Determination of Organic Carbon and Nitrogen In Particulate and Particle Size Fractions of Brookston Clay Loam Soils Using Soil Infrared Spectroscopy.

The objective of this study was to determine if models developed from infrared spectroscopy of Brookston clay loam soils could be used to estimate organic C and N contents and the C/N ratio in particulate organic matter (POM) and particle size fraction samples of the same soils. Soils used for model development included 165 samples for the POM model and 221 samples for the particle size fraction model. Soil organic C and total N contents in the POM and particle size fractions (sand, 2000-53 μm; silt, 53-2 μm; clay, 2 μm) were determined using dry combustion techniques. The bulk soil samples were scanned from 4000 to 500 cm^-1 for mid-infrared (MIR) spectra and from 8000 to 4000 cm^-1 for near-infrared (NIR) spectra. Partial least squares regression (PLSR) analysis and the “leave one out” cross-validation procedure were used for the model development. Organic C and N contents and C/N ratio in the POM were well predicted using both MIR- and NIR-PLSR models (R²_cal = 0.84-0.92; R²_val = 0.78-0.87). The predictions of organic C content in three size fractions were good with high coefficient of determination for model fit (R²_cal = 0.84 – 0.94 for the MIR model and R²_cal = 0.86 to 0.92 for the NIR model) and model validation (R²_val = 0.79 – 0.94 for the MIR and R²_cal = 0.84 – 0.91 for the MIR model). The prediction for the N content and the C/N ration in the sand and clay fractions was sound too (R²_cal = 0.73 – 0.88; R²_val = 0.67 – 0.85). However, the predictions for the N content and C/N ratio in the silt fraction were poor (R²_cal = 0.23 - 0.55; R²_val = 0.20 – 0.40). The results indicate that NIR model can be used as an alternative method for estimating organic C and N in the POM and sand fractions as the MIR model used for estimating organic C in the silt and clay and N in the clay fractions.