Pyrolisis-MS and FT-IR analysis of fresh and decomposed dairy manure

The ability to predict manure nitrogen mineralization is essential for optimizing crop growth while preventing N losses to the environment. However, estimating mineralizable manure N is problematic because of the wide variety of organic manure N forms, as well as the lack of a rapid standardized method. Fourier-transformed infrared spectroscopy (FT-IR) is a promising technology, since it can detect the absorbance of proteins and primary amines, in addition to being instantaneous and non-destructive. Likewise, analytical pyrolysis-mass spectrometry (Py-GC/MS) has been used to study lignin and protein composition of forages. We carried out a 10-week laboratory incubation of manure-amended soil in order to test the sensitivity of FT-IR and Py-GC/MS to detect changes in manure during decomposition in soil. Four different dairy or beef manures were included. The manures (0.15 mg manure-N g−1 soil) were placed inside mesh bags during the soil incubation to allow for the recovery and analysis of fresh as well as decomposed manure. Infrared spectroscopy of the fresh and incubated manures showed that this technique is sensitive to changes in manure organic N after soil application. Bands associated with primary amines decreased during the incubation, while bands associated with proteins increased during manure decomposition. Bands associated with fatty acids tended to decrease during the incubation, possibly due to utilization as C sources. The spectroscopic data also showed that lignin-specific signals increase during manure decomposition, suggesting that the relative amount of lignin in manure increases as it decomposes in soil. The changes in peak areas of pyrolyzates were related to changes in manure constituents during decomposition. The Py-GC/MS data showed qualitative changes in manure lignin during decomposition. The relative amounts of nitrogen containing pyrolyzates, such as phenylacetonitrile and methylindole, changed during the decomposition, but the changes were not consistent across manures. In summary, this experiment showed that the use of manure bags is valuable in discerning between the N cycling dynamics of manure and soil, and that infrared spectroscopy, as well as Py-GC/MS show potential as analytical tools to study manure decomposition in soil.