Short-term soil carbon dynamics of humic fractions in low-input and organic cropping systems

Observing changes in soil organic matter (SOM) is a fundamental part of defining the carbon cycle in natural and cultivated environments. However, relying on changes in the mass of soil C over short periods often produces conflicting results because of errors associated with sampling and analysis. In addition, C mass balance studies provide little interpretation of processes or turnover of specific C fractions. In the following study, we used C isotope and chemical separation of soil organic C to observe short-term soil C dynamics. With corn as the source of tracer C in two cover crop-based agricultural systems, natural abundance C measurements were used to identify changes in soil humic fractions (humic acid, fulvic acid, and humin) during two seasons under organic or low-input management treatments. All three fractions showed significant accumulation or turnover of C, with the fulvic acid fraction showing the most frequent but the smallest changes. The fulvic acid fraction showed a 5–9% turnover of C compared to 16% C turnover in the humic acid fraction. The stable soil C fraction defined as humin also exhibited an 8% turnover of C. The different humic fractions were affected at different times in the two treatments, supporting the idea that individual humic fractions may have different roles in C cycling depending on inputs and seasonal conditions. D 2003 Elsevier Science B.V. All rights reserved.