ObjectiveExploring the impact of tree on site quality is an important research field to sustainable management of plantations. Studying the soil structure and nutrient changes of Robinia pseudoacacia plantations during the process of multi-generation replacement management can provide theoretical basis for formulating control measures on soil fertility under the multi-generation management object of Robinia pseudoacacia plantations.

MethodIn this study, the comparison research in soil nutrients and structure characteristics was carried out on 1−3 generations of Robinia pseudoacacia forest and non-forest land of the same age in the shallow hilly area of western Henan Province, central China.

Result(1) The content of soil nutrients in the second generation Robinia pseudoacacia woodlands was significantly higher than that in the first generation one (P < 0.05). In the topsoil layer (0−5 cm), the contents of organic matter, total nitrogen, and nitrate nitrogen in the second generation woodlands increased by 94.0%, 91.0% and 169.4%, respectively, increased by 82.77%, 61.14%, and 343.35%, respectively in the 10−20 cm deep soil layer, and increased by 53.25%, 21.60% and 556.20%, respectively in the 40−60 cm deep soil layer compared with those in the first generation woodlands. The total nitrogen content in each soil layer was 0.63, 0.39, 0.29 and 0.28 g/kg from the surface to deep layer for 1st generation Robinia pseudoacacia forestland. Totally, there was no significant difference in soil nutrient content between the second and third generation of Robinia pseudoacacia woodlands. The contents of organic matter, total nitrogen and total phosphorus in the first generation Robinia pseudoacacia woodlands was all significantly lower than that in the control land (P < 0.05), while the content of nitrate nitrogen in the control land was significantly lower than that in the first generation woodlands (P < 0.05). Nutrient content in the surface soil layer was higher than that in the deeper layer, and with generation increasing, the increment of nutrient content in the upper layer was greater than that in the deeper layer. From the perspective of stoichiometry, in each soil layer, C/P and N/P in the first generation woodlands were higher than those in the second and third generation woodlands. Compared with organic matter and total nitrogen contents, C/N value was relatively stable with generation increasing. C/P and N/P in the deeper soil layer were relatively stable. C/P showed a stronger increase compared with N/P in the topsoil layer and 10−20 cm deep soil layer from the the first generation forest to the third generation forest. (2) In terms of soil structure reflected by soil bulk density, porosity and aggregates, non-capillary porosity at the depth of 10−20 cm in the second generation and third generation woodlands increased by 11.4% and 21.4%, respectively compared with the first generation woodlands. It also showed an upward trend at the depth of 40−60 cm with generation increasing. The deeper the soil layer was, the greater the change range of soil bulk density was among intergenerational forestlands. Except for the surface layer, generally the soil density showed a decreasing trend in the other layers with generation increasing. The quantity and stability of soil surface water stable aggregates were in the order of control land > second and third generation woodlands > first generation woodlands. On the whole, in terms of soil structure, the surface layer was superior to the deeper layer, and the second and third generation woodlands were superior to the first generation woodlands. (3) Principal component analysis based on soil nutrients and structural property indicated that forest had a significant effect on the deeper soil. The soil conditions in the second and third generation woodlands were significantly better than those in the first generation woodlands. The surface soil properties in the second generation woodlands were better than those in the third generation woodlands, while the deeper soil properties in the third generation woodlands were better than those in the second generation woodlands.

ConclusionIn the shallow mountain area of western Henan Province, the management generation replacement of Robinia pseudoacacia forest has a significant impact on soil nutrients and structure. During the management process from the first generation forest to the second generation forest, soil structure is improved significantly. In addition, the soil nutrient content increases significantly, and the increment in the surface layer is greater than that in the deeper layer, and the soil structure is improved during the process. The soil nutrients and structure in the third generation forest remain relatively stable. In the process of generation increasing, carbon accumulates faster than nitrogen, and the supply capacity of carbon and nitrogen is smaller than that of phosphorus. Besides, the problem of soil nutrient imbalance becomes serious during the process of generation replacement.