Soils play a key role in the global carbon cycle, and can be a source or a sink of atmospheric carbon (C). Thus, the effect of land use and management on soil C dynamics needs to be quantified. This study was conducted to assess: (1) the role of aggregation in enhancing soil organic carbon (SOC) and total soil nitrogen (TSN) concentrations for different mulch rates, (2) the association of SOC and TSN with different particle size fractions, and (3) the temporal changes in the SOC concentration within aggregate and particle size fractions with duration of mulching. Two experiments were initiated, one each in 1989 and 1996, on a Crosby silt loam (Aeric Ochraqualf or Stagnic Luvisol) in central Ohio. Mulch treatments were 0, 8, and 16 Mg ha-1 yr-1 without crop cultivation. Soil samples from 0-5 cm and 5-10 cm depths were obtained in November 2000; 4 and 11 years after initiating the experiments. Mulch rate significantly increased SOC and TSN concentrations in the 0-5 cm soil layer only. The variation in the SOC concentration attributed to the mulch rate was 41 percent after 4 years of mulching and 52 per cent after 11 years of mulching. There were also differences in SOC and TSN concentrations among large aggregate size fractions, up to 2 mm size after 4 years and up to 0.5 mm after 11 years of mulching. There were also differences in SOC and TSN concentrations among particle size fractions. Variation in the SOC concentration in relation to particle size was attributed to clay by 45-51 per cent, silt by 34-36 per cent, and to sand fraction by 15-19 per cent. Bulk of the TSN (62-67 per cent) was associated with clay fraction and the rest was equally distributed between silt and sand fractions. The enrichment of SOC and TSN concentrations in the clay fraction increased with depth. The C:N ratio was not affected by the mulch rate, but differed significantly among particle size fractions; being in the order of sand silt clay. 2003 John Wiley and Sons, Ltd.
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