Climatic effects on soil organic matter composition in the great plains

To examine temperature and moisture effects on the chemical composition of soil organic matter (SOM), six soil profiles were studied from a climosequence in the native prairie of the Great Plains of the USA. After the removal of plant debris, the vertical distributions (0-60 cm) of soil organic carbon (SOC), N, lignin, and cellulosic and noncellulosic polysaccharides were determined. Samples of the topsoil horizons were additionally characterized by means of C-13 and H-1-Nuclear Magnetic Resonance (NMR) spectroscopy. As annual temperature increased from 7 to 23 degrees C at sites with <500 mm precipitation per year, the amount of polysaccharides decreased from 605 to 422 g kg(-1) SOC in the topsoil (0-15 cm) and from 516 to 278 g kg(-1) SOC in the subsoil (30-40 cm). Polysaccharide contents increased with increasing precipitation, reaching 726 g kg(-1) SOC in the top 15 cm and 876 g kg(-1) SOC at 50-cm soil depth (830 mm annual precipitation, 12.6 degrees C annual temperature). At all sites, the content of lignin-derived phenols per kilogram SOC decreased with increasing soil depth. Polysaccharides decreased much less. It is suggested that polysaccharides were resynthesized during SOM alteration within the soil profiles and thereafter preferentially to lignin protected from decay in the mineral soil. The NMR spectra suggested that both polysaccharides and alkyl compounds reflected climatic influences on the SOhl composition of the Great Plains.