Effects of long-term fertilization on fractal features for soil aggregates in semi-arid farmland ecological system

Fractal characteristic of soil aggregates is an important index to describe soil structure.In this study,soil samples,collected from the 25-year long-term fertilizer experiment carried out on a manual loessial soil situated sub-humid farmland ecological system,were used to study the relationships among the fractal features,structures,and fertility of soil aggregates under different types of long-term fertilization.The results indicated that there were distinct differences among the fractal features of soil aggregates under different types of long-term fertilization,even though the soils are belonged to the same group which is Eum-Orthic Anthrosols.The fractal dimension values of soils in the seven treatments ranged from 2.4388 to 2.6363.The value for soils under the treatment of applying chemical fertilizer and stable manure together was the highest,and the lowest was the one without applying any fertilizer.The above results revealed that applying chemical fertilizer with organic matters together in a long-term had pronounce effects on the structure distribution of soil aggregates. The correlation analysis revealed that high significant positive correlation exited between the fractal dimensions of soil aggregate and 5-2 mm soil aggregate class,with correlation coefficient being 0.994.Moreover,there were significant positive correlations between the fractal dimensions of soil aggregates and concentrations of organic carbon and total nitrogen and nitrate nitrogen and available phosphorus in soils,while the correlation between carbon and nitrogen ratio(C/N) in soils and the fractal dimensions of soil aggregates was significant negative.Under different long-term fertilization,there were obvious differences among the marginal values of soil property change according to fractal dimensions of soil aggregates.With one unit of increment for the fractal dimension,the increment units for soil organic carbon,total nitrogen,C/N,nitrate nitrogen,and available phosphorus were 31.628%、2.404%、-6.014%、90.370% and 172.760% respectively.The marginal analysis indicated that the change of fractal dimension of soil aggregates influenced soil available phosphorus and nitrate nitrogen mostly.