Tillage and N-fertilizer influences on selected organic carbon fractions in a North Dakota silty clay soil

Physical, chemical, and biological fractions of SOC pools, such as coarse particulate organic matter C (CPOM-C), permanganate oxidizable C (KMnO4-C), microbial biomass carbon (MBC), and mineralizable C (Cmin) respond to changes in management practices and provide sensitive indication of changes in the SOC dynamics than commonly reported total soil C alone. We hypothesized that tillage and N-fertilizer managements induced changes in SOC at the surface 0–15 cm soil would predominantly be reflected by parallel changes in different fractions of SOC. Three field experiments (Expt1: 2008–2011, Expt2: 2005–2011, Expt3: 2005–2011) were conducted in a Fargo (Typic Epiaquerts)-Ryan (Typic Natraquerts) silty clay complex in Fargo, North Dakota, USA. Our objectives were (i) to evaluate the effects of tillage (conventional till [CT], strip till [ST] and no-till [NT] and different N-fertilizer managements on SOC, CPOM-C, KMnO4-C, MBC, and Cmin and (ii) to determine any relationships among these C fractions within corn (Zea mays)-sugarbeet (Beta vulgaris)-soybean (Glycine max) rotation. Compared with CT, ST and NT had significantly higher SOC concentration by 3.8 and 2.7%, SOC stock by 7.2% and 9.2%, CPOM-C by 22 and 25%, and KMnO4-C by 4.8 and 4.1%, respectively in Expt2 and had significantly higher SOC concentration by 3.9 and 6.6%, SOC stock by 11.9 and 8.7%, and CPOM-C by 33 and 45%, respectively in Expt3. The KMnO4-C and 30 d cumulative Cmin were greater under ST than CT by 3.3 and 23%, respectively in Expt3. The amounts of Cmin were consistently higher under ST and NT than CT throughout the incubation period except at 7 d, in Expt3. Across the study, CPOM-C was 16.3–22.1%, MBC was 3.4–4.5%, cumulative Cmin was 0.7–1.4%, and KMnO4-C was 1.6–1.7% of the total SOC. Significant correlations were observed among SOC, CPOM-C and Cmin in all the experiments. CPOM-C was the most sensitive fraction to tillage changes. Tillage influences on SOC fractions followed the order: physical (CPOM-C) > biological (cumulative Cmin) > chemical (KMnO4-C), however, the sequence change with soil type and crop rotation requires further investigation.