Long-term stability of soil spectral libraries with chemical and spectral insights.

Soil spectral libraries (SSLs) are physical soil samples that are stored under different conditions by many users for decades. Yet the long-term stability of soil properties under these stored conditions remains largely unexplored. This study investigates the chemical and spectral stability of the Israeli legacy SSL, established in 1987, stored under uncontrolled indoor conditions for 34-37 years. Ninety-one Mediterranean soils from this collection were reanalyzed for soil organic matter (SOM), calcium carbonate (CaCO), using identical protocols and spectroscopic methods in 1987 and 2024 (chemical) and 2004 and 2024 (spectral). Results demonstrate minimal changes in SOM and CaCO, supported by strong linear correlations between historical and contemporary datasets (R of 0.925 and 0.962 for SOM and CaCO respectively). Spectroscopic analysis showed superior precision and reliability compared to wet chemistry. Additionally, spectral stability over time was confirmed using the modified average spectral difference stability (mASDS) metric, Principal Component Analysis (PCA) and partial least squares regression (PLSR), underscoring the robustness of spectroscopic approaches. Spectral modeling of the chemical data from both years revealed outliers which we assume emerged from analytical accuracy differences and not from spectroscopy errors. This study highlights that Mediterranean soils stored under simple un-controlled conditions maintain their physical and chemical integrity, enabling reliable longitudinal studies. These findings advocate for broader SSL archiving efforts to support soil health monitoring, climate change studies, and sustainable land management practices by utilizing old collections of stored soils that can be measured spectrally to enrich SSLs worldwide. Future research should focus on other climatic regions and soil types to generalize these findings and address possible microbial activity impacts during storage. This work underscores SSLs as critical resources for soil science, offering insights into temporal soil dynamics and facilitating global soil monitoring efforts.