Carbon and Nitrogen in the Enriched Labile Fraction along a Climosequence of Zonal Steppe Soils in Russia

Abstract

Breaking aggregates by plowing resulted in the decomposition of formerly physically protected soil organic matter (SOM), including the enriched labile fraction (ELF), but it was unknown to what extent such effects were controlled by climate. To investigate this question, aggregate‐size fractions were obtained from each of five native and adjacent long‐term cultivated topsoils across a climosequence in the Russian steppe. After ultrasonic dispersion of small macro‐ (250–2000 μm) and microaggregates (53–250 μm) 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 two lightest fractions. Cultivation reduced the C and N contents in all aggregates. In small macroaggregates, C losses occurred primarily as ELF, whereas 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 related to climate. Losses in ELF were also not related to climate, suggesting that ELF represents a C pool that is site‐specifically influenced by cultivation. The C losses from fractions <2.07 g cm−3, however, increased as the climate became dryer and warmer, suggesting that they reveal interactive effects of climate and land use on physically stabilized SOM.