Inactive and inefficient: Warming and drought effect on microbial carbon processing in alpine grassland at depth

Subsoils contain >50% of soil organic carbon (SOC) globally yet remain under-investigated in terms of their response to climate changes. Recent evidence suggests that warmer, drier conditions in alpine grasslands induce divergent responses in SOC decomposition and carbon accrual in top- versus subsoils. However, longer term effects on microbial activity (i.e., catabolic respiration vs. anabolic growth) and belowground carbon cycling are not well understood. Here we utilized a field manipulation experiment on the Qinghai-Tibetan Plateau and conducted a 110-day soil incubation with and without C-13-labeled grass litter to assess microbes' role as both SOC decomposers and contributors in the top- (0-10 cm) versus subsoils (30-40 cm) after 5 years of warming and drought treatments. Microbial mineralization of both SOC and added litter was examined in tandem with potential extracellular enzyme activities, while microbial biomass synthesis and necromass accumulation were analyzed using phospholipid fatty acids and amino sugars coupled with C-13 analysis, respectively. We found that warming and, to a lesser extent, drought decreased the ratio of inorganic nitrogen (N) to water-extractable organic carbon in the subsoil, intensifying N limitation at depth. Both SOC and litter mineralization were reduced in the subsoil, which may also be related to N limitation, as evidenced by lower hydrolase activity (especially leucine aminopeptidase) and reduced microbial efficiency (lower biomass synthesis and necromass accumulation relative to respiration). However, none of these effects were observed in the topsoil, suggesting that soil microbes became inactive and inefficient in subsoil but not topsoil environments. Given increasing belowground productivity in this alpine grassland under warming, both elevated root deposits and diminished microbial activity may contribute to new carbon accrual in the subsoil. However, the sustainability of plant growth and persistence of subsoil SOC pools deserve further investigation in the long term, given the aggravated N limitation at depth.

Automated summary

Subsoils globally contain more than 50% of soil organic carbon, yet their response to climate changes is under-investigated. A study on the Qinghai-Tibetan Plateau found that warming and drought treatments reduced organic carbon mineralization in subsoils, leading to decreased microbial activity and efficiency, while topsoils remained unaffected. This suggests that subsoil microbes may become inactive and inefficient under warming and drought conditions.