Decomposition dynamics of five soil amendments and the development of total carbon and total nitrogen predictive models using MIR spectroscopy

Abstract

Land application of organic amendments (OAs) preserves nutrient-rich materials and enhances soil health, but OAs vary widely in feedstock, maturity, and processing. This variability complicates understanding their decomposition dynamics despite decades of research. In this study, five OAs were evaluated for their soil decomposition over 397 and 21 days: straw (Str), cattle bedding and manure (Str-DCB), agricultural in-vessel compost (Str-IVC), municipal windrow compost (Mun-WRC), and municipal biosolids (Mun-ATB). Mid-infrared (MIR) spectroscopy with PLSR modeling was used to predict total C and N, assessing both OA effects on model accuracy. Results indicate OA C degradation depended most on material maturity, with composts being more stable. While MIR spectroscopy effectively predicted incubation total C and N, it performed poorly with volatile OAs and dissimilar soils with similar treatments. Given correlation amongst samples, care was needed to prioritize validation set independence, highlighting the need to minimize bias in future spectroscopy-based soil surveys.