Sequestration Mechanisms Of Organic Carbon In Black Soils Under Conservation Tillage

The improvement of soil organic carbon （SOC） sequestration not only helps maintain soilfertility and sustainable development of agriculture, but also contributes to the reduction ofCO₂release from soil, which in turn retards the greenhouse effect. As the sequestration of CO₂in farmland soil becoming a hot research spot, the effects of different tillage managements onSOC sequestration and the sequestration mechanisms is the focus of research in this field. Inthis study, we mainly determined the storage and structure stability of SOC fractions indifferent tillage systems and analyzed the roles aggregates played in SOC sequestration, basedon a short-term tillage trial in black soils in northeast China and a long-term trial in blacksoils in southwest Ontario, Canada. The results are helpful in selecting a suitable tillagesystem in favor of SOC sequestration and in understanding the mechanisms.Conservation tillage favors the formation of stable aggregates, improves soil structure,and increases SOC contents in macro-aggregates. The effects of ridge tillage （RT） is moreobvious than no-tillage （NT）. RT improves SOC sequestration in black soils of northeastChina in the form of particulate organic carbon （POC） and micro-aggregates occluded inmacro-aggregates. Compared with NT, RT is more favorable to the improvement of soilfertility and turning farmland soil from C “source” into C “sink” in the poorly-drained clayloam soil of northeast China.The concentration of humic acid in soil and C concentration in humic acid aresignificantly higher in soil under conventional tillage （CT） than under long-term NT. With theincrease of tillage intensity, the humic acid becomes more humified, and showes a trend oftransformation from microbial decomposition product towards lignin.The mid-infrared spectroscopy combined with curve-fitting technique is a good approachto study SOC structure features in soil fractions. The curve-fitting technique makes it possibleto further interpret peak and functional group information in mid-infrared spectra. Comparedto smaller-sized fractions, the SOC in sand-sized fraction responds faster to tillagemanagements. Concentration of aliphatic-C and aliphatic-to aromatic-C ratio are significantly higher in long-term NT soils than in CT soils.More CO₂release is observed from NT soil aggregates than CT in aerobic incubation,indicating that the SOC associated with aggregates in NT soil is more easily mineralizable,especially with0.25-0.053mm aggregates. Carbon mineralization from aggregates increaseswith aggregates size. The CO₂release from crushed aggregates （from4-1mm and1-0.25mmto below0.25mm） is higher than from the4-1mm. These results show that C mineralizationincreases when it loses the protection from macro-aggregates structure.The SOC concentration and stability differ greatly from the inner cores to the externallayers of aggregates, especially in the well-developed woodlot and grassland soil aggregates.The CO₂released during aerobic incubation was significantly greater from aggregate externallayers than from the cores. The SOC in the cores was associated with a longer half-life and anotably greater stable C （aromatic-C） pool than that in aggregate external layers, whichsuggests that the SOC was more decomposable in the external layers than the cores. Theseresults illustrated the mechanisms of SOC sequestration from an internal aggregate structurepoint of view.