DYNAMICS OF SOIL ORGANIC-MATTER STUDIED BY PYROLYSIS FIELD-IONIZATION MASS-SPECTROMETRY

Abstract

Soil organic matter (SOM) dynamics was studied by pyrolysis-field ionization mass spectrometry (Py-FIMS) of primary organic matter (POM) (rye straw, farmyard manure), of whole soil samples taken (i) weekly in an agricultural fertilization experiment and (ii) in similarly managed soils from different experimental sites. The pyrolysis-field ionization (Py-FI) mass spectra of rye straw from different fertilization treatments indicated that the soil nutrient status had a significant influence on the chemical composition of the organic matter in the straw. In the FI spectrum of farm-yard manure, the most intense signals were assigned to lipids, lignin subunits, carbohydrates and nitrogen compounds. Pronounced seasonal variations were observed in the soil organic carbon and total nitrogen concentrations of two differently fertilized plots in a 90-year-old experiment. The variations corresponded to differences in the chemical composition of SOM. In the unfertilized soil, the decline of SOM content from the 24th to the 32nd week was accompanied by decreased abundances of phenols, lignin monomers, alkylaromatics and carbohydrates, and by increased amounts of nitriles. In the fertilized soil, only minor changes in the chemical composition of SOM were observed between the 25th and the 29th week. A refined evaluation of abundances and evolution temperatures of marker molecules gave new insight into the thermal behaviour of lignin subunits, carbohydrates and nitrogen compounds which was also strongly affected by the seasonal variations in SOM quality. Medium-term SOM dynamics was reflected by the influence of the 90-year management with and without fertilization on the quality of SOM. In the unfertilized soil, the mass spectra indicated a particular enrichment of heterocyclic N compounds. Moreover, the comparison of equivalent fertilization treatments of a Haplic Chernozem with an Orthic Greyzem of similar age, but different texture and mineral composition, also showed the most clear differences among heterocyclic N compounds. Thus, it was concluded that these compounds are especially involved in medium- and long-term dynamics of SOM.