Seasonal and interannual variability in precipitation is
substantial in tallgrass prairies (Knapp, 1984). This variability
is a major factor affecting grassland structure and function, and
most key system processes from the microbial to the ecosystem level
vary with precipitation. Indeed, this variability may be
responsible for many of the characteristics that define tallgrass
prairie (Seastedt and Knapp, 1993). In 1990, we began experimental
supplementation of water across annually burned topographic
gradients on Konza Prairie. Transects were replicated in 1993. This
experiment was designed to help us to evaluate several important
issues related to climatic effects in this ecosystem. Questions
that are being addressed at upland, hillside and lowland
topographic positions include:<P>

Can site productivity be maintained under an annual fire regime
with net primary production maximized each year? Or will soil N
limitations become more severe over time as predicted by Century
model output (Ojima et al., 1990)?<P>

Will reproductive effort of the dominant grasses be enhanced by
long-term irrigation? Or will allocation to reproduction decline as
belowground resources become limited?<P>

Will significant plant species composition shifts occur? More
specifically, is annual fire sufficient to keep woody species from
increasing along these wet transects? Will increased competition
for N and light reduce the abundance of forbs?<P>

How will soil processes such as decomposition and N mineralization
be affected by irrigation? Will readily mineralizable C and N pools
decrease (Ojima et al., 1990)? How will the temporal dynamics of
microbial biomass be affected?<P>

     The planned minimum duration of this study is 10 years.
Variables to be measured along irrigated and control transects
include: soil moisture, plant water potential, NPP, reproductive
effort, plant species composition, soil organic C and N, microbial
biomass, N mineralization, plant C and N content, and litter
decomposition. Water supplementation is scheduled to offset
evapotranspiration losses and minimize moisture deficits during the
growing season.<P>

Knapp, A.K. 1984. Water relations and growth of three grasses
     during wet and drought years in a tallgrass prairie.
     Oecologia 65: 35-43.<BR>
Ojima, D.S., W.J. Parton, D.S. Schimel and C.E. Owensby. 1990.
     Simulated impacts of annual burning on prairie
     ecosystems. In: Collins, S.L. and L.L. Wallace (eds),
     Fire in North American Tallgrass Prairies. University of
     Oklahoma Press, Norman, OK.<br>
Seastedt, T.R. and A.K. Knapp. 1993. Consequences of
     non-equilibrium resource availability across multiple
     time scales: the transient maxima hypothesis. American
     Naturalist 141: 621-633.<br>