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Abstract
Evaluation of soil organic carbon (SOC) dynamics is often limited by the complexity of soil matrix. Quantitative information on the distribution of SOC within aggregate hierarchy will help elucidate the carbon flow in soil matrix. However, this knowledge still needs to be documented. Soils were sampled from a surface Mollisol with plots under 100 years of continuous cropping, 31 years of simulated overgrazing to severely degraded bareland, and grassland restoration from cropped soil. A combined density and chemical fractionation procedure within water-stable aggregate was utilized to quantify the distribution of OC after long-term different land use patterns. Results showed that grassland significantly increased total SOC and mean aggregate associated OC compared to initial soil in 1985 with total SOC (g kg−1 soil) from 46.1 to 31.7 and mean aggregate associated OC (g kg−1 aggregate) from 31.6 to 44.7. Converting cropland to grassland also enhanced the formation of macroaggregates (>0.25 mm) (from 34.7% to 52.2%) and increased the OC concentrations in density and humic fractions by 48.3%-75.9% within aggregates. But the proportions of OC in density and humic fractions to SOC only increased in macroaggregates in grassland. Alternatively, converting cropland to bareland caused substantial depletion of total SOC, macroaggregates and their associated OC concentrations. The SOC (g kg−1 soil) and mean aggregate associated OC (g kg−1 aggregate) significantly decreased from 31.7 to 25.7 and from 31.6 to 26.2, respectively. While the OC concentration of density and humic fractions within aggregates in bareland did not show significant decreases. Principal component analysis demonstrated that the soils were developed by contrasting land use changes, with the grassland soil being more associated with labile OC fractions within macroaggregats and bareland soil more associated with recalcitrant OC fractions within microaggregates and silt-clay units. These findings highlighted the favorable preservation of plant-derived carbon within soil aggregates, particularly in the labile OC fractions within macroaggregates under high plant inputs with 31 years of grassland conversion. For the cropland and bareland soils without organic inputs, more OC was stabilized within fine aggregates via organo-mineral interactions, tending to be more recalcitrant.
Highlights
The processes governing the formation, transformation, and stabilization of soil organic carbon (SOC) are closely tied to soil aggregate architecture[1]
SOC decomposition rate reduced within soil aggregates in comparison to bulk soil; the interactions between SOC dynamics and aggregate turnover were affected by organic manuring and other changes in land use managements varying in the quantity and quality of organic matter entering soil[9,13]
We reported the results of a combined fractionation scheme where density and humic fractions within water-stable aggregates were separated from surface Mollisols under 31-year contrasting land use patterns
Summary
The processes governing the formation, transformation, and stabilization of soil organic carbon (SOC) are closely tied to soil aggregate architecture[1]. A relatively large proportion of organic residue was accumulated in the coarse fraction within macroaggregates, but the OC in microaggregate and silt-clay units did not continue to increase with additional organic manure inputs[14] These results suggested that the OC saturation occurred in a hierarchical fashion. Combining the physical and chemical fractionation methods within soil aggregates may assist in determining the effects of long-term land use change on the physicochemical sequestration mechanisms of OC in specific soil matrixes[3,18]. The aims of this study were to: (1) quantify and compare the OC changes in density and humic fractions within aggregates of different sizes, (2) determine the effects of contrasting land use patterns with different quantity and quality of organic inputs, and (3) try to elucidate the stabilizing mechanisms of OC within soil aggregate matrixes
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