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- vs. subsoils. However, longer-term effects on microbial activity (i.e. catabolic respiration versus 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 13 C-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 five 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 13 C 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 was 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 on a longer term, given the aggravated N limitation at depth.