Soil organic matter (SOM) as represented in mathematical simulation models involves several hypothetical pools of differing resistance to decay. These conceptual pools satisfy requirements of modeling, but usually have little in common with existing information on physical and chemical properties of SOM. Using our data on turnover times for soil C in fractions of natural aggregates and primary particles, we attempted to relate age of C in physical fractions with that in widely accepted theoretical pools. Soil from a field experiment with 14C‐labeled soybean residues was sampled periodically and separated into physical fractions. The amounts of 14C associated with these fractions at different times provided data for calculation of decay rates and turnover times. The most labile fraction of SOM was plant fragments with turnover time ranging from 1 to 3 yr, which was inversely related to fragment size. Soil aggregates were found to be enriched in C compared with whole soil. This was most pronounced for coarser aggregates whose construction apparently involved the relatively labile plant fragments in some progressive state of decay. The macroaggregates with partially processed C showing turnover from 1 to 3 yr contrasted with microaggregates that included more highly humified C having a longer residence time of ≈7 yr. Various soil fractions differing in residence time of associated C were assembled into several groups that demonstrated consistency with conceptual pools of two widely accepted simulation models. Data from 13C natural abundance studies of soil and of primary fractions were in harmony with models requiring at least two pools of stable SOM.
The CO2 concentration of the gaseous phase of a silt loam soil (Udollic Ochraqualf) was studied under cultivation of wheat (Triticum aestivum), corn (Zea mays L.), and soybeans (Glycine max L. Merr.) over a 2‐year period. Disposable chromatographic tubes for direct field measurements of CO2 in soil air were used. Dynamics of CO2 in the soil air were observed to be conditioned by both biological and abiotic factors. Under wheat, periods of highest CO2 concentration (6–8%) corresponded to times of intensive decomposition of plant residue within the soil profile. Under corn and soybeans, highest CO2 concentrations were related to the periods of intensive plant growth. Soil moisture and soil temperature combined, were found to be responsible for just > 50% of CO2 fluctuations. The influence of water tension on CO2 in the soil atmosphere was more significant (r2 = 0.83) if data were restricted to that where temperature was 20 ± 2°C, and in transformations of the two parameters were used. The ln‐to‐ln dependence was linear within the limits from field capacity to wilting point. Under conditions of optimum soil water content, ln of CO2 concentration increased linearly with ln of soil temperature within the limits of 10 to 20°C.
Long‐term data from Sanborn Field, one of the oldest experimental fields in the USA, were used to determine the direction of soil organic carbon (SOC) dynamics in cultivated land. Changes in agriculture in the last 50 years including introduction of more productive varieties, wide scale use of mineral fertilizers and reduced tillage caused increases in total net annual production (TNAP), yields and SOC content. TNAP of winter wheat more than doubled during the last century, rising from 2.0–2.5 to 5–6 Mg ha–1 of carbon, TNAP of corn rose from 3–4 to 9.5–11.0 Mg ha–1 of carbon. Amounts of carbon returned annually with crop residues increased even more drastically, from less than 1 Mg ha–1 in the beginning of the century to 3–3.5 Mg ha–1 for wheat and 5–6 Mg ha–1 for corn in the 90s. These amounts increased in a higher proportion because in the early 50s removal of postharvest residues from the field was discontinued. SOC during the first half of the century, when carbon input was low, was mineralized at a high rate: 89 and 114 g m–2 y–1 under untreated wheat and corn, respectively. Application of manure decreased losses by half, but still the SOC balance remained negative. Since 1950, the direction of the carbon dynamics has reversed: soil under wheat monocrop (with mineral fertilizer) accumulated carbon at a rate about 50 g m–2 y–1, three year rotation (corn/wheat/clover) with manure and nitrogen applications sequestered 150 g m2 y–1 of carbon. Applying conservative estimates of carbon sequestration documented on Sanborn Field to the wheat and corn production area in the USA, suggests that carbon losses to the atmosphere from these soils were decreased by at least 32 Tg annually during the last 40–50 years. Our computations prove that cultivated soils under proper management exercise a positive influence in the current imbalance in the global carbon budget.
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