基于三种空间预测模型的海南岛土壤有机质空间分布研究

为探索适合热带地形复杂区土壤有机质(SOM)含量的空间预测方法,以海南岛为研究区域,结合地形因子、归一化植被指数、土壤类型、土地利用类型变量,选用普通克里格法(OK)、回归克里格法(RK)、随机森林模型(RF)三种方法对训练集128个样点SOM含量的空间分布规律进行预测,并通过验证集32个验证点比较了三种方法的预测精度。结果表明:(1)0~5 cm土层三种方法的平均预测误差(ME)均接近于0,从均方根预测误差(RMSE)来看,RF(0.8867)RK(0.910 4)OK(0.9641),从决定系数(R~2)来看,RF(0.214 1)RK(0.171 5)OK(0.070 8)。综合以上三个参数,该土层最优拟合模型为RF。同理得出0~20、20~40、40~60 cm土层的最优拟合模型分别为RF、RF、OK。RK和RF能够更好地描述SOM含量局部变异信息;(2)四个土层SOM含量的均值分别为19.67、15.89、10.30、8.07 g kg~(-1),呈现出西南、东北高,西部、东南沿海地区低的空间分布趋势。

Research on Spatial Distribution of Soil Organic Matter in Hainan Island Based on Three Spatial Prediction Models

The knowledge about spatial distribution characteristics of soil organic matter (SOM) contents is of important significance to development of the agriculture. Models different in simulation mechanism are used to predict distribution of SOM contents, because different regions have soils of different soil properties and different environmental factors affecting soil properties. It is, therefore, essential to choose an optimal suitable model site-specifically for the prediction, this paper is oriented to explore for a model that is capable of predicting spatial distribution of SOM contents in a tropical region complicated in relief, like Hainan Island. Ordinary kriging (OK), regression-kriging (RK) and random forest (RF) were the three models used to predict spatial distribution of SOM contents in the Hainan Island based on the training set of 128 samples, combined with topographic factors, normalized vegetation index, soil types and landuse types, and then the predictions were validated with a validating set of 32 validation points in the study area for comparison of prediction accuracy. Results show: (1) The optimal fitting model for prediction of distribution of organic matter contents in the 0 ~ 5,0 ~ 20, 20 ~ 40 and 40 ~ 60 cm soil layer is RF, RF,RF and OK, respectively. RK and RF can better describe local variation of SOM contents; (2) The mean value of SOM contents in the four soil layers was 19.67, 15.89, 10.30 and 8.07 g kg^-1, respectively, showing a spatial distribution trend that SOM contents are higher in the southwest and northeast parts and lower in the west and southeast parts of the island.