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Crop yields directly affect carbon (C) inputs into soils. Tillage management can influence crop performance, and as such should be considered when quantifying soil organic carbon (SOC) change, and thus net greenhouse gas emissions from croplands for national greenhouse gas inventory reporting. We conducted a meta-analysis of the effects of no-tillage (NT) and conventional tillage (CT) on crop yields for multiple crop species, soil types, and climatic regions of Canada. Yield response to NT varied between western and eastern Canada. Regardless of crop type, experiment duration, soil texture, and residue management, experiments in eastern Canada showed an average of 6% lower yields (p ≤ 0.005) under NT compared to CT. In western Canada, crop type had an important effect on yields between NT and CT with wheat, canola, and legumes exhibiting 10% (p ≤ 0.001), 7% (p ≤ 0.05), and 9% (p ≤ 0.05) higher yields on average under NT compared to CT, respectively. In western Canada, higher yields would be reflected in a similar scale of higher C inputs to NT systems. A recent meta-analysis of the effects of tillage management on SOC in Canada showed an 8% higher storage of SOC under NT systems compared to CT, a difference limited to western Canadian soils. Incorporating the effect of tillage on C inputs will lead to improvements in the accuracy of the effects of tillage management on SOC change in Canadian cropland. The activity data can be improved by applying weightings to the yield data by site-specific assessment of tillage practices across Canada. [ABSTRACT FROM AUTHOR]

There is abundant evidence that minimizing soil disturbance reduces mineralization of organic matter and can result in larger storage of soil organic carbon (SOC) relative to conventional tillage. However, little is known about the persistence of SOC when no-till lands are plowed periodically. This study set out to determine the change in SOC when a long-term (22 yr) no-till field in southern Ontario, Canada, was plowed once. Four plots were located within three textural classes [sandy loam (SL), sandy clay loam (SCL), and silty clay loam (SiCL)] within two hydrologic conditions (well- and poorly drained) in the field. The plots were sampled before and three times after (3 d, 7 mo, and 18 mo) the no-till field was moldboard plowed. The single tillage event homogenized the SOC through the profile and reduced the stratification. When calculated on an equivalent mass basis beyond the plow depth, there was no significant change in SOC 18 mo after plowing the SCL, SiCL, and SL high SOC plots. However, in the SL plot with low SOC, there was a loss of about 3 Mg SOC [ha.sup-1] after 18 mo, and the loss occurred primarily between the 15- and 30-cm depths in the profile. The loss may have accounted for as much as two-thirds of the SOC gained from no-tillage. This study also emphasized the need for additional care to account for changes in bulk density when comparing the quantity of soil constituents, such as SOC, after plowing.

National estimates of changes in the amount of soil organic carbon (SOC) in cropland requires an assessment of uncertainty for accounting and reporting under the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol. Canada has data sets on SOC stocks in croplands, historical changes in SOC levels due to management practices, and historical changes in the area of land devoted to certain soil management practices. We conducted an analysis of uncertainty of the change in SOC levels due to management practices in Canada from 1991 to 2001 using Monte Carlo analysis and a simple model. Probability distribution functions were determined for each of the inputs of the model, enabling us to assess the uncertainty for the output. The storage rate of SOC in cropland soils of Canada for the 10-year period ranged from 3.2 to 8.3 Mt C yr−1 at 95% confidence, with a mean of 5.7 Mt C yr−1. Approximately 67% (about 3.8 Mt C yr−1) of the increase in SOC storage in Canada occurred in Saskatchewan where the cropland area under no-till increased from 10% to 35%, and the area of summer-fallow declined from 43% to 20% during the study period. The large uncertainty in the effect of no-till on SOC stock changes in the Gray-Brown Luvisols of Ontario contributed most to the variance in the model output. If trends in agricultural management continue for the next 10-year census period, the estimated SOC storage would comprise between 7% and 19% of the gap required to achieve the 6% reduction in 1990 greenhouse gas emission levels for Canada under the Kyoto Protocol. [ABSTRACT FROM AUTHOR]

Abstract: Agricultural soils can constitute either a net source or sink of the three principal greenhouse gases, carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). We compiled the most up-to-date information available on the contribution of agricultural soils to atmospheric levels of these gases and evaluated the mitigation potential of various management practices in eastern Canada and northeastern USA. Conversion of native ecosystems to arable cropping resulted in a loss of ∼22% of the original soil organic carbon (C)—a release of about 123Tg C to the atmosphere; drainage and cultivation of organic soils resulted in an additional release of about 15Tg C. Management practices that enhance C storage in soil include fertilization and legume- and forage-based rotations. Adopting no-till did not always increase soil C. This apparent absence of no-till effects on C storage was attributed to the type and depth of tillage, soil climatic conditions, the quantity and quality of residue C inputs, and soil fauna. Emission of N2O from soil increased linearly with the amount of mineral nitrogen (N) fertilizer applied (0.0119kgN2O-NkgN−1). Application of solid manure resulted in substantially lower N2O emission (0.99kgN2O-Nha−1 year−1) than application of liquid manure (2.83kgN2O-Nha−1 year−1) or mineral fertilizer (2.82kgN2O-Nha−1 year−1). Systems containing legumes produced lower annual N2O emission than fertilized annual crops, suggesting that alfalfa (Medicago sativa L.) and other legume forage crops be considered different from other crops when deriving national inventories of greenhouse gases from agricultural systems. Plowing manure or crop stubble into the soil in the autumn led to higher levels of N2O production (2.41kgN2O-Nha−1 year−1) than if residues were left on the soil surface (1.19kgN2O-Nha−1 year−1). Elevated N2O emission during freeze/thaw periods in winter and spring, suggests that annual N2O emission based only on growing-season measurements would be underestimated. Although measurements of CH4 fluxes are scant, it appears that agricultural soils in eastern Canada are a weak sink of CH4, and that this sink may be diminished through manuring. Although the influence of agricultural management on soil C storage and emission of greenhouse gases is significant, management practices often appear to involve offsets or tradeoffs, e.g., a particular practice may increase soil C storage but also increase emission of N2O. In addition, because of high variability, adequate spatial and temporal sampling are needed for accurate estimates of greenhouse gas flux and soil C stock. Therefore a full accounting of greenhouse gas contributions of agricultural soils is imperative for determining the true mitigation potential of management practices. [Copyright &y& Elsevier]