In sustainable and organic agriculture, farmers typically add carbon-rich soil amendments to boost nitrogen and to maintain organic matter levels. These amendments -- manure and decaying plants -- undergo fairly rapid decomposition as soil microbes get to work.Tsutomu Ohno, an associate professor of soil chemistry at the University of Maine in Orono, said that changes to the amendments and the dissolved organic matter they release are important. “This is what we are interested in, since the changes can alter reactivity to soil surfaces and dissolved ions in soil solution,” he said.Researchers used the differences between full-scan excitation-emission fluorescence spectra for fresh (left) and decomposed (right) dissolved organic matter derived from corn to follow what happens to various constituents of the plant as they undergo decomposition. Images reprinted with permission of the American Chemical Society. To track such changes, Ohno and graduate student James F. Hunt employed excitation-emission matrix fluorescence spectroscopy to characterize fresh and decomposed dissolved organic matter from a variety of plant and animal sources. In the technique, a sample’s emission spectra are measured over a wide range of excitation wavelengths. The method is rapid, nondestructive and sensitive. The latter aspect was particularly important in these studies because the concentration of dissolved organic matter in a soil solution is fairly low.To evaluate the data, the scientists made use of parallel factor analysis, a technique developed by Rasmus Bro, professor at the University of Copenhagen in Denmark.The researchers started with a variety of dissolved organic matter, including oat, millet, soybean, wheat, hairy vetch, crimson clover, alfalfa, canola and lupine for plant sources, as well as dairy, poultry and swine manure for animal sources. Researchers performed the same analysis using decomposing animal manure. Shown is a full-scan excitation-emission fluorescence spectrum for fresh (left) and decomposed (right) dissolved organic matter derived from poultry manure. After preparing samples, the scientists chemically characterized them by, among other things, using a spectrophotometer from Agilent Technologies Inc. of Santa Clara, Calif., to measure UV absorbance at 254, 280 and 365 nm. They also used a spectrofluorometer made by Hitachi High Technologies America Inc. of San Jose, Calif., for fluorescence measurements, sweeping the excitation from 240 to 400 nm and capturing the emission from 300 to 500 nm in 3-nm increments. They did this before and after controlled decomposition.The Maine researchers found that fluorophores inside the biomass changed during decomposition. Moreover, certain classes of compounds were more resistant to decomposition than others, a result Ohno described as interesting.In analyzing their results, the investigators used a three-way model consisting of fluorophore concentration, and emission and excitation wavelength. They found that the best fit had seven fluorophore components. Earlier studies with just fresh plant biomass and animal extract but without decomposition had revealed a five-fluorophore fit. “So microbial decomposition of the soil amendments does produce fluorophores of a different type,” Ohno said.The group also found that, for most of the extracts, absorptivity at 254 nm increased after decomposition, indicating that some compounds were more resistant to the breakdown process than others. Understanding the mechanism behind that will take more research.The group is improving its equipment and techniques, including the use of mass spectrometry to permit a wider range of compounds to be tracked and an extension of the existing excitation-emission spectroscopy.“The next generation will include lifetime measurements at each excitation-emission wavelength pair,” Ohno said.Journal of Agricultural and Food Chemistry, March 21, 2007, pp. 2121-2128.