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Despite few workers recording black C or charcoal in soils from the USA, historically fire has been extensive High levels of charcoal C resulting from repeated historical burning in the North American grasslands. For example, Collins of grasslands, open woodlands, and agricultural crop residues have (1990) reported that because of "periodic droughts, high been reported in soils from Australia and Germany. In this study, temperatures, and strong winds, the North American five U.S. soils were selected from long-term research plots in widely different agricultural areas. The charcoal C content was estimated on grasslands provide an ideal environment for ignition each soil using a combination of physical separation, high energy photoand perpetuation of fire." Collins (1990) also concluded oxidation and solid-state 13 C nuclear magnetic resonance (NMR) specthat "most evidence suggests that Indians frequently troscopy. These analyses showed that all five soils contained measurstarted grassland fires to modify habitat and either drive able amounts of charcoal C, Ͻ53 m in size and ranging from 1.8 to or attract wild game" and that "fires, caused by human 13.6 g C kg Ϫ1 soil and constituted up to 35% of the soil total organic C activity or otherwise, were historically a frequent com-(TOC). Scanning electron microscopy showed that the charcoal mateponent of the disturbance regime in grasslands." It might rial had a plant-like morphology but were blocky and had fractured be expected therefore that a legacy of this frequent edges. These particles were similar in morphology to those separated burning would be the presence of significant quantities from Australian and German soils. The implications of this material, of charcoal in the soil profiles under this activity. which must be highly resistant to microbiological decomposition, to the soil C cycle are discussed.

Significant increases in root density,

The increasing concern for rising CO2 concentrations from agricultural activities has prompted the need to better understand the flux of greenhouse gases to the atmosphere. This work determines the effect of four fall tillage methods on short‐term CO2 flux from a Hamerly clay loam (fine‐loamy, frigid Aeric Calciaquoll) in the northern Corn Belt. Moldboard plow only, moldboard plow plus disk harrow twice, disk harrow once, and chisel plow once using standard tillage equipment following a wheat (Triticum aestivum L.) crop were compared with no‐tillage. The CO2 flux was measured with a large portable system commonly used to measure canopy gas exchange of field crops. Measurements of CO2 flux were initiated within 5 min after tillage completion for each tillage treatment and continued intermittently for 19 d. Moldboard plow had the roughest soil surface and the highest initial CO2 flux (29 g m−2 h−1) and maintained the highest flux throughout the study. Moldboard plow plus disking twice and chisel plow had similar initial rates (7 and 6 g m−2 h−1, respectively) that were greater than disk harrow and no‐tillage. The high initial CO2 fluxes were more related to depth of soil disturbance that resulted in a rougher surface and larger voids than to residue incorporation. The differences in CO2 flux between tillage treatments were small but consistent 19 d after initial tillage and 64 mm rain. Lower CO2 flux rates caused by tillage were associated with low soil disturbance and/or small voids. Tillage methods affected the initial CO2 flux differently and suggest improved soil management can minimize agriculture's impact on global CO2 increase.