Soil Pore Structure Of Aggregates And Its Relation With Organic Carbon Mineralization In Black Soils

Soil aggregates are the basic unit of soil structure and indispensable for soil function. Soil aggregate structure is closely related to dynamics of soil organic carbon(SOC). The study on their relationship will be helpful to comprehensively explain the mechanism of C sequestration within aggregates. A new technology, computed tomography technology(CT), has provided an approach to study soil structure and aggregate structure. In this study, soil samples were collected from a long-term conservation tillage trial established on typical middle- layer black soils and undisturbed soils as a control, we had detailed studies on the pore structure of both inter- and intra-aggregates of 1-2 mm(macroaggregate) and 0.053-0.25 mm(microaggregates), as well as the pore structure of the intact soil cores. Correlation analyzes between pore structure and soil organic carbon(SOC) content in aggregate, readily oxidized organic carbon(ROC) content and SOC mineralization rate were done to dig out the mechanism of SOC sequestration through soil aggregates. The main conclusions were as follows:(1) Significant differences in soil pore structure existed among undisturbed soils and cultivated soils. Within macroaggregates, the porosities of 10-30 μm and >100 μm, and total porosity were significantly higher in undisturbed soils than in cultivated soils, but the number of pores was diverse. To sum up, whatever for intra- macroaggregates or intra- microaggregates, the greatest differences in pore structure occurred between undisturbed soils and no-tilled soils. The ridge-tilled soils had a pore structure closely approximate to undisturbed soils, indicating that ridge tillage improved soil aggregate stability and has a potential to restore soil structure approaching to that in undisturbed soils.(2) There were significant differences in the porosity of 100-500 μm, pore length and specific surface area between in undisturbed intact soils and no-tilled intact soils. The pore structure of inter- macroaggregates was very similar to that of intact soil core, and became the main frame constituting bulk soil structure. Within inter- microaggregates, both the number and the porosity of 100-500 μm were dominated, and the pore structure of inter- microaggregates was remarkably different from those of inter- macroaggregates and the intact soil cores. The pore structure of inter- microaggregate was not as important for the entire soil structure as that of inter-macroaggregates, which just could adjust soil structure.(3) Within macroaggreagtes, the porosity of >100 μm and total porosity both were closely related to SOC and ROC in undisturbed soils(P<0.01), but the relationships were not significant in conventional-tilled, no-tilled and ridge-tilled soils. The porosity of 10-30μm, the number of pores and specific surface area of pore were all positively related to SOC and ROC. The total porosity, the porosity of > 100 μm, pore length, the radius and area of pore throats, and tortuosity were negatively related to SOC and ROC, respectively. There was weak relationship between pore structure and SOC within microaggregates, only occurred in total porosity and the porosity of 10-30 μm.(4) The relationships between pore structure of inter- macroaggregates and SOC were more closely than that of inter- microaggregates. Within inter- macroaggregates, the porosities of 30-60 μm, 100-500 μm and >500 μm were all significantly related to SOC, but within inter- microaggregates, the close relationship existed between the porosity of 30-60 μm, the radius and area of pore throats, respectively. Meanwhile, under a steady aggregation environment within enough carbon input, microaggregates were adsorbed on the surface of macroaggregates or formed within the macroaggregates, which was supposed to be effective for C sequestration, that is, avoiding external C of microaggregates from decomposing and being accumulated as internal C.(5) On the scale of intact soils cores, the pore structure including the porosities of 60-100 μm and 100-500 μm, the number of pores, pore length, the number of pore throats, specific surface area of pore, and tortuosity, had a close relationship with SOC. The porosity of 60-100 μm and 100-500 μm were beneficial to SOC sequestration. The number of pores and pore throats, pore length and tortuosity could significantly affect SOC and ROC. As well as, the middle axle length and tortuosity were positively related to SOC(P < 0.01), its numerical growth increased the scope of the organic matter distribution after enter into soil. While specific surface area was negatively related to SOC(P < 0.01), the increased ratio reduced the fixed pore area of organic matter entered into soil.(6) On the different- level scales, the relationships between pore structure and SOC were different. However, obviously, the pore structure of inter- and intra- macroaggregates had great impacts on soil structure and SOC. Due to the stability of microaggregates, the pore structure of inter- and intra-aggregates didn’t greatly affect soil structure. Comprehensively, the relationships between soil structure and SOC were determined by several parameters characterizing pore structure, rather than a single parameter. Consequently, it needs to further study to develop a comprehensive indicator characterizing the relationship between soil pore structure and SOC.