Plant roots exert critical controls over microbial decomposition, but
climate change factors may alter these effects. Yet, the direction and
magnitude of interactive climate change effects, as well as the
underlying mechanisms, remain unclear. Here we show evidence from a
field experiment demonstrating that climate change factors strengthen
root controls over litter decomposition in a semi-arid grassland. While
plant roots significantly increased soil microbial biomass, they
suppressed microbial activities, C-cycling enzymes and litter
decomposition. Both precipitation reduction and warming reduced
decomposition regardless of the root presence, but precipitation
increase stimulated decomposition only in the absence of roots,
suggesting that plants outcompete microbes for water and constrain
microbial activities. Together, warming-enhanced plant competition for
water, combined with N-suppression of microbes, may provide a unique
mechanism through which moderate increases in precipitation, warming and
N inputs interactively enhance root controls over microbial
decomposition, thereby facilitating soil C sequestration in the
water-limiting grasslands.