizer and potash fertilizer in this area.(2) Cokriging with organic matter as subsidiary variable was used to study the spatial characteristics and prediction accuracy of available phosphorus and available potassium under different sampling number. The prediction accuracy of Cokriging was higher than that of Ordinary Kriging. When the sample number of available nitrogen was reduced to 1846(50%) and available phosphorus was reduced to 2215(60%), the prediction accuracy of Cokriging interpolation was still higher than that of initial 3692 samples by Ordinary Kriging. The Cokriging method of available potassium did not show obvious advantage relative to that of Ordinary kriging. Therefore the sampling number was not reduced.(3) Soil fertility quality was evaluated by using the weighted sum model in fuzzy mathematics and then was classified. Soil fertility quality grades of cropland in Hengnan County were mainly distributed in the II, III and IV accounting for 92.92% of total cropland area. Among them, the III grade cropland accounted for 43.05%, with the II grade for 28.49% and IV grade for 21.38%. The overall level of soil fertility quality of cropland was low in Hongshan Town and Baogai Town.(4)Soil erosion had a certain effect on soil fertility. With the decrease of soil fertility grade, the proportion of micro erosion area accounting for in this grade was also decreasing and the proportion of the mild erosion area was increasing. The results of analysis of variance showed that soil erosion had significant effects on the soil integrated fertility index, the contents of OM and AN. No significant difference could be found between the micro erosion and slight erosion in the soil integrated fertility index, in the content of OM, in the content of AN, but they were significantly higher than those of moderate erosion. |
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