Objective  Soil aggregates play an important role in improving the stability of soil organic carbon (SOC). In recent decades, the Qinghai-Tibet Plateau has experienced significant warming, but there are few studies on the effect of warming on the stability of soil aggregates. Whether the increase in temperature affects SOC changes by affecting the turnover and stability of soil aggregates remains to be determined.

  Method  Based on a 10-year long-term warming field experiment, the effects of long-term warming on the formation, turnover and stability of soil aggregates in Qinghai Tibet alpine grassland were explored by using the method of combined physical and chemical fractionation. At the same time, revealed aggregate physical protection effect on the change of SOC.

  Result  The results showed that long-term warming had no effect on SOC content. Long-term warming had no significant effect on the content of large macroaggregate (> 2 mm). But reduced the mass fraction of small macroaggregates (2 - 0.25 mm), microaggregates within macroaggregates (mM), non-occluded fine POM inside macroaggregates but outside microaggregates (M(f)POM) and silt and clay-sized fractions inside mM (mM-silt + clay) by 15.4%, 25.1%, 52.7% and 40.5%, respectively. The contents of free microaggregates (0.25 - 0.053 mm) and non-aggregated silt + clay (< 0.053 mm) increased by 14.8% and 43.5%, respectively. Long-term warming decreased Geometric mean diameter, Mean weight diameter and Proportion of > 0.25 mm aggregate by 23.0%, 11.1% and 19.5%, respectively, implying the decreasing of stability of soil aggregates. The SOC was mainly distributed in small macroaggregates (2 - 0.25 mm) and free microaggregates (0.25 - 0.053 mm), which accounted for 71.6% - 83.2% of the total SOC of the whole soil. The long-term warming increased the SOC content of 0.25 - 0.053mm, non-aggregated silt + clay (< 0.053 mm) and mM-silt + clay by 16.8%, 61.4% and 86.5%, respectively. The long-term warming decreased the SOC content of mM and M(f)POM 27.4% and 20.6%, respectively. It was found that the ratio of M(f)POM /M(c)POM (coarse particulate organic matter inside macroaggregates but outside microaggregates) decreased significantly by 20.6%, indicating that the macroaggregate turnover rate increased.

  Conclusion  In conclusion, long-term warming increases the mass fraction of carbon-poor 0.25 - 0.053 mm microaggregates, but reduces the mass fraction of carbon-rich 2 - 0.25 mm macroaggregates, and at the same time accelerates the turnover rate of soil macroaggregates and reduces the stability of aggregates. Therefore, in the long run, the physical protection of SOC may be reduced. The results of this study help to improve our understanding of the response of SOC stability in alpine meadow ecosystems to future climate change.