Background & Objectives
top 30 cm of the soil profile in each mesocosm contains 3-rod wave-guides
connected with an automated time domain reflectometry system (TDR 100,
Campbell Scientific, Logan,
The TDR system is set up to determine soil water content (volumetric) at
2-hour intervals. Wireless networking capability at the mesocosm facility
allows soil moisture data to be downloaded remotely.
Soil CO2 flux is measured monthly using an 850 cm3
cylindrical chamber with a 40.7 cm2 circular opening coupled to a
closed-flow gas exchange system (Li-Cor 6200, LiCor Inc., Lincoln, NE, USA).
Grass/forb plant responses
Leaf level photosynthesis and
midday leaf water potential have been conducted at varying intervals,
focusing on the C4 grass Andropogon gerardii
and the legume Lespedeza capitata.
Photosynthesis measures are made with an LI-6400 photosynthesis system
(Li-Cor 6400, LiCor Inc., Lincoln, NE) and water potential is measured with a
pressure chamber (PMS-1000, PMS Instruments Inc., Corvallis, OR, USA).
Aboveground NPP and species composition
Annual net primary productivity (ANPP) is estimated from end of season
harvests of three 0.1 m2 quadrats per container. All biomass is
sorted by species and dried to constant weight. All aboveground standing dead
biomass is removed immediately prior to the growing season start (early
April), so end of season biomass is an estimate of ANPP.
Root distribution and turnover
One minirhizotron tubes are installed per container (45º angle, 1 m
depth). Images are captured three times per growing season with a Bartz
Technology Co. (model BTC-2, Santa
Barbara, CA, USA) digital root imaging system
to document root production. The images are digitized with RooTracker
software (Version 2.0, The National Phytotron at Duke
University, Durham, NC, USA). Root image data are
augmented by root biomass depth profiles determined from soil cores (5 cm
diameter x 60 cm deep) that will be taken during the 2006 season. Soil cores
(12 per treatment) will be washed and the roots collected on screens for
biomass and C and N determinations.
Plant and soil C and N
Root, soil and ANPP samples are ground and analyzed for C and N content using
a Carlo-Erba NA 1500 C/N analyzer (Carki Erba Instruments, Milan, Italy);
results are archived for possible future analysis. Resin bags will be used
during 2006 to estimate N availability under the precipitation treatments.
This approach provides an integrated measure of N availability, and requires
less destructive sampling than buried soil cores. Characterization of
concentrations and depth distributions of extractable N (NO3-
and NH4+) will be done on soil from the root cores. All
inorganic N analyses will be done by automated analysis (Alpkem FlowSolution
System, Alpkem, Milwaukie,