CLIMATIC EFFECTS ON SOIL ORGANIC-MATTER COMPOSITION IN THE GREAT-PLAINS

To examine temperature and moisture effects on the chemical compositio n of soil organic matter (SOM), six soil profiles were studied from a climosequence in the native prairie of the Great Plains of the USA. Af ter the removal of plant debris, the vertical distributions (0-60 cm) of soil organic carbon (SOC), N, lignin, and cellulosic and noncellulo sic polysaccharides were determined. Samples of the topsoil horizons w ere additionally characterized by means of C-13 and H-1-Nuclear Magnet ic Resonance (NMR) spectroscopy. As annual temperature increased from 7 to 23 degrees C at sites with <500 mm precipitation per year, the am ount 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 precipitatio n, 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 t emperature). At all sites, the content of lignin-derived phenols per k ilogram SOC decreased with increasing soil depth. Polysaccharides decr eased much less. It is suggested that polysaccharides were resynthesiz ed during SOM alteration within the soil profiles and thereafter prefe rentially to lignin protected from decay in the mineral soil. The NMR spectra suggested that both polysaccharides and alkyl compounds reflec ted climatic influences on the SOhl composition of the Great Plains.