Water availability drives fine root dynamics in a Eucalyptus woodland under elevated atmospheric CO₂ concentration

Abstract

Fine roots are a key component of carbon and nutrient dynamics in forest ecosystems. Rising atmospheric [CO2] (eCO2) is likely to alter the production and activity of fine roots, with important consequences for forest carbon storage. Yet empirical evidence of the role of eCO2 in driving root dynamics is limited, particularly for grassy woodlands, an ecosystem type of global importance. We sampled fine roots across seasons over a 2‐year period to examine the effects of eCO2 on their biomass, production, turnover and functional traits in a native mature grassy Eucalyptus woodland in eastern Australia (EucFACE). Fine root biomass, production and turnover varied greatly through time, increasing as soil water content declined. Despite a lack of consistent effects of eCO2 on fine root biomass, production or turnover across the 2‐year sampling period, we found enhanced production pulses under eCO2 between 10‐ and 30‐cm soil depth. In addition, eCO2 led to greater carbon and phosphorus concentrations in fine roots and increased root diameter, but no detectable effects on other morphological traits. Synthesis. We found minor quantitative effects of eCO2 on fine root biomass dynamics that were largely driven by temporal variations in soil water availability. Our results suggest that in this mature grassy woodland, and perhaps also in other similar forested ecosystem types, eCO2 effects are small and transient. This also implies a limited ability of these systems to mitigate climate change through below‐ground mechanisms. A free Plain Language Summary can be found within the Supporting Information of this article.

The influence of roots on mycorrhizal fungi, saprotrophic microbes and carbon dynamics in a low‐phosphorus Eucalyptus forest under elevated CO2

“…It has been proposed that the extra C assimilated under eCO 2 is rapidly respired without changing soil C dynamics . This is supported by the lack of changes in root biomass observed by Piñeiro et al (2020) and our observation of the lack of impacts of eCO 2 on mycorrhizal fungi biomass. Changes in microbial community composition and enhancements of C losses and mycorrhizal biomass with eCO 2 have been found to be seasonal and substrate-dependent (Castañeda-Gómez et al, 2020).…”

Phosphorus and water supply independently control productivity and soil enzyme activity responses to elevated CO2 in an understorey community from a Eucalyptus woodland

“…In contrast (H1b), we expect stronger positive effects of P addition than of water addition under eCO 2 due to the particularly strong P-limitation of these soils (Crous et al 2015). We also predict that (H2) eCO 2 will increase fine root production and rhizosphere enzyme activity under low P and water supply conditions as a functional response to greater nutrient demand (Piñeiro et al 2020), but not under high levels of these resources. Finally, we hypothesize that (H3) eCO 2 will lead to a greater net CO 2 uptake rate (i.e.…”

“…During the first two weeks of the experiment, all pots were watered to gravimetric soil moisture contents (gSMC) of 21% (i.e., ∽90% of soil water holding capacity) to ensure homogeneous seed germination across water treatments, after which pots were held at treatment targets for soil water content (12% and 20%). The lower moisture level used in our experimental set up (∽60% WHC) serves as reference as it falls within the range for the experimental site during the growing season (Piñeiro et al 2020). Our higher moisture level (∽90% WHC) was selected to provide enough contrast with reference soil moisture levels.…”

The influence of elevated CO2 and soil depth on rhizosphere activity and nutrient availability in a mature Eucalyptus woodland

“…Root litter decomposition can thus be an important source of nutrient release at depth. Further, eCO2 has been found to increase the rate of root turnover in this system (Piñeiro et al, 2020), which is one of the main sources of C supply to the deeper soil, other than increased root exudation.…”

“…Though enzyme activities decrease with depth, they are more abundant per unit soil C deeper in the profile. Given the rather low eCO2 fertilisation effect found on photosynthetic rate (Ellsworth et al, 2017;Jiang et al, 2020) and root production in this system (Piñeiro et al, 2020) the presumed limited increase in C release belowground is likely turned over without affecting the SOM decomposition. Mineral adsorbed P forms however, are sensitive to root derived changes in pH (Jones and Darrah, 1994), representing a different mechanism for affecting the P cycle separate from SOM decomposition.…”