Volume 54, Number 2, 2000 APPLIED SPECTROSCOPY 221 0003-7028 / 00 / 5402-0221$2.00 / 0 q 2000 Society for Applied Spectroscopy FT-IR Spectroscopy of Organic Matter in Tropical Soils: Changes Induced through Deforestation G. HABERHAUER, * B. FEIGL, M. H. GERZABEK, and C. CERRI Department of Environmental Research, Austrian Research Centers, A-2444 Seibersdorf, Austria (G.H., M.H.G.); and Centro de Energia Nuclear na Agricultura, Universidade de SaÄ o Paulo, Piracicaba, Brazil (B.F., C.C.) This article describes a series of transmission Fourier transform infrared (FT-IR) spectroscopy measurements of soil layers, espe- cially organic soil layers, originating from a tropical forest and from a pasture, which was developed after deforestation of the tropical soil in 1987. Spectral information obtained from forest and pasture soil layers from central RondoÃnia, Brazil, was analyzed and com- pared. A variety of bands characteristic of molecular structures and functional groups have been identi®ed for these complex samples. Cluster analysis revealed that such land use change affects the spec- troscopic behavior of the organic soil layers. A signi®cantly closer relationship between the pasture soil layers in comparison to the forest soil layers was obtained. This result indicates a higher ho- mogeneity of the intoduced litter from pasture vegetation compared to forest. The application of regression models enabled the estima- tion of soil parameters, such as organic carbon and nitrogen; the identi®cation and differentiation of organic forest soil horizons; and the determination of the decomposition status of soil organic matter in distinct layers. On the basis of the data presented in this study, it may be concluded that FT-IR spectroscopy is a powerful tool for the investigation of decomposition dynamics and litter quality in tropical soils. Index Headings: FT-IR spectroscopy; Litter; Tropical soils; Soil analysis; Deforestation. INTRODUCTION Land use changes have many consequences in terms of element cycling, soil conservation, and sustainable de- velopment of rural areas. The intense deforestation pro- cess which the Amazon forest is undergoing has already led to large areas of land being abandoned 1 or used for agricultural production. The conversion of tropical forest into pasture induces signi®cant changes in soil organic carbon (SOC) cycling. The importance of SOC in global change processes is already a major area of discussion. 2 Through losses or accumulation, soil organic matter plays an important role as a source or sink of atmospheric CO 2 , the most important greenhouse gas. 3,4 Thus, to obtain fur- ther insight into the dynamics of litter and soil organic matter accumulation processes and the effects induced through deforestation, scientists require the development of methods that are able to follow and compare decom- position of litter in soils. Decrease of C-stock in pasture after deforestation depends on both the stability of forest- derived organic matter and organic carbon introduced from the pasture. Structural information, therefore, is im- portant to understand the mechanisms. Besides stable isotope techniques that have been de- veloped to quantify turnover rates of SOC, 5,6 advances in instrumentation and techniques that permit structural de- Received 23 April 1999; accepted 22 September 1999. * Author to whom correspondence should be sent. termination of organic substances have provided new in- sights into the chemical composition of soil organic mat- ter. 7±9 The development of Fourier transform Raman (FT- Raman) spectroscopy has resulted in improved vibration- al spectra of lignin polymers 10 and humic substances obtained from various sources. Infrared (IR) analysis, which contributed substantially to the knowledge of the chemistry of soil humus, 11,12 has been used to follow the composting process of forest litter 13 and manure 14 and to determine the maturity of asphaltene samples. 15 We used a simple KBr-pellet transmission technique to analyze the IR characteristics of forest soil organic matter (SOM) of various decompositional stages and to compare them with the IR characteristics of a pasture site succeeding the for- est in 1987. By applying nonlinear statistical models, we attempted to prove the applicability of FT-IR and che- mometric analysis 16 to determine the decompositional stages of SOM along with other soil parameters and to reveal the impact of land use changes on FT-IR spectro- scopic properties of soil organic matter. MATERIALS AND METHODS Soil pro®les from two different sitesÐone forest site and one pasture siteÐfrom an area in central RondoÃnia (10820 9 S, 62830 9 W), Brazil were used. The annual mean temperature is 25.5 8C with a seasonal variation of less than 5 8C. 17 The pasture was created by the slash and burn technique directly from forest in 1987. 18 Five indi- vidual soil pro®les of each site were collected down to a depth of mineral soil of approximately 5 cm from an undisturbed area. The predominant soil type is a Tropo- dult (PodzoÂlico Vermelho-Amarelo according to the Bra- zilian classi®cation). Organic horizons were divided into L (fresh litter), F (fermentation horizon), and H (humi- ®cation horizon) (only for forest soil) layers, respectively; in the case of thicker horizons, the plots were divided vertically into several subsamples. Only the ®rst centi- meter of the underlying Ah horizon (humic mineral soil layer) was included for FT-IR-measurement. Altogether 35 soil samples were analyzed. The soil samples were air-dried and sieved to remove material .2 mm. Then the samples of sieved soil were powdered in an agate mill. Further sample preparation for the transmission FT-IR de- terminations was as follows: In all cases the total soil was used. One milligram of the homogenized agate- milled soil sample was mixed thoroughly with 100 mg of KBr (FT-IR grade). A pellet was prepared with the use of a press. Afterwards, the pellet was immediately put into the sample holder and FT-IR spectra were recorded. Total C (organic C content) and total N content were determined by an automatic combustion method with the