Calibration of near infrared spectra for measuring decomposing cellulose and green manure in soils

Plant residues are the major organic input to soils. Therefore, a method for monitoring the degradation of cellulose as a major component of plant residues is expected to be a useful aid in studying their turnover in soil. In order to examine whether the decay of cellulose in soil can be monitored by near infrared spectroscopy (NIRS), we analysed soil samples from an incubation experiment using this rapid and inexpensive method. A soil amended with and without cellulose (2 g cellulose kg⁻¹ soil) was incubated under aerobic conditions in the dark at 15 °C for 70 days. The soil samples, which were taken at the start and twelve times within the incubation period were spectrally analysed with a NIR-spectrometer. The decay of cellulose was simulated using negative exponential functions. These simulated cellulose concentration in the soil was used for the calibration of NIRS-equations. Although the cellulose comprises only a very small part of the total organic carbon in the soil, the decay of cellulose could be clearly monitored by NIRS. Ninety-five percent of the variation in the soil cellulose concentration as simulated by the negative exponential function could be explained by the NIRS-equation (r²=0.95), when using appropriate rate constants in the exponential decay. The spectral signature, represented by this NIRS-equation could be proved to represent cellulose by comparing it with the spectrum of pure powdered cellulose not mixed with soil. We applied the NIRS-equation from the cellulose treatment to soil samples of a green manure treatment. The coefficient of determination for residual green manure in soil, predicted by this NIRS-equation versus residual green manure contents as simulated by a negative exponential function, was r²=0.84 and 0.94 for a sandy and a clay soil, respectively. Our results confirm that NIRS provides a useful tool for keeping track of a specific and relatively small organic fraction among the background of a large amount of total soil organic matter.