Effect of grassland degradation on aggregate‐associated soil organic carbon of alpine grassland ecosystems in the Qinghai‐Tibetan Plateau

One of the important mechanisms for the stabilization of soil organic carbon (SOC) is its spatial inaccessibility for microbial biodegradation within soil aggregates. However, little has been documented regarding soil aggregate stability with grassland degradation in the alpine region of the Qinghai‐Tibetan Plateau (QTP). In this study, we used physical and density fractions to elucidate the mechanisms of differences in SOC in non‐degraded and degraded grasslands of two grassland biomes, alpine meadow and alpine steppe in the QTP. There were considerable differences between non‐degraded and degraded grasslands for the soil physical and chemical properties, aggregate distribution and aggregate‐SOC content. The non‐degraded alpine meadow (AMND) had the largest value among all the alpine grasslands for the SOC content of the microaggregate fraction, with values of 31.83 g kg⁻¹. The degraded grasslands showed significantly larger SOC content of macroaggregates than non‐degraded grasslands. The degraded alpine steppe (ASD) had the largest SOC content, with the value of 25.51 g kg⁻¹. The aggregate distribution of the macroaggregate, microaggregate and free silt+clay fractions was consistent with the variation in SOC content of these three aggregate fractions. The fine intra‐aggregate particulate organic matter (fiPOM) content was significantly less in degraded grasslands, indicating that grassland degradation might have disrupted the fiPOM‐C. The increase in CO₂ emissions was related to the destruction of soil aggregates in the alpine grasslands of the QTP. HIGHLIGHTS: Effects of alpine grassland degradation on aggregate‐associated SOC were investigated. Used soil aggregate fractions to evaluate the variation of aggregate‐associated SOC. Grassland degradation promoted the destruction of soil aggregates in the alpine grasslands of the QTP. Destruction of soil aggregates promoted CO₂ emissions in the degraded alpine grasslands of the QTP.