Wheat straw composition and spectral reflectance changes during decomposition

Quantification of crop residue cover is required to assess the extent of conservation tillage. Our objectives were to measure the changes in wheat straw composition and spectral reflectance during decomposition and to assess impact of these changes on remotely sensed estimates of residue cover. Mesh bags filled with wheat straw were placed on the soil surface and removed at intervals over 22 months. The relative proportions of cellulose and hemicellulose in the straw declined while lignin increased. Reflectance spectra of wheat straw and two soils were measured over 350-2500 nm region. Absorption features in the reflectance spectra associated with cellulose diminished as the straw decomposed. The Cellulose Absorption Index (CAI) was a robust estimator of crop residue cover. Advanced multi-spectral sensors with multiple relatively narrow shortwave infrared bands or hyperspectral sensors are needed to assess crop residue cover reliably over diverse agricultural landscapes.