The Monsoon Rainfall Manipulation Experiment (MRME) is to understand changes in ecosystem structure and function of a 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 alter grassland productivity, ecosystem processes, and plant community dynamics. In particular, 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 and soil respiration by providing sufficient deep soil moisture to sustain plant growth for longer periods of time during the summer 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, August, September, and October through June) soil N, (2) plant species composition and ANPP, (3) seasonal root and fungal dynamics in minirhizotrons, and (4) 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 are measured prior to and up to 5 days after scheduled precipitation events.Soil Measurements
Soil temperature and water content were measured with ECH2O soil sensors. Soil CO2 was measured with solid state CO2 sensors. For each plot, soil sensors were placed under the canopy of B. eriopoda to a depth of 2, 8, and 16 cm. Measurements were recorded every 30 minutes. Daily soil water content at each depth was calculated as the average soil water content for measurement times between the hours of 0000-06:00.
Instrument Name: Solid State Soil CO2 sensorManufacturer: VaisalaModel Number: GM222
Additional Study Area InformationStudy Area Name: Monsoon siteStudy Area Location: Monsoon site is located just North of the grassland Drought plotsVegetation: dominated by black grama (Bouteloua eriopoda), and other highly prevalent grasses include Sporabolus contractus, S.cryptandrus, S. lexuosus, Muhlenbergia aernicola and Bouteloua gracilis.North Coordinate:34.20143South Coordinate:34.20143East Coordinate:106.41489West Coordinate:106.41489
Additional Information on the Data Collection Period
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