Belowground responses to altered precipitation regimes in two semi-arid grasslands

Predicted climate change extremes, such as severe and prolonged
drought, may profoundly impact biogeochemical processes like carbon
and nitrogen cycling in water-limited ecosystems. To increase our
understanding of how extreme climate events impact belowground
ecosystem processes, we investigated the effects of five years of
severe growing season drought and two-month delay in monsoon
precipitation on belowground productivity and biogeochemical
processes in two semi-arid grasslands. This experiment takes place
during the fifth year of the Extreme Drought in Grassland Experiment
(EDGE) at the Sevilleta National Wildlife Refuge (SNWR), a Long-Term
Ecological Research in central New Mexico, USA. The two grassland
sites a Chihuahuan Desert grassland dominated by Bouteloua eriopoda
and Great Plains grassland dominated by B. gracilis are ~5km apart
in the SWNR. The EDGE platform was established in the spring of 2012
(pre-treatment). Each site contains three treatments (ten
replicates): ambient rainfall, extreme growing season drought, and
delayed monsoon. The extreme drought treatment reduces growing
season rainfall (April through September) each year by 66%, which
equates to a 50% reduction of annual precipitation while maintaining
natural precipitation patterns. There are 10 replicates per
treatment within each site. All plots are 3 x 4 m in size and are
paired spatially into blocks with treatments assigned randomly
within a block. We measured an array of belowground and
biogeochemical variables. Each variable was measured either once,
twice, or three times (specific information on sampling scheme for
each measured variable in methods section). Belowground net primary
productivity, standing crop root biomass, total organic carbon, and
total nitrogen were measured once. Extractable organic carbon,
extractable total nitrogen, microbial biomass carbon, microbial
biomass nitrogen and extracellular enzymes were measured twice.
Available soil nitrate, available soil ammonium, and available soil
phosphate were measured three times.