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.

No abstract

A better understanding of C turnover, with estimates of root‐derived C, is needed to manage soil C sequestration. The objective was to evaluate the long‐term treatment and environmental effects on unharvestable soil C components. Two N fertilizer treatments and a control were imposed during 29 yr of continuous corn (Zea mays L.) with stover removal as silage vs. stover return during grain harvest with moldboard plow (MB) tillage. Soil organic carbon (SOC) declined and natural 13C abundance (δ13C) increased during the 29‐yr period. Field averages of SOC and δ13C (0–30 cm) were 96.4 Mg ha−1 and −17.3‰ in 1965; respective values in 1995 were 78.9 Mg ha−1 and −16.6‰. Loss of SOC was greater with stover removed or no fertilization, but δ13C increased for all treatments. Stover yield (SY), SOC, and δ13C data were applied to a model to estimate unharvestable C and predict total source C (SC) input from corn. The SC for 29 yr totaled 172 to 189 Mg ha−1 when stover was harvested and 268 to 284 Mg ha−1 when stover was returned. The SC input from unharvestable sources was 1.8 times more than SC from aboveground stover when N was added and 1.7 when N was not added. The root‐to‐shoot ratio was 1.1 when N was added and 1.2 with no N. Only 5.3% of the SC was retained as SOC. Unharvestable C contributions to rhizodeposition are much larger than suggested from controlled studies including C‐enriched CO2 followed by soil respiration or CO2 efflux measurements.

No abstract

Application of exogenous N fertilizers provides agronomic benefits but carries environmental liabilities. Managing benefits and liabilities of N-based fertilizers in conventional (CNV) and organic (ORG) cropping systems might be improved with better knowledge of nutrient dynamics, the generation of intrinsic N, and maintenance of soil organic matter. This study evaluated mineral N dynamics, yields, residue inputs, and change in soil organic C (SOC) and total N (TN) in strip-tilled, four-year crop rotations [corn (Zea mays L.)-soybean (Glycine max [L.] Merr.)-wheat under-seeded with alfalfa (Triticum aestivum L./Medicago sativa L.)-alfalfa] over eight years of production under CNV management using mineral-N (NO3NH4) and chemical pesticides or ORG management using organic-N (animal manure) and no chemical treatments. In ORG, N availability increased over time, but did not benefit ORG yields due to poor control of insects and weeds. Corn, soybean, and wheat grain yields were 1.9 to 2.7 times greater in CNV. In general, SOC was lost in CNV but did not change in ORG. Cumulative yield N removals exceeded cumulative fertilizer-N inputs by an average of 78% in CNV and 64% in ORG. These results indicated ORG management supported N availability by generating intrinsic N.