基于不同模型的区域尺度耕地表层土壤有机质空间分布预测

Spatial Prediction of Topsoil Organic Matter of Arable Land by Different Models at the Regional Scale

  • 摘要: 研究不同模型对土壤有机质空间预测的性能差异对制定更加科学合理的采样策略、提升采样效率和提高土壤空间预测精度有着重要的指导意义。本研究将6496个土壤样点按8∶2的比例分层随机分成训练集与验证集,应用普通克里格、随机森林以及随机森林-回归克里格三种有代表性的数字化土壤制图(Digital Soil Mapping,DSM)模型,对河南省许昌市耕地表层土壤有机质含量及空间分布进行预测,对三种模型性能表现进行综合评价。三种模型输出的预测结果显示:研究区耕地表层土壤有机质含量水平一般,均值为18.70 ~ 18.81 g kg−1,变异系数0.15 ~ 0.17,属中等强度变异;空间分布总体格局为西北与西南部分山地褐土区、东南部砂姜黑土区表层有机质含量高,中北部脱潮土、石灰性潮土区表层有机质含量低。验证结果表明:三种模型性能表现无明显差距,预测精度基本一致,输出结果对研究区耕地表层土壤有机质变异解释百分比在33% ~ 34%之间,在相同和相近尺度土壤有机质空间预测案例研究里属中等水平。在协变量有限且样点分布较为均匀的情况下,普通克里格模型便于快速获得研究区目标变量的空间分布;如果协变量比较丰富且易于收集利用,或是进行空间预测的同时还需要甄别不同因素对目标变量的影响大小,则建议采用随机森林模型;协变量有限,但样点密度较大时,随机森林-回归克里格模型可能是对目标变量进行空间预测的不错选择。

     

    Abstract: The research on the spatial prediction of soil organic matter by different models has an important guidance for the strategies and efficiency of scientifical formulating sampling and the accuracy improvement of soil spatial prediction. The total 6496 of the observed soil sites were divided into training and validation datasets stratified randomly according to the ratio of 8 to 2, and then ordinary kriging, random forest and random forest-regression kriging were employed to predict spatial variation of topsoil SOM in arable land in Xuchang, a prefecture-level region of Henan Province. The prediction accuracy was validated and the model performance was evaluated. The factors dominating topsoil SOM content and spatial variability in the study region were analyzed and identified through a Boruta feature selection approach. According to the prediction results produced from the three models, topsoil SOM contents of arable land in the region were moderately low, ranging from 18.70 to 18.81 g kg−1, and a coefficient of variation from 0.15, to 0.17. Arable land with higher topsoil SOM content was concentrated in the mountainous areas of the northwestern and southwestern parts where the cinnamon soil formed on the loess parent materials was found mainly, and in the southeastern part where Shajiang black soil was distributed. The area of arable land with lower topsoil SOM content was mainly found in the central and northern of the study region. The validation results revealed that three models showed similar performance and prediction outputs, which could explain 33%-34% variance of topsoil SOM content of arable land. This result shows that the spatial prediction of SOM was of medium level in the case study at the same and similar scales. When the covariates are limited and the sample points are relatively evenly distributed, the ordinary kriging model is convenient to quickly obtain the spatial distribution of the target variables in the study area. If the covariates are abundant and easy to collect and use, the random forest model is recommended. The covariates are limited, but when the sample density is high, the random forest-regression kriging model may be a good choice for spatial prediction of the target variable.

     

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