Soil moisture response to multiple, interacting factors of global change in an old-field ecosystem

Jake F. Weltzin1, Philip B. Allen1, Richard J. Norby2 and Lara A.C. Souza1

1 University of Tennessee and 2 Oak Ridge National Laboratory

Leaf Area Index (LAI) was estimated monthly during the growing season at six locations within each plot using a line-integrating ceptometer (AccuPAR) below the plant canopy coupled to an external quantum sensor above the canopy. We analyzed LAI by date with a mixed model ANOVA. In contrast with VWC, LAI was little affected by the warming treatment. LAI was most affected by the irrigation treatment, particularly later in the growing season, when it was greater in ‘wet’ than in ‘dry’ plots. LAI tended to be greater under elevated than ambient [CO2], particularly during the second growing season. As for VWC, there were no interactions among irrigation, temperature, and [CO2].

For each soil depth, we analyzed monthly mean VWC for main and interactive effects of irrigation, temperature, and [CO2] by date using a mixed model ANOVA. For each of the 17 months analyzed, there were no 2- or 3-way interaction effects (P > 0.16). To date, the top 30 cm of soil has been consistently drier in warmed plots than in ambient plots; at 55 cm, effects of warming are apparent only during the growing season. During the second growing season in particular, VWC in shallow soil was typically greater under elevated than ambient [CO2], although this effect was not observed at deeper depths. VWC was consistently greater in ‘wet’ than in ‘dry’ treatments on every date after August 2003.

After we adjusted our irrigation regime, VWC at 0-15 cm differed clearly between wet and dry treatments throughout the experiment. VWC in treated plots generally tracked VWC in the unchambered controls, although treated plots exhibited less of a range in VWC than did control plots. During the non-growing season, VWC in control plots was typically about 3% (absolute) greater than VWC in ‘wet’ plots; however, during the growing season, VWC in control plots exhibited greater variation, and would often dry to levels commensurate with the low range of the ‘wet’ plots.

Open-top chambers with fixed precipitation shelters were used to manipulate [CO2] (ambient, + 300 ppm), air temperature (ambient, + 3° C), and soil moisture (‘wet’, ‘dry’). Details on the treatments and the plant community are on nearby posters; this poster is focused on methodology and response of the system in terms of soil moisture. Hand watering of the split-plot watering treatment initially followed a computer-generated year of precipitation events, with varying event size and frequency, based on the local 50 year average precipitation record. ‘Dry’ plots received 33% less irrigation than each computer-generated event while ‘wet’ plots received 33% more. This regime resulted in a two-fold difference in irrigation amount, but failed to maintain differences in soil moisture between ‘dry’ and ‘wet’ plots, even during the peak of the 2003 growing season. In mid-August 2003, we adjusted our irrigation regime: on a weekly basis during the growing season, and weekly to twice-monthly during the non-growing season, dry plots receive 2 mm of irrigation water and wet plots receive 25 mm of irrigation water. Further, during the late growing season of each year, when foliar cover of vegetation and temperature of air are greatest, we periodically increase the amount of irrigation water applied to the ‘dry’ plots to avoid over-drying of soils.

Plots were prepared by trenching to a depth of 75 cm, installing time domain reflectometry (TDR) probes vertically from 0-15cm at two locations near the center of each plot, and horizontally within the trenches at depths of 30 cm and 55 cm at two locations within each plot. We then lined the trenches with insulative foam and 6-mil PVC film, and backfilled the trenches.

Changes in atmospheric [CO2], coupled with increases in tropospheric temperatures and changes in precipitation regimes, are likely to affect the structure and function of managed and natural communities and ecosystems. There have been few investigations of how these factors may interact to affect in-situ communities in natural field settings. We are investigating interactive effects of [CO2], temperature, and soil water availability within open-top chambers that contain constructed old-field communities near Oak Ridge, Tennessee. One of our key hypotheses has been that the response of the plant community to elevated [CO2] and increased air temperature will be mediated by the availability of soil moisture, with an awareness that the biotic community itself may exert feedback controls over soil moisture contents. Moreover, we hypothesized that there would be strong interactions among these driving variables in terms of most response variables, including soil moisture contents. Here, we outline the response of our old-field system, in terms of soil moisture through time and space, between experiment initiation in May 2003 and October 2004.

Introduction

Methods

             

Leaf-level transpiration was determined for two individual plants of each of four dominant herbaceous species (Plantago lanceolata, Trifolium repens, Solidago canadensis, and Dactylis glomerata) four times during the 2004 growing season using a portable photosynthesis system. Transpiration rates were integrated across a 24-hour period, and were analyzed for each date using a mixed model ANOVA. In April, June, and September, transpiration for each species differed little among treatments. At peak growing season (in July), transpiration by Trifolium and Solidago was greater in ‘wet’ than ‘dry’ plots; transpiration for other species was unaffected by irrigation. Transpiration for all species in July was unaffected by temperature or [CO2]. Similar to VWC and LAI, there were few interactions among the three driving variables.

In sum, warming decreased VWC, whereas elevated [CO2] and irrigation increased VWC; the paucity of interactions among treatments suggests that each driving variable is sufficiently important to override potential feedbacks among treatments (e.g., elevated [CO2] was insufficient to overcome dry conditions caused by irrigation or warming).

Warming-induced reductions in VWC were not attributable to changes in transpiration by the dominant species; this observation, coupled with a trend towards reductions in LAI with warming, suggests that warming controls VWC through effects on evaporation. Elevated [CO2] increased both VWC and LAI, but did not apparently affect transpiration. These data suggest that VWC in plots with elevated [CO2] was controlled less by leaf- or plot-level transpiration than by reductions in evaporation caused by increases in LAI and plant foliar cover (Engel, data not shown). Stomatal conductance data are currently being analyzed.

Our understanding of controls over soil moisture in this experiment will be strengthened by partitioning evapotranspiration (ET) into component fluxes of evaporation and transpiration. ET can be scaled to the level of the plot from leaf-level measurements, although we have yet to determine species-specific leaf areas. Alternatively, we are investigating the use of Keeling plots to partition ET using plot-level chamber measurements; we will need to develop an approach to extend this technique to gas exchange chambers nested over plots within our OTCs, which has seldom been done.

Soil moisture response - treatment effects Leaf Area Index and leaf-level transpiration

Research was supported by U.S. Department of Energy, Office of Science, Biological and Environmental Research