Comparison of spectroscopic techniques for the determination of Kjeldahl and ammoniacal nitrogen content of farmyard manure

The feasibility of determining the nitrogen content of farmyard manure usin g infrared spectroscopy was investigated. Fifteen samples each of cattle, p ig, and turkey manure were analyzed by three infrared techniques: Fourier t ransform mid-infrared (MIR), using attenuated total reflection (ATR); near- infrared reflectance (NIR-R); and near-infrared optothermal photoacoustic ( NIR-OT). The near- infrared measurements were made at wavelengths determine d: respectively by four (NIR-OT) and five (NIR-R) band-pass filters. The to tal nitrogen (using the Kjeldahl method) and volatile (ammoniacal) nitrogen contents of all samples were measured by wet chemistry. Internally cross-v alidated (ICV) partial least-squares (PLS) regression was then used to obta in calibrations for the nitrogen content. The data sets obtained by each te chnique were treated separately. Within these sets, data from each manure t ype were treated both separately and combined: the best predictive ability was obtained by combining data from all three manure types. From the combin ed data set, the residual standard deviations and correlation coefficients for the ICV-predicted versus actual Kjeldahl nitrogen content were, respect ively, 6772 mg/kg dry wt, 0.862 (MIR); 9434 mg/kg dry wt, 0.771(NIR-OT); an d 8943 mg/kg dry wt, 0.865 (NIR-R). For the ammoniacal nitrogen content, th e residual standard deviations and correlation coefficients were 3869 mg/kg dry wt, 0.899 (MIR); 6079 mg/kg dry wt, 0.820 (NIR-OT); and 3498 mg/kg dry wt, 0.961 (NIR-R).