Warming-dominated climate change impacts on soil organic carbon fractions and aggregate stability in Mollisols

Mollisols contain high amounts of soil organic carbon (SOC), which is highly susceptible to climate change; thus, climate change could indirectly influence soil aggregate stability, but the dominant factor affecting aggregate stability remains controversial. Here, a soil transplanting test from high-latitude to low-latitude locations was initiated in 2004 to investigate the influences of warming-dominated climate change (approximately 3–4.7 ℃) on the quantity and molecular composition of OC fractions in surface (0–20 cm) soils, aggregate stability changes and underlying mechanisms. Different initial soil organic matter (SOM) contents of 50.6 g kg−1 (SOM5), 58.8 g kg−1 (SOM6), and 108.9 g kg−1 (SOM11) were established in situ soils and in transplanted soils to simulate warming. The 15-year warming-dominated climate change presented no noticeable change in the SOC content in the lower SOM Mollisols (SOM5 and SOM6) but increased the SOC content by 13.3% in the higher SOM Mollisol (SOM11). In terms of labile OC fractions, warming-dominated climate change significantly increased the dissolved organic carbon (DOC) content by 20.1%–47.7% but reduced the easily oxidizable organic carbon (EOC) and microbial biomass carbon (MBC) contents by 22.1%–33.6%. Irrespective of any treatment, warming-dominated climate change decreased soil aggregate stability, as evidenced by the reduction in mean weight diameter (MWD) and geometric mean diameter (GMD) of 41.7%–79.3% and an increase in fractal dimension (D) of 28.6%–58.5%. For hierarchically organized soil aggregates, warming-dominated climate change increased the proportion and OC content of particulate organic matter inside free microaggregates (Fm-POM) as well as nonaggregated silt + clay-sized organic matter (nA-MOM). However, climate change decreased the proportion and OC content of silt + clay-sized fractions inside microaggregates within macroaggregates (mM-MOM). Of importance, warming-dominated climate change increased the amount of carbohydrates and decreased the amount of lignin in the mM-MOM, Fm-POM, and nA-MOM fractions. Therefore, we speculated that chemical protection by mineral association within macroaggregates and nA-MOM fractions and physical protection by the occlusion of POM within free microaggregates might be the primary mechanisms for SOC stabilization under long-term climate change in Mollisols. The long-term warming-dominated climate change results demonstrated a positive effect on SOC content in higher SOM Mollisols, DOC, carbohydrate C, free microaggregate-associated C and nonaggregated silt + clay-sized associated C but exhibited a negative effect on EOC, MBC, lignin C, silt + clay-sized inside microaggregates within macroaggregates C and aggregate stability. These variables all contributed to the reduction in soil aggregate stability and might act as sensitive indicators of warming-dominated climate change in Mollisols, which in turn affect farmland ecosystem C fluxes in response to further climate change.