Carbon and nitrogen in the enriched labile fraction along a climosequence of zonal steppe soils in Russia

Breaking aggregates by plowing resulted in the decomposition of formerly ph ysically protected soil organic matter (SOM), including the enriched labile fraction (ELF), but it was unknown to what extent such effects were contro lled by climate. To investigate this question, aggregate-size fractions wer e obtained from each of five native and adjacent long-term cultivated topso ils across a climosequence in the Russian steppe. After ultrasonic dispersi on of small macro (250-2000 mu m) and microaggregates (53-250 mu m) at 100 and 500 J mL(-1) (only for so-called stable macroaggregates) and a particle -size fractionation, density fractions <1.85 g cm(-3), 1.85 to 2.07 g cm(-3 )), 2.07 to 2.22 g cm(-3) (= ELF for the small macroaggregates) and >2.22 g cm(-3) were obtained from the fine silt-sized particles. In all fractions, C and N contents were determined. The stable aggregates were found only at native sites, were almost free of ELF, and showed their C maximum in the t wo lightest fractions. Cultivation reduced the C and N contents in all aggr egates. In small macroaggregates, C losses occurred primarily as ELF, where as microaggregates lost C in the fine silt density range of 1.85 to 2.22 g cm(-3). The C partition among the fine silt density fractions was not relat ed to climate. Losses in ELF were also not related to climate, suggesting t hat ELF represents a C pool that is site-specifically influenced by cultiva tion. The C losses from fractions <2.07 g cm(-3), however, increased as the climate became dryer and warmer, suggesting that they reveal interactive e ffects of climate and land use on physically stabilized SOM.