Soil macroaggregates determine soil organic carbon in the natural grasslands of the Loess Plateau

Soil aggregates are generally thought to be the nucleus of all carbon sequestration mechanisms. However, the Loess Plateau, a typical region where a fragile ecosystem has gradually recovered, has high variability in soil aggregates due to differences in climate and soil properties; thus, the driving mechanism of soil aggregates to changes in soil organic carbon (SOC) content remain unclear. To determine the dominant mechanisms affecting SOC content, we performed a large-scale investigation of the aggregate size fractions, aggregate-associated OC, OC in bulk soil, and environmental factors of the natural grasslands across a climatic gradient (semi-arid to semi-humid) on the Loess Plateau. The results showed that the proportion of macroaggregates (MA) decreased but that of microaggregates (MI) increased in the soil from south to north on the Loess Plateau. MA content decreased with soil depth, whereas MI and the silt + clay fraction (SC) contents increased with soil depth. In addition, aggregate-associated OC decreased with soil depth. The contribution rate of macroaggregate-associated OCwf (MAFC) to OC in bulk soil from south to north in the Loess Plateau showed a trend of decreasing first and then increasing, and the contribution rates were 73.77 % (Fufeng), 62.29 % (Huangling), 32.78 % (Suide), 28.31 % (Shenmu), and 47.62 % (Dalad Banner), respectively. For the entire Loess Plateau, MAFC contributed the most to SOC in different soil layers and the contribution rates were 64.68 % (0-5 cm), 49.54 % (5-10 cm), 44.43 % (10-20 cm), and 37.23 % (20-30 cm), respectively. Across the considered depths and sites, the MA, MAFC, and OC content of MA had significantly (p < 0.05) positive direct effects on the OC in bulk soil. Furthermore, MAFC was mainly dependent on the OC content and proportion of MA. Therefore, these results indicated that the proportion of MA and OC content within MA dominated the OC dynamics in bulk soil in the Loess Plateau.