Abstract:The soils were collected from Robinia pseudoacacia pure forest, Pinus tabulaeformis pure forest and apple orchard on the same slope, soil structure stability (aggregate composition), organic carbon, inorganic carbon and soil respiration were determined, and then the effects of artificial forest types on soil properties in the loess hilly area of Weibei of Shaanxi and the roles of soil organic and inorganic carbon in the formation of water stable aggregates were discussed. The results showed that disaggregation rates of soil aggregates were high under the three forest types. Water stability indexes of soil aggregates, soil organic carbon and respiration in the three forest types were in the order of Pinus tabulaeformis > Robinia pseudoacacia > Malus pumila, soil inorganic carbon was in the order of Robinia pseudoacacia > Malus pumila > Pinus tabulaeformis. Negative correlation existed between soil organic carbon and inorganic carbon, and they were positively and negatively correlated with water stability of aggregates respectively. Meanwhile, inorganic carbon had a threshold value, positive or negative correlation with water stability of aggregates existed when it was lower or higher than the threshold value. Therefore, when inorganic carbon is high, measures should be taken to decrease it and increase soil organic carbon for the improvement of soil structure and the alleviation of soil erosion. Significant positive correlations existed among different stability indexes of soil structure and between stability indexes of soil structure with soil organic carbon and respiration. In conclusion, artificial forest type can significantly affect soil properties and Pinus tabulaeformis is better than Robinia pseudoacacia in improving soil quality.