Linkages between soil respiration and microbial communities following afforestation of alpine grasslands in the northeastern Tibetan Plateau

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Knowledge gaps exist in our current understanding of the linkages among soil respiration, microbial communities, and carbon sequestration following grassland afforestation. Here, we investigated the effects of forest planting on soil respiration, microbial communities, and associated soil properties in alpine grasslands of the northeastern Tibetan Plateau, and explored microbial mechanisms of the soil respiration response to afforestation. Generally, afforestation increased the richness and diversity of soil microbial communities. Redundancy analysis indicated that the variability in bacterial and fungal communities was mainly explained by soil organic carbon (SOC, R2bacteria = 0.85, R2fungi = 0.89), nitrate nitrogen (NO3−-N, R2bacteria = 0.64, R2fungi = 0.82), available phosphorus (R2bacteria = 0.91, R2fungi = 0.91), and soil pH (R2bacteria = 0.66, R2fungi = 0.90). Afforestation also favored a significant increase in mean soil respiration rates during the growing season, and high correlations were detected between soil respiration and microbial community abundances and diversity, SOC, NO3−-N, and soil pH. Because afforestation did not affect fine root biomass, increased soil respiration can be attributed to accelerated microbial respiration and elevated substrate availability. In addition, bacterial communities transitioned from Actinobacteria (oligotrophs)-dominant to Alphaproteobacteria (copiotrophs)-dominant communities. Copiotrophs are generally expected to have higher respiration rates than oligotrophs. Thus, the shift from Actinobacteria-dominant to Alphaproteobacteria-dominant bacterial communities may have also contributed to the increase in soil respiration following afforestation.