Estimation of soil phosphorus availability via visible and near-infrared spectroscopy

ABSTRACT: Spectroscopic techniques have great potential to evaluate soil properties. However, there are still questions regarding the applicability of spectroscopy to analyze soil phosphorous (P) availability, especially in tropical soils with low nutrient contents. Therefore, this study evaluated the possibility to estimate P availability in soil and its pools (labile, moderately labile and non-labile) via Vis-NIR spectroscopy based on intra-field calibration. We used soils from two different locations, a plot experiment that received application of phosphate fertilizers (Field-A) and a cultivated field where a grid soil sampling was performed (Field-B). We used the technique of diffuse reflectance in the visible and near-infrared (Vis-NIR) to obtain the spectra of soil samples. Predictive modeling for P availability and labile, moderately labile and non-labile pools of P in soil were obtained via partial least squares (PLS) regression; classification modeling was performed via Soft Independent Modeling of Class Analogy (SIMCA) on three P availability levels in order to overcome the limitation on quantifying P via Vis-NIR spectroscopy. We found that isolating P contents as the only variable (Field-A), Vis-NIR spectroscopy does not allow estimating P pools in the soil. In addition, quantification of P available in the soil via predictive modeling has limitations in tropical soils. On the other hand, estimating P content in soil through classes of availability is a feasible and promising alternative.

摘要：光谱技术在土壤属性评估领域具备巨大应用潜力，但目前关于光谱技术用于分析土壤磷（P）有效性的适用性仍存在争议，尤其针对养分含量偏低的热带土壤。为此，本研究基于田间内校准的可见-近红外（Vis-NIR）光谱技术，评估了估算土壤磷有效性及其磷库（易分解态、中等易分解态与难分解态）的可行性。本研究采集了两处不同地点的土壤样品：一处为施加过磷肥的小区试验田（田块A），另一处为开展网格土壤采样的耕作田（田块B）。采用可见-近红外漫反射光谱技术获取土壤样品的光谱数据；通过偏最小二乘（PLS）回归，构建了土壤磷有效性及各形态磷库的预测模型；同时，为克服可见-近红外光谱定量分析磷含量的局限性，针对三类磷有效性等级，采用软独立建模分类法（SIMCA）开展分类建模。研究发现，若仅以磷含量作为单一变量（田块A），可见-近红外光谱无法有效估算土壤中的磷库。此外，通过预测模型量化土壤有效磷含量的方法，在热带土壤中存在一定应用局限。相较而言，基于磷有效性等级估算土壤磷含量的方案，是一种可行且极具应用前景的替代途径。