Abstract In the C 4 ‐dominated semiarid grasslands of the western Great Plains (Colorado, USA), spring soil moisture (soil volumetric water content, Soil VWC ) 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) Soil VWC with standard drought shelters. Our goal was to reduce rainfall inputs from May to early July in a native grassland dominated by C 4 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 Soil VWC . Indeed, the drought shelters were able to reduce Soil VWC 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 CO 2 efflux and canopy greenness, and no effect on ANPP, consistent with Soil VWC 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 CO 2 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 Soil VWC 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.
Condon et al. (Fri,) studied this question.