ELSEVIER Geoderma 74 (1996) 193-206 GEODE~ Organic species in ped surface and core fractions along a climosequence in the prairie, North America W. Amelung *, W. Zech Institute of Soil Science and Soil Geography, University ~?f Bavreuth 95440 Bayreuth, Germany Received 9 April 1996; accepted 30 July 1996 Abstract To test the hypothesis that preferential decomposition of soil organic matter (SOM) at ped surfaces results in different SOM quality at ped surfaces compared to the cores, small scale heterogeneities of the lignin and carbohydrate signature were assessed in peds from native topsoils along a temperature and precipitation transect across the North American prairie. The peds were separated into a 0.5 mm ped surface (PSF) and ped core fraction (PCF) prior to chemical analysis. Soil organic carbon (SOC) and C/N are significantly depleted in the surface fractions relative to the core ones at the native sites. On average the PCF contains 8.6% more SOC than the PSF, ranging from - 8 to + 20%. In cultivated soils no such differences were found. Analyses of lignin and of individual saccharides in the peds from the native sites suggest that SOC losses from ped surfaces are caused by a favourable environment for microbial decay. Lignin was preferentially oxidized in the PSF which on average contains 25% less lignin derived phenols than the corresponding PCF. Saccharide analysis confirmed significantly more microbial alteration of SOM at ped surfaces than in the core, indicated by higher ratios of hexoses to pentoses and a significant enrichment of acidic sugars. With increasing mean annual temperature the amounts of microbe-de- rived saccharides increase in the PSF relative to PCF, thereby contributing to the small scale heterogeneity of SOM species within mollic A horizons. In general, the PSF thus characterises a C-Pool of higher organic matter turnover than the core of aggregates > 2 mm. As ped surfaces are zones of contact with plant roots the results may have ecological importance for the preferential release of nutrients or the decay of organic pollutants. Keywords: soil organic matter; carbohydrates; lignin; aggregates; grassland soils; climate * Corresponding author. Fax: +49(921)552246, E-mail: wulLamelung@uni-bayreuth.de 0016-7061/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0016-706 1(96)00063-8