Soil fertility evaluation in mixed forests of Pinus massoniana and Camellia oleifera

Taking the mixed forest of Pinus massoniana and Camellia oleifera in the state-owned Paiyangshan Forest Farm of Guangxi, China as an example, studying the differences in soil fertility between the mixed coniferous and broad-leaved P. massoniana forest and the pure forest is significant for the broad-leaved transformation of P. massoniana plantations. The differences in eight soil chemical indicators, including pH value, organic matter, total nitrogen, total phosphorus, total potassium, alkali-hydrolyzable nitrogen, available phosphorus and available potassium content, between the mixed forest of P. massoniana and C. oleifera and the pure forest of P. massoniana were analyzed. Five evaluation methods, namely the rank- sum ratio comprehensive evaluation method, technique for order preference by similarity to ideal solution, the principal component analysis method, the multi-criterion compromise solution ranking method and the entropy method, were used to conduct a comprehensive evaluation of soil fertility, to determine the weights of different chemical indicators in soil quality assessment and compare the differences in soil fertility between mixed forests of P. massoniana and C. oleifera and pure forests. The evaluation grades of soil chemical indicators for the mixed forest of Pinus sasanqua and C. oleifera （7∶3） are as follows: available phosphorus level 2, organic matter level 4, pH value, total nitrogen, total potassium and total phosphorus level 5, available potassium and alkali-hydrolyzable nitrogen level 6. All the chemical index values of the pure P. massoniana forest were lower than those of the mixed forest. Among them, the differences in total phosphorus and available phosphorus were significant, both reaching the sixth grade level. The comprehensive evaluation results of soil fertility show that the soil fertility of the mixed forest of P. massoniana is higher than that of the pure forest. The mixed ratio of P. massoniana and C. oleifera in 7∶3 is better than 5∶5, and the fertility of the surface soil is higher than that of the sub-surface soil. The analysis of weight results of soil chemical indicators shows that available phosphorus has the highest weight and plays a key discriminating role in the comprehensive evaluation, followed by total phosphorus and available potassium. The three occupy a core position in the evaluation of soil fertility. The multi-layered intercropping of C. oleifera and P. massoniana at different ages helps increase soil organic matter and nutrient content, alleviate acidification trend in the acidic red soil of the Southern subtropical zone, and enhance soil fertility. It is an effective management strategy for the broad-leaved transformation of P. massoniana plantations.