Response of a grassland species to dry environmental conditions from water stable isotopic monitoring: no evident shift in root water uptake to wetter soil layers

Abstract

Aims We aimed at assessing the influence of above- and below-ground environmental conditions over the performance of Centaurea jacea L., a drought-resistant grassland forb species. Methods Transpiration rate, CO2 assimilation rate, leaf water potential, instantaneous and intrinsic water use efficiency, temperature, relative humidity, vapor pressure deficit and soil water content in one plant and root length density in four plants, all grown in custom-made columns, were monitored daily for 87 days in the lab. The soil water isotopic composition in eleven depths was recorded daily in a non-destructive manner. The isotopic composition of plant transpiration was inferred from gas chamber measurements. Vertical isotopic gradients in the soil column were created by adding labeled water. Daily root water uptake (RWU) profiles were computed using the multi-source mixing model Stable Isotope Analysis in R (Parnell et al. PLoS ONE 5(3):1–5, 2010). Results RWU occurred mainly in soil layer 0–15 cm, ranging from 79 to 44%, even when water was more easily available in deeper layers. In wet soil, the transpiration rate was driven mainly by vapor pressure deficit and light intensity. Once soil water content was less than 0.12 cm3 cm− 3, the computed canopy conductance declined, which restricted leaf gas exchange. Leaf water potential dropped steeply to around − 3 MPa after soil water content was below 0.10 cm3 cm− 3. Conclusion Our comprehensive data set contributes to a better understanding of the effects of drought on a grassland species and the limits of its acclimation in dry conditions.

Deep roots mitigate drought impacts on tropical trees despite limited quantitative contribution to transpiration

“…The high deep-water uptake percentages observed in the B2WALD drought demonstrate that RWU from overlying soil was strongly limited, even though unbound water (Ψ soil > −1.5 MPa) was still available in the middle soil compartment. This could indicate that few fine roots were present therein and/or the measured trees developed higher root length densities above the impermeable concrete floor as observed in a column experiment ( Deseano Diaz et al, 2022 ). This deep root growth might be a response to the current or previous droughts ( Fan et al, 2017 ), and could be the reason for the observed mid-drought decrease in deep-water, thus indicating the below-ground drought adaptability of trees in the humid tropics.…”

Relevance of near-surface soil moisture vs. terrestrial water storage for global vegetation functioning

“…We find that vegetation preferentially utilizes the water from shallow soil in semi-arid regions and in regions with low tree cover, which is related to the continuous availability of near-surface water and a lack of deep rooting systems, respectively. The stronger correlation of NIRv with SSM than with TWS is supported by sitelevel studies that find higher root water uptake of surface soil moisture (Brinkmann et al, 2019;Gessler et al, 2022;Deseano Diaz et al, 2023;Kulmatiski and Beard, 2013) when deeper water is also available. Some local studies however find a higher root water uptake from deeper layers (Zhu et al, 2023).…”

“…This means that the uptake of deeper soil layers cannot compensate for the loss of water uptake from the dry topsoil. Contrary to trees, grasses do not shift their uptake depth (Deseano Diaz et al, 2023) or even extract water from the most shallow soils (Prechsl et al, 2015;Kulmatiski and Beard, 2013).…”

Challenges in studying water fluxes within the soil-plant-atmosphere continuum: A tracer-based perspective on pathways to progress

“…(Deseano Diaz et al, 2022a;Volkmann et al, 2016b;Volkmann and Weiler, 2014) and higher temporal resolution (daily to sub daily scale e.g., Mennekes et al, 2021;Kübert et al, 2020;Piayda et al, 2017;Volkmann et al, 2016b) for relatively low monitoring costs with high accuracy (Volkmann and Weiler, 2014). Beyer et al (2020) provided a broad review of insitu measuring methods for water stable isotopes.…”

“…However, isotopic nonsteady state conditions were encountered that limited measurements in the morning and evening. Indeed, the prevalence of isotopic non-steady state during most parts of the day (Dubbert et al, 2017(Dubbert et al, , 2014Kübert et al, 2022) has been a hindrance to utilize chamber measurements coupled to laser spectrometers in the field to infer xylem isotopic composition in high temporal resolution (Deseano Diaz et al, 2022a;Kühnhammer et al, 2020). Nevertheless, a recent study clearly demonstrated that integrated to daily resolution, in-situ chamber measurements of transpiration are very well suited to reflect steady state and hence dynamics in the xylem isotopic composition of both woody and non-woody species (Kübert et al, 2022).…”