Soils on the Tibetan Plateau are extremely sensitive to global climate change. They are young in terms of development, forming a thin, skeletal soil layer.
The lack of understanding of soil organic matter (SOM) transformation processes in this region hinders the prediction of SOM stocks under future climate conditions.
Recently, Prof. ZHANG Gengxin's group from the Institute of Tibetan Plateau Research (ITP) of the Chinese Academy of Sciences investigated the responses of the alpine SOM to global climate change using pyrolysis-gas chromatography/tandem-mass spectrometry combined with microbial analysis methods.
The climate change factors include Nitrogen (N) deposition, warming and increased precipitation.
The researchers selected five typical alpine ecosystems on the Tibetan Plateau for pre-test to explore which soil fraction was most sensitive to external environment changes. They found that labile fraction SOM (LF-SOM) with density ≤2.25 g/cm3 was the most sensitive fraction.
Based on the pre-test results, artificial N additions, warming and increased precipitation experiments were conducted to study the degradation stability of LF-SOM.
Through a 2-yr multiple N addition experiment, the researchers found that N additions had significant effects on the content and composition of LF-SOM.
There was a nonlinear response of LF-SOM to N addition levels, that is, the LF-SOM increased at the lowest N addition levels and decreased at higher N addition levels, but the decrease of LF-SOM was weakened at 160 kg N ha-1 yr-1 addition.
Further study showed that the abundance of microbial functional genes, as one of the indicators of microbial activity, had significant positive correlations with LF-SOM.
In addition, by a three-year field manipulation experiment with warming (+2 °C above ambient temperature) and increased precipitation (+15% and +30% above ambient precipitation) treatments, they found that warming and increased precipitation indirectly affect LF-SOM turnover by directly affect the balance between vegetation input and microbial degradation.
More specifically, increased precipitation promoted LF-SOM accumulation, which were mainly due to the positive effect of increased precipitation on vegetation productivity.
In contrast, warming alone intensified the effect of drought on the alpine grassland, which had negative effects on both vegetation and microorganisms and reduced LF-SOM.
Warming plus increased precipitation not only alleviated water loss but also increased soil temperature, which was more favorable for the growth of microorganisms.
The researchers predicted that, in the future, N deposition would inhibit the activity of microorganisms and contribute to the accumulation of LF-SOM. In contrast, microbial activity would increase as temperature increases, which would promote LF-SOM degradation only if precipitation also increases.