Subtle differences in spring soil moisture can substantially influence annual productivity in a semiarid grassland
Condon, K. V. and A. K. Knapp . 2026.
Abstract
In the C4-dominated semiarid grasslands of the western Great Plains
(Colorado, USA), spring soil moisture (soil volumetric water content, SoilVWC)
is thought to be a critical determinant of aboveground net primary production
(ANPP) and overall carbon cycling. However, because evidence for this is
based on observational studies, we attempted to test this sensitivity by experimentally reducing spring (May-June) SoilVWC with standard drought shelters.
Our goal was to reduce rainfall inputs from May to early July in a native grassland dominated by C4 Bouteloua spp. and assess ANPP and related carbon
cycle responses. Although we were able to reduce ambient rainfall inputs
directly into the plots, spring precipitation was unusually high (~40% above
average) in the year of this experiment (2021), and our treatments had negligible impacts on SoilVWC. Indeed, the drought shelters were able to reduce
SoilVWC below ambient levels for only 2 weeks, after a large natural precipitation event (~65 mm) fell in late June. As expected, by mid-July, we could
detect only minor impacts of our treatments on soil CO2 efflux and canopy
greenness, and no effect on ANPP, consistent with SoilVWC remaining at
non-limiting levels throughout most of the first half of the growing season.
Despite the seeming ineffectiveness of the spring drought treatments, we challenged these plots with an experimental deluge (60 mm of water addition)
after the drought shelters were removed. Surprisingly, in plots subjected to the
drier spring treatment, we measured significant reductions in soil CO2 efflux
immediately after the experimental deluge as well as reduced canopy greenness throughout much of the remainder of the growing season. Moreover,
end-of-season ANPP was reduced by 32%, and regrowth after defoliation
("simulated grazing" at mid-season) was reduced by 45% in the treatment with
only minor reductions in SoilVWC in the spring. Our largely serendipitous
results confirm the strong sensitivity of this grassland to alterations in early
season rainfall inputs, even during a wet spring when measurable impacts on
soil moisture are minimal.
