Structural Characteristics and Stability of the Aggregates in the Orchard Soils with Different Cultivation Years in Northern China

With dry sieving and wet sieving methods, we studied the distribution characteristics and stability of the aggregates in the typical orchard soils with different cultivation years in northern China under long-term freezing-thawing conditions. The results show that: the contents of the aggregates with particle size above 10 mm or between 0.25 and 10 mm are significantly increased with cultivation years and annual growth rates were respectively 1.28% and 0.078%. Fractal dimension in the orchard soils was gradually decreased and lower than that in uncultivated area near the orchard in corresponding cultivation year. MWD of orchard soils shows annual increasing trend of 0.76%. The content of the water stable macro aggregates in orchard soils was increased with the cultivation years and was about 11.65%-20.18% higher than that of uncultivated soils near the orchard. Average annual growth of orchard soils was 0.37%. Fractal dimension was decreased year by year on the water stable macro aggregates. MWD of the water stable macro aggregates was significantly greater than that of uncultivated soils near the orchard and increased with cultivation years. Aggregate destruction rate was gradually decreased to 31.61% of that in the 60th year and annual average decrease rate was 0.91%. Soil bulk density was gradually decreased.