Objective  The influence of different factors on forest soil nutrients was analyzed to explore the causes of forest soil nutrient diversity and support forest management and site quality evaluation.

  Method  Based on the soil data of forest inventory in Guangdong Province of southern China, organic matter (OM), alkali-hydro-nitrogen (AN), available phosphorus (AP) and available potassium (AK) were taken as dependent variables, and 20 variables (10 qualitative factors and 10 quantitative factors) were taken as independent variables in four categories, namely geographical factor, topographic factor, soil factor and stand factor. The main effector was selected based on the general linear model and backward elimination method. The influencing factors and degree of forest soil nutrient were analyzed by the interaction of qualitative and quantitative factors. The difference was analyzed by multiple linear regression analysis of quantitative factors and multiple comparative analysis of qualitative factors.

  Result  (1) The soil nutrients were mainly affected by three geographical factors (longitude, latitude and watershed), three topographic factors (altitude, slope position and slope grade), and four stand factors (vegetation coverage under forest, mean DBH, mean age and dominant tree species). And the factors on different soil nutrients were various. (2) The influencing factors had different degrees of interpretation for different soil nutrients. The interpretation degree of AN was more than 0.30, the interpretation degree of OM and AK was more than 0.25, and the interpretation degree of AN was more than 0.20. Interaction prominently enhanced the interpretation of soil nutrients, with an average increase of 0.141. AN, OM and AK were up to about 0.50, 0.45 and 0.40, respectively, AP was more than 0.30. (3) Through multiple comparative analysis, there were significant differences in soil nutrients among dominant tree species, watershed and slope position.

  Conclusion  The factors affecting forest soil heterogeneity in Guangdong Province include geographical factors, topographic factors and stand factors. The influencing factors of different soil nutrients are not exactly the same. Latitude, watershed, altitude, slope position, parent rock, mean DBH, mean age and dominant tree species have influences on most soil nutrient factors. For different soil nutrients, OM and AN have a higher degree of interpretation, while AP and AK have a lower degree of interpretation. Interaction significantly improves the degree of interpretation of soil nutrients. In this study, the main effect screening method with qualitative factors is proposed and the interaction between first-order qualitative factors and quantitative factors is introduced, so that it can highly increase the interpretation of soil nutrients.