Scientists from Alberta Agriculture and Rural Development (Canada), the National Institute of Agricultural Technology, the University of Buenos Aires (Argentina), and University of California, Davis (USA) have investigated soil C balance in distinct agroecosystems under different management practices including soil tillage, N fertilization, elimination of fallow, and establishment of grass.

In each case, C sequestration occurred in response to higher C input to soil; however, increase in SOC was confined to labile fractions such as the light fraction and larger soil aggregates.

Investigation: Canada
In southeastern Alberta, a long-term study showed previously that eliminating summer fallow or establishing grass significantly increased soil organic C after 6 yr. In the 12th year of the study, total organic C and light fraction C were determined in three rotations with summer fallow, two continuously cropped rotations and grass. All rotations had subtreatments with different levels of fertilization.

The light fraction of soil C was obtained using density separation and consisted mostly of non-decomposed root and straw fragments.

Although soil organic C was increased by elimination of summer fallow, fertilization, and establishment of grass, gains in soil organic C between Years 6 and 12 were negligible in all treatments except the fertilized grass treatment.

Most of the gains in total soil organic C were due to increased light fraction C. The results indicate that much of the gain in soil organic C in response to improved practices on semiarid prairie soils likely occurs within one decade.

Investigation: Argentina
In the semiarid portion of the Pampas, scientist compared no-till management to a conventional tillage system (disk-tillage). Emissions of CO2-C from the soil and crop C inputs were determined, estimating soil C balance under both tillage systems.

As a part of this study, a field experiment was performed during 6 yr on an Entic Haplustoll where no-till and disk-tillage was applied to a soil cropped under a common rotation in the region (oat + hairy vetch, corn, wheat, oat). From Year 3 to 6 in situ CO2-C fluxes were measured and C inputs from above and below ground plant residues were estimated.

Results showed that in the semiarid environment of the study C sequestration occurred under no-till. The sequestration process was attributed to the effect of tillage systems on crop productivity rather than on the mineralization intensity of soil organic pools.

Investigation: United States
In Kentucky (USA) a study was conducted in a corn agroecosystem experiment to test the soil C saturation concept which postulates that there is an upper limit to the equilibrium soil C level of mineral soils even when soil C input is increased.

In this experiment, a gradient of soil C input was produced with four N fertilizer application rates under two tillage systems, no-till and moldboard plowing. To investigate if physical protection of organic C leads to soil C saturation, C stabilization in soil fractions that differ in C stabilization potential was determined, and the relationship between soil C input and soil organic C was analyzed.

Total soil organic C was positively related to C input, and this was primarily due to C stabilization in larger soil aggregates. In both tillage systems, however, C in the two smallest soil size fractions did not increase with greater C input.

Moreover, in three soil fractions further separated from larger soil aggregates, C associated with particulate organic matter and microaggregates increased with C input, but there was no increase in C associated with silt-plus-clay, which was the smallest soil size fraction.

Haegeun Chung at University of California, Davis, the first author of the study conducted in Kentucky (USA), stated "Our results indicate that soil fractions with low C stabilization potential show C saturation. Therefore, we need to consider soil C saturation levels to better predict the change in C sink capacity and fertility of soils when soil C input increases under higher plant production or organic amendment."

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