ORGANIC-SPECIES IN PED SURFACE AND CORE FRACTIONS ALONG A CLIMOSEQUENCE IN THE PRAIRIE, NORTH-AMERICA

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 lig nin and carbohydrate signature were assessed in peds from native topso ils along a temperature and precipitation transect across the North Am erican 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 fracti ons 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 cul tivated soils no such differences were found. Analyses of lignin and o f individual saccharides in the peds from the native sites suggest tha t SOC losses from ped surfaces are caused by a favourable environment for microbial decay. Lignin was preferentially oxidized in the PSF whi ch on average contains 25% less lignin derived phenols than the corres ponding PCF. Saccharide analysis confirmed significantly more microbia l alteration of SOM at ped surfaces than in the core, indicated by hig her ratios of hexoses to pentoses and a significant enrichment of acid ic sugars. With increasing mean annual temperature the amounts of micr obe-derived saccharides increase in the PSF relative to PCF, thereby c ontributing to the small scale heterogeneity of SOM species within mol lic A horizons. In general, the PSF thus characterises a C-Pool of hig her organic matter turnover than the core of aggregates > 2 mm. As ped surfaces are zones of contact with plant roots the results may have e cological importance for the preferential release of nutrients or the decay of organic pollutants.