Monsoon Rainfall Manipulation Experiment (MRME)

The objective of the Monsoon Rainfall Manipulation Experiment (MRME) is to understand changes in ecosystem structure and function of semiarid grassland caused by increased precipitation variability, which alters the pulses of soil moisture that drive primary productivity, community composition and ecosystem functioning. The overarching hypothesis being tested is that changes in event size and variability will have differential effects on grassland dynamics and creosote invasion of desert grassland. Specifically, we are testing the hypothesis that increased frequency of small rainfall events will increase grassland above and belowground NPP, relative to plots receiving a small number of large events during the monsoon season. In addition, we predict that many small events will increase soil CO2 effluxes by stimulating microbial processes but not plant growth, whereas a small number of large events will increase aboveground NPP by providing sufficient deep soil moisture to sustain plant growth for longer periods of time during the monsoon.

MRME contains three ambient precipitation plots and five replicates of the following treatments: 1) ambient plus a weekly addition of 5 mm rainfall, 2) ambient plus a monthly addition of 20 mm rainfall. Rainfall is added during the monsoon season (July-Sept) by an overhead (7 m) system fitted with sprinkler heads that deliver rainfall quality droplets. At the end of the summer, each treatment has received the same total amount of added precipitation, delivered in different sized events. Each plot (9x14 m) includes subplots (2x2 m) that receive 50 kg N ha-1 y-1. Each year we measure: (1) seasonal (July thorugh October) soil N using Plant Root Simulator Probes, (2) plant species composition, above- and belowground NPP in two permanently located fertilzied and control quadrats in each replicate plot, and (3) soil temperature, moisture and CO2 fluxes (using in situ solid state CO2 sensors). In addition, soil N2O fluxes, and predawn and mid-day (10-12 AM) water potential and mid-day leaf photosynthetic gas exchange and stomatal conductance of black grama have been measured prior to and up to 5 days after scheduled precipitation events.